A study of Thermosbaena mirabilis (Malacostraca, …...261 A study of Thermosbaena mirabilis (Malacostraca, Peracarida) and its reproduction By D. BARKE R (From the Department of Zoology,

261

A study of Thermosbaena mirabilis (Malacostraca,Peracarida) and its reproduction

By D. BARKER

(From the Department of Zoology, University of Hong Kong. Present address: Depart-ment of Zoology, Durham Colleges, University of Durham)

With a plates (figs. 6 and 8)

SummaryKnowledge of the anatomy of Thermosbaena is extended as follows: (i) The topographyof the head and anterior thoracic region, and the orientation and relationships of themouthparts and anterior appendages are described, (ii) A branchial chamber islocated underneath each lateral carapace flap, (iii) 'Skin' glands are demonstrated tooccur at the bases of the paragnaths, maxillules, and maxillipedes; in the labrum and inthe mandibular palps; in the groove leading from the food basin to the paragnathbases; and in the basal segments of the endopodites of peraeopods II-V. The functionof these glands is obscure; histochemical tests indicate that they do not secrete mucus,(iv) The posterior aorta is traced from the heart to the level of the first abdominalsegment, where it bifurcates into 2 lateral vessels which diverge to each side of theintestine, (v) Four conditions of the female reproductive system are distinguished,namely, the immature, mature, copulatory, and brood-pouch conditions; temporaryseminal receptacles are formed by dilatation of the oviducts in the copulatory condi-tion.

An inhalant respiratory current is produced by oscillation of the maxillipedeepipodites in the branchial chambers. The exhalant respiratory current emerges viacarapace lappets on either side of the head.

Thermosbaena feeds on particles of detritus suspended in a mucilaginous matrixthat is probably derived from the mucilaginous sheaths of the Cyanophyceae in thediet. A main food-stream appears to be driven forwards between the peraeopods,through a channel formed by the maxillipede bases to the food basin, and fromthence along a ventral groove and between the paragnath bases to the oral cavity. Itis possible that the main food-stream is augmented by detritus brought by the inhalantrespiratory current, and by lateral feeding currents drawn in from the sides.

Following internal fertilization, early segmentation of the embryo occurs within theovaries. The blastomeres are first formed within the body of the yolk and sub-sequently rise to the surface, the segmentation thus resembling that of Hemimysisand Mesopodopsis. The embryos are transferred to the dorsal brood-pouch shortlyafter segmentation begins, probably by expulsion from the posteriorly directed vaginaewhile the female is upside down, followed by suction into the brood-pouch with theinhalant respiratory current. The embryos emerge from the brood-pouch as miniatureadults with the full complement of peraeopods. There appears to be no transitory7th abdominal ganglion.

Thermosbaena is probably paedomorphic in certain respects and less primitive thanMonodella. There are insufficient grounds for establishing a new malacostracandivision Pancarida, as advocated by Siewing (1958), to receive the Thermosbaenacea.

[Quarterly Journal of Microscopical Science, Vol. 103, part 2, pp. 261-86, June 1962]

262 Barker—Thermosbaena mirabilis and its reproduction

IntroductionT H E relict malacostracan Thermosbaena mirabilis Monod was discovered bySeurat in a hot-spring at the oasis of El Hamma, near Gabes, Tunisia, in 1923,and was named and described by Monod (1924 a, b). Three cavernicolousspecies of a related form, Monodella, have since been discovered; 2 in Italy, M.stygicola (Ruffo, 1949 a, b), and M. argentarii (Stella, 1951 a, b); and 1 in Yugo-slavia, M. halophila (Karaman, 1953). The order Thermosbaenacea (Monod,1927a), which embraces these forms, is generally regarded as belonging to thePeracarida, though Siewing (1958) has established a new malacostracan divi-sion to receive it, the Pancarida, and syncaridan affinities have been urged byTaramelli (1954) and Stella (1959).

So far as can be gathered from published information, Thermosbaena hasbeen collected from the hot-springs (maximum temperature 480 C) at ElHamma on 6 occasions during the past 39 years. Seurat obtained 3 specimenson his first visit in 1923 (see Monod, 19246), and a further 15 specimensin 1925 (see Monod, 19276), when Omer-Cooper and Hill also visited theoasis and collected 'rather more than 20 specimens' (Omer-Cooper, 1928).Subsequent collections were made by Absolon in 1927 (number of specimenscollected not stated; Absolon, 1935); by Bruun in 1938 (158 specimens;Bruun, 1939); and by Russell and myself in 1950 (714 specimens; Barker,1956, 1959, i960). Our collection contained breeding female specimens,hitherto unknown, and was made from two Roman baths fed by hot-springsknown as Ain el Bordj, the type-source, and Ain Sidi Abd el Kadar, where theanimal had not previously been recorded. In an account of the collection ofthese specimens (Barker, 1959), the distribution and systematic position ofthe Thermosbaenacea are discussed, and it is suggested that Thermosbaena isan interstitial organism living in thermal brackish ground-water at El Hamma,the habitat being of comparatively recent origin. In the present study, certainnew features of the morphology of Thermosbaena are presented, the methodof feeding is discussed, and an account is given of the reproduction and de-velopment, which have not previously been described.

MethodsThe specimens collected at El Hamma were preserved in Bouin's fluid.

In addition to observations made of the living animal, the study has includedthe examination of 8-/x paraffin-wax serial sections cut in transverse, sagittal,and horizontal longitudinal planes and stained by Mallory's method; thepreparation of whole mounts stained with paracarmine; the dissection andmounting of individual mouthparts and appendages; and the binocular ex-amination of preserved specimens. Histochemical tests for the presence ofmucus were carried out on serial sections of 2 specimens using alcian blueand toluidine blue. In order to gain further knowledge of the morphology ofthe carapace and mouthparts, a X 160 reconstruction was made from cameralucida drawings of 65 8-̂ t horizontal longitudinal sections. Each drawing was

Barker—Thermosbaena mirabilis and its reproduction 263

traced on to a sheet of 'vinagel 118' (a polyvinyl acetate compound) of theappropriate thickness, and then cut out. Models of each successive sectioncreated in this way were built up to form the reconstruction, which on com-pletion was hardened at a temperature of 1500 C. After photographs hadbeen taken, the reconstruction was bisected in the sagittal plane and one sideof the carapace cut away to reveal the branchial chamber. In studying thefemale reproductive system and embryos, graphic reconstructions were madein a number of instances from amalgamated camera lucida drawings.

External features

Thermosbaena is a small, unpigmented crustacean 2 to 3 mm long. Rudi-mentary eyes have been described by Siewing (1958): they are evident insection but are not visible externally. The carapace is fused with the firstthoracic segment and normally extends back only as far as the fourth, thoughin the breeding female it becomes greatly enlarged to form a brood-pouch.It is prolonged anteriorly into two small lappets on either side of the rostrumin the neighbourhood of the bases of the antennae and antennules. Thebody-form is elongated and cylindrical, though there is some degree of dorso-ventral flattening. The cephalocaudal axis of the animal is slightly flexed sothat the body has a ventral curvature. The general appearance is shown infigs. 4, E; 6, A, E.

Most adult specimens are between 2-5 and 3-0 mm long; specimens lessthan 2-0 mm long may be regarded as juvenile (see p. 282). The maximumlength attained by either sex in the specimens collected was 3 -5 mm. Thereis no significant difference in length between the sexes, as implied by Bruun(1939), who figures a male 2-5 mm long and a female 3-0 mm long. Monod(1940) supposed that breeding female specimens would be the longest, but infact they are no longer than other adults. As compared with other Thermos-baenacea, Thermosbaena is larger than M. stygicola (1-5 mm), but slightlysmaller than M. halophila (4-0 mm) and M. argentarii (3-0 to 3-5 mm).

In common with species of Monodella, Thermosbaena possesses a biramousantennule; a mandibular lacinia mobilis; a maxillipede epipodite dorsally andbackwardly dirocted in the branchial chamber; and 2 pairs of vestigial pleopods.In contrast to Monodella, it possesses 5 pairs of peraeopods instead of 7; thetelson is fused with the sixth abdominal segment and not articulated with it;and the pair of uropods, though well developed, is slightly smaller and lessplumose. The maxillipede of male Thermosbaena bears no copulatory endopo-dite, and thus differs from the males of M. halophila and M. argentarii (themale of M. stygicola remains to be discovered).

The only constant external difference between the sexes is the pair of penesborne by the male on the eighth thoracic segment. It is unreliable, how-ever, to differentiate between the sexes solely on the presence or absence ofthese organs; an apparent female, by this criterion, may be a male with thepenes flattened against the abdomen and hidden from view. In fresh material


the most readily recognizable specimens are the breeding female, with itsenlarged carapace, and the mature male when it possesses full, pink-colouredampullae (see p. 274); the sex of other specimens, however, is often obscure.Thus while 103 specimens could not be sexed with certainty by microscopicalexamination at El Hamma, subsequent sectioning revealed 21 mature males,5 immature males, 9 mature females, 23 immature females, and 45 juveniles(gonads not developed). The sex analysis made by Bruun (1939) of hiscollection (15 males, 109 females, and 34 juveniles) should be treated withcaution since the presence or absence of penes and size were his sole criteria.Bruun concluded from this analysis that males were rare, but of the 381specimens in our collection that were sexed, 43% were male, 32-3% female,and 247% juvenile.

Abnormalities. One of the juvenile specimens collected possessed an addi-tional pleopod on the left side. It was attached to the hind border of thethird abdominal segment, thus making 3 pleopods on this side instead of theusual 2, the supernumerary one being the largest. In 4 male specimensthe penes were of an aberrant bifid nature with a single or multi-segmentedappendage springing from the base. In 3 cases the anomaly was common toboth sides, and in the fourth case it occurred on the right side only. In 2 ofthe males the extra portion was a simple palp with 3 or 4 terminal setae; inone the palp was borne by a basal segment; and in the other the extra portionwas in the nature of a slender 4-segmented limb, twice as long as the penisjoined to it, and without setae.

External morphology of head and thorax

The general features of the external morphology of this region are typicallyperacaridan, and there are obvious similarities between my findings and thoseof Cannon and Manton (1927) on Hemimysis, and Dennell on Diastylis (1934)and Apseudes (1937). The mouthparts and appendages have been describedby Monod (1927a, 1940), but there is little information available as to theorientation of the various parts and their relation to the body; the descriptionsthat follow concentrate chiefly on this aspect.

FIG. I . External morphology of the carapace, mouthparts, and anterior thoracic regionof Thermosbaena. The figures are based on a x 160 reconstruction supplemented by theexamination of serial transverse and sagittal sections and preparations of individual append-ages. A, the head is viewed from the left side and the carapace (limit indicated by broken line)has been cut away to reveal the branchial chamber. The right-hand appendages are omitted,and the left antennule, antenna, and peraeopods III and IV are truncated at their bases.Spines and setae omitted from mouthparts other than the maxillipede. B, internal sagittalview of the right half of the bisected head. Internal organs are omitted and the right antennuleand peraeopod exopodites are truncated at their bases. In both figures 'skin' glands are shownas black patches, mxp ep, maxillipede epipodite (double-headed arrow indicates movement inbranchial chamber); th 2 to 5, second to fifth thoracic segments. Peraeopods indicated by

roman numerals.


branchialchamber

exhalant

mandibleparagnath

mandibu/ar palpproximal endite of maxilla

inhalantrespiratorycurrent

peraeopod endopodite

coxa/ endite of maxillipedeI

basal endite of moxillipedeFIG. 1

doctylus


crp Ipt

FIG. 2


The carapace and branchial chambers

The carapace extends back from the head so as to form an arched openingover the hind border of the fourth thoracic segment (fig. 1, A, B). In sectionedmaterial the extent of this opening is approximately 0-2 mm deep and 0-5 mmwide. The sides of the carapace are formed by lateral flaps which cover thebases of the mouthparts and the first pair of peraeopods. Fusion with thehead is along a line extending from the hind border of the first thoracic segmentto the lateral bases of the mandibles and maxillules. The lateral flaps extendanteriorly as two lappets which lie on either side of the rostrum and coverthe lateral bases of the antennules and antennae. The lappets fuse with thehead along a line which extends dorsally from the lateral base of each man-dible to the side of the rostrum (figs. 1, A; 2, C-E).

Two narrow channels diverge ventro-laterally from the point where thecarapace becomes detached from the dorsal mid-line of the body, and leadinto a branchial chamber on each side (figs. 1, A; 2, A-D). The chambers areformed owing to the narrow width of the first thoracic segment as comparedwith the second, which results in there being a shallow depression underneaththe carapace on either side. The limits of each chamber are formed posteriorlyby the outer walls of the second thoracic segment, and anteriorly by the lineof fusion of the carapace flaps with the head. Exit from the branchial cham-bers is via the carapace lappets.

The ventral surface

From the level of the eighth to the second thoracic segment the mid-ventralsurface immediately below the nerve-cord is marked by a groove which,though shallow for the most part, is deep and well-formed at its hind end(fig. 3). At the level of the first thoracic segment there is a pronounced de-pression of the ventral body surface (fig. 1, B); this segment is thus not onlynarrower in width but also shallower in depth than those immediately behindit, with the result that the entire thorax in this region is constricted to form a

FlG. 2. Camera lucida outline drawings of 9 horizontal longitudinal 8-/x sections of Thermos-baena selected from a total series of 118 sections and 65 drawings made in creating thereconstruction of the carapace, mouthparts, and anterior thoracic region referred to in thetext. The drawings shown in A to j have been selected for their illustration of features ofparticular interest, and represent the following sections (numbering serially from dorsal toventral surface): A, 26; B, 30; c, 39; D, 44; E, 46; F, 48; G, 54; H, 56; 1, 61; j , 69. In A to Dthe right half of the section only is shown; E, F, and H include some part of the left half inaddition, while G, 1, and j are complete. 'Skin' glands indicated as black patches, peraeopodsby roman numerals, a J, antennule; az, antenna; br ch, branchial chamber; crp, carapace;crp Ipt, carapace lappet; dtr, detritus (distribution of detritus particles in each section isaccurately shown); fd bn, food basin; Ibr, labrum; mnd, mandible; mnd p, mandibular palp;mx 1, maxillule; mx 1 p, maxillulary palp; mx 2, maxilla; mxp, maxillipede; mxp bsl end,basal endite of maxillipede; mxp cxl end, coxal endite of maxillipede; mxp ep, maxillipedeepipodite; mxp ex, maxillipede exopodite; or cv, oral cavity; per end, peraeopod endopodite;per ex, peraeopod exopodite; pr, paragnath; th 1 to 4, first to fourth thoracic segments; v gr,

ventral groove.


'neck'. From here to the oral cavity a median ventral path may be traced(figs, i, B; 2, G-i; 3) which leads between the bases of the maxillipedes to aspace bounded laterally by the bases of the maxillae; this appears to correspondto the 'food basin' of Hemimysis (Cannon and Manton, 1927). A mediangroove descends from this basin, passes between the paragnath bases, andthence leads dorsally into the oral cavity.

The maxillipede

The maxillipede is the largest and most elaborate mouthpart. It arises inthe branchial chamber as a ridge-like protuberance of the body-wall (figs. 1,A; 2, c, D) which increases in size ventrally and finally becomes detached asthe limb base situated in the anterior part of the ventral depression of thefirst thoracic segment (fig. 1, B). On their inner sides the bases of each append-age are concave and become closely apposed to form a distinct channel (fig. 8,F, G). At this level, on the lateral side, the epipodite is given off, a broad thinblade which sweeps out laterally towards the wall of the carapace and thencurves upwards and backwards into the branchial chamber (figs, i, A; 2, C-E;4, E; 8, F). The outer margin of the blade is covered with a fringe of finesetae which extends around the tip for a short distance. Monod (1927a,fig. 4, E) shows setae equally distributed along both margins of the blade, butthe presence of setae on the outer margin and lack of them on the inner marginis apparent in both sectioned and dissected material. Immediately below thejunction of the epipodite with the limb base, a feebly developed, 2-segmentedexopodite branches off bearing 3 long, feathered spines at its distal end. Itis the most laterally placed of all the mouthparts, and in the reconstruction itlay parallel to the ventral limit of the carapace, extending forwards as far asthe level of the maxillules (figs. 1, A; 2, G-i). The limb base in then prolongedventrally into a large flat basal endite which becomes progressively thinnerand has a lateral margin that is slightly curled forwards (figs. 1, A, B; 2, 1, j).Finally, the limb base, on its inner aspect, gives rise to the coxal endite, asimple protuberance which bears 3 long spines, the longest of which, togetherwith those on the opposite side, extend forwards in the mid-ventral line as faras the bases of the paragnaths (figs. 1, B; 2,1). The distribution of spines andsetae on the appendage has been described by Monod (1927a), but theorientation of the various parts is not evident from his figure, which shows theappendage as seen when flattened by a coverslip after removal by dissection.Examination of sectioned and dissected mature male specimens failed to revealthe presence of a copulatory endopodite as occurs on the male maxillipede ofM. argentarii (Stella, 1951 a, b, 1953) and M. halophila (Karaman, 1953).

Other mouthparts

The positions occupied by the mouthparts [in relation to the oral regionand food basin is shown in ventral view as a schematic ground-plan in fig. 3.


A comparable figure has been published by Siewing (1958, fig. 41) showingthe mandibles in the position occupied by the paragnaths, which are omitted,and is altogether misleading.

FIG. 3. Schematic ground-plan of head and anterior thoracicregion of Thermosbaena as seen in ventral view, with all mouth-

parts and appendages truncated at or near their bases.

The sides of the median ventral groove leading forwards from the foodbasin merge anteriorly with the medial aspects of the paragnath bases (fig. 2,G-I) . Each paragnath assumes the form of a lobe with a depressed concavesurface facing forward and a fringe of fine setae clothing the inner margin.The oral cavity is formed anteriorly by the inner base of the labrum, laterallyby the medial bases of the mandibles, and posteriorly by the front of theparagnath bases. The molar processes of the mandibles reach up into the oralcavity for a short distance and meet in the mid-line with their grinding surfacesclosely apposed. The mandibular lobes are placed immediately below the oralcavity and are fringed with a row of 6 or 7 strong setae on their inner marginswhich approximate in the mid-line in front of the paragnaths. Their distalextremities bear the incisor processes and laciniae mobiles, of which, in bothcases, those borne by the left mandible are larger than those on the right. Eachincisor process and lacinia mobilis interdigitates with its partner on the oppositeside, crossing over at the level of the ventral tip of the labrum. On its lateralaspect, the base of the mandible gives rise to a well-developed palp of 3 seg-ments which extends ventro-laterally bearing a few feathered setae (figs. 1, A ;2, j). As judged from a number of sectioned specimens, the arc of movementof these palps would appear to lie from the level of the maxillules and thenceforwards between the antennae on either side of the labrum. The labrum is asimple flap, convex on its forward aspect, with the base springing anteriorly


from between the bases of the antennules. The tip bears a fringe of setae, andas the flap narrows down ventrally, its anterior surface becomes corrugatedinto 4 ridges. The depth of the labrum from tip to ventral border of oralcavity is approximately 0-15 mm; it is a much deeper flap than shown bySiewing (1958, figs, i, A, 28).

The base of each maxilla and maxillule arises as a ridge on the innerwall of the branchial chamber anterior to the base of the maxillipede (figs,i, A; 2, D). The medial aspect of each limb base, as in the case of themandibles and maxillipedes, is the last to separate from the head and becomeindividuated (figs. 1, A; 2, E-H). Both mouthparts bear 2-segmented palpswhich are directed forwards and ventrally; that of the maxilla extends be-tween the paragnath and mandibular palp (figs. 1, A; 2, j). The proximalendites of both mouthparts bear long spines which are directed forwards inthe ventral mid-line; those of the maxillae lie to either side of the spines ofthe coxal endites of the maxillipedes (figs. 1, B; 2, 1), the longest extendingup to the paragnaths, while those of the maxillules are the most laterallyplaced and extend between the paragnaths. The nature of the spines andsetae of both mouthparts have been described and figured in detail by Monod(1927a, 1940).

The peraeopodsEach peraeopod consists of a 2-segmented exopodite, which bears long,

feathered setae and is ventrally and backwardly directed; and a 4-segmentedendopodite bearing a strong dactylus directed forwards (figs. 1, A, B; 4, E; 6, E).Siewing (1958, fig. 1, A) shows each dactylus directed backwards, but theforward position may be observed in both sectioned and preserved specimens;even at the extreme limit of the backswing of the endopodite, the dactyluspoints forwards. The first peraeopod is smaller than the rest and its base isso orientated that the outer aspect lies anterior to the inner, in contrast to therest where the reverse is true. Thus horizontal longitudinal sections at thelevel of the peraeopod bases show the bases of II—V orientated obliquelyforwards towards the mid-line, while the base of I lies obliquely backwards(figs. 2, G; 3). In the reconstruction, the endopodites of each first peraeopodcurved downwards close together immediately behind the maxillipedes, andthen bent laterally so that each dactylus was placed outside, but close to, theouter tip of the basal endite of the maxillipede. The endopodites of the otherperaeopods curved downwards and forwards, clustered close together belowthe midventral space.

The respiratory currentIn living specimens the maxillipede epipodites may be observed beating in

the branchial chambers, swinging in towards the mid-line (dorso-median up-stroke) and back again (ventro-lateral downstroke). In females with brood-pouches this movement tends to agitate the most anterior embryos as the


epipodite tips come into contact with them (fig. 4, E). The position of theepipodite at the end of the downstroke is between the carapace flap and thebody-wall of the second thoracic segment (figs. 1, A; 2, c), which probablyaccounts for the absence of setae on its inner margin. The function of theepipodite is clearly comparable to that of the epipodite of the first thoraciclimb of Hemimysis; of the first maxillipede of Diastylis; and of the maxilli-pede of Apseudes, i.e. to draw an inhalant respiratory current into the bran-chial chambers, in this case through the posterior opening of the carapace(fig. 1, A). The epipodite may also function as a gill, as Monod (1940)supposed, for it contains vascular lacunae and offers a relatively large surfacearea. The main site of respiratory exchange, however, is the underside of thecarapace wall, where a thick cuticle covers a system of vascular lacunae in thecarapace epithelium which communicates with the perivisceral and pericardialsinuses (Siewing, 1958). The outflow of water from the branchial chamberswould appear to be by way of the carapace lappets (compare Hemimysis).

'Skin' glands

Siewing (1958) draws attention to the occurrence of small 'skin' glands(Hautdriisen) at the bases of the paragnaths, maxillules, and maxillipedes.I find their distribution more widespread, there being glands situated in thelabrum, in the mandibular palps, in the groove leading from the food basin tothe bases of the paragnaths, and in the endopodites of all but the first pairof peraeopods, as well as in the locations cited by Siewing (see figs. 1, A, B; 2,G-j; 8, H). Each gland consists of a small group of cells whose cytoplasm, inBouin-fixed sections treated by Mallory's method, stains dense blue andsurrounds a nucleus containing a large red-stained nucleolus. No other cellin the body has these characteristics. The labral gland is the largest, con-sisting of approximately 15 cells grouped together and extending for a depthof 64 /u. in the anterior basal region. In the metastome a gland consisting of8 cells is situated at the posterior border of the base of each paragnath, and1 or 2 cells lie on each side of the groove leading to them. The mandibularpalps bear 2-celled glands located on the posterior side of the second segment.In the maxillules the gland is composed of 8 cells located on the lateral sideof the base of the distal endite; and in the maxillipedes the gland consists of4 cells situated at the base of each appendage on the medial side just belowthe nerve-ganglia in this segment. Finally, glands consisting of 2 to 3 cellsoccur within the basal segments of the endopodites of peraeopods II-V.

The maxillipede glands are the most obvious and contain cells which areslightly larger than those of the others, measuring from 8 to 24 p across andfrom 16 to 32 /x deep. Siewing (1958) found 6 cells in the paragnath glands,5 to 6 in those of the maxillules, and 3 to 4 in those of the maxillipedes. He isof the opinion that such glands possess ducts, but has been unable to demon-strate their existence; neither have I. I do find, however, that each gland isusually associated with a space lined and traversed with strands of trabecular


tissue, and in a number of instances what appears to be a pore guarded by asimple valve leads from this space to the exterior. In the case of the maxil-lipede glands, for example, the pores open on the inner aspect of the limbbases, dorsal to the channel formed by their apposed concave surfaces (fig. 8, G).It seems probable that there is no duct, and that the secretory productsaccumulate in an ampulla prior to discharge. The presence of such an ampullais evident in the photograph of the maxillipede gland shown in fig. 8, H, and itmay also be seen in Siewing's figure 39 (1958). The function of these glandsis obscure. Their location suggests that they play some part in feeding, andI had previously assumed (i960) that they secrete mucus; subsequent histo-chemical tests, however, do not support this assumption.

Nature of food and probable feeding methodAn examination of the contents of the alimentary canal indicates that

Thermosbaena is a detritus feeder, and that algae belonging to the Cyano-phyceae form a substantial part of the diet. An analysis of the plant contentsshowed that portions of Nostoc and Oscillatoria were the most prevalent.Other Cyanophyceae identified were Lyngbya, Spirulina, Chroococcus, Sytn-ploca, and Anabaena. The diatoms Asterionella and Navicula, and angiospermpollen grains (probably mostly from date palms) were also seen. Other contentsthat could be identified were bacteria (coccus and bacillus types), pieces ofchitin (probably from lice and other arthropods drowned in the pools), andsand-grains. The contents of the alimentary canal in specimens from theopen baths at Ain el Bordj have a greenish tinge and are rich in algae, whilethe contents in specimens from Ain Sidi Abd el Kadar, where the baths areroofed over and enclosed, are pallid, with fewer algae present. The baths atthese springs are the ones most frequently used by bathers, and no doubt therich supply of detritus here permeates the surrounding ground water and leadsto a concentration of the Thermosbaena population. The open baths at Ain elBordj are the most favourable for collecting specimens, probably owing to theabundance of Cyanophyceae, but it may be noted that these algae form animportant element of subterranean flora down to a depth of 2 m (Smith, 1950).

Since the branchial chambers are full of detritus (fig. 2, A-D), it is clear thatthe medium surrounding Thermosbaena is full of suspended detritus particles.The sweeping motions of the antennae and antennules, and the movements ofthe peraeopod endopodites, may serve to dislodge particles from the sub-stratum and contribute to this suspension. In the living animal the peraeopodendopodites have a metachronal flailing motion, back and forth, so that eachdactylus is downwardly and forwardly directed on to the substratum. Insections tested histochemically for the presence of mucus, the particles ofdetritus present among the mouthparts, and in the branchial chambers andalimentary canal, proved to be suspended in a mucilagenous matrix, derived,in all probability, from the mucilagenous sheaths which enclose many of theCyanophyceae that the animal feeds upon.

Unfortunately, studies of current movements with live specimens were not


carried out at El Hamma, and it is not possible, in the present study, toestablish whether Thermosbaena possesses a typical peracaridan filter-feedingmechanism. It may be said, however, that the presence of detritus in the ven-tral thoracic groove and mid-ventral space between the peraeopods suggeststhe existence of a food-stream flowing forwards along the mid-ventral spacetowards the maxillipedes as the main feeding current. In Hemimysis a food-stream of this kind 'is narrowed down as it passes between the bases of thefirst thoracic limbs' (Cannon and Manton, 1927). In Thermosbaena the mainstream apparently passes through the channel formed by the apposed basesof the maxillipedes; in section, this channel is invariably full of detritus(fig. 8, G). The 'skin' glands located here are well placed for their secretion toaffect the detritus streaming through into the food basin. Similar glands inthe ventral groove leading from this basin; in the paragnath bases; and in thelabrum, are strategically placed to contribute secretion to food passing for-wards along the groove, between the paragnaths, and into the oral cavity.

It is possible that the main food-stream is augmented by detritus broughtin by the inhalant respiratory current, and by lateral feeding currents drawnin from the sides. While some of the detritus drawn into the branchialchambers must inevitably pass out with the exhalant current, some may alsopass ventrally into the space between the maxillipedes and first peraeopods tojoin the main food-stream in its passage through the channel between themaxillipede bases. Detritus from this source could also join the main streamby passing ventrally between the maxillae and maxillipedes. The structureof the basal endites of maxillipedes suggests that by pivoting back and forth ontheir medial axes they could draw in a food-stream from each side and drive itforwards. The backward orientation of the first paraeopod limb bases wouldfacilitate such a movement, and it is perhaps significant that in section the inter-limb space between each basal endite and maxilla is invariably found to befull of detritus, large particles often being present (fig. 2, j).

Posterior aorta

Accounts of the vascular system in Thermosbaenacea have been given byTaramelli (1954) for M. argentarii and by Slewing (1958) for Thermosbaena.Taramelli describes the posterior aorta in Monodella as extending from theheart to the level of the seventh or eighth thoracic segment. Siewing is of theopinion that the posterior aorta ends in the second thoracic segment inThermosbaena, but found difficulty in tracing it in serial sections, and em-phasized the need for observing the living animal. In one live specimen I wasable to observe the intermittent movement of fluid in this vessel in the pos-terior thoracic region, and in serial transverse sections (fig. 8, E) I havetraced it as far as the first abdominal segment. At this level it appears tobifurcate into 2 lateral vessels which diverge to either side of the intestine. Thecourse of the posterior aorta is marked, apart from the lumen of the vesseland the precipitated plasma within it, by the sporadic occurrence of 'fat-cells'


as described by Taramelli (1954) in the heart of Monodella and shown by herfigure 19.

Male reproductive system

The morphology of the male reproductive system has been described byMonod (1927 a, b, 1940) and Siewing (1958). The spherical testes are situatedin the head and first thoracic segment just behind the cerebrum. Each vasdeferens originates ventrally from the testis, and in sections which show thisorigin, three types of cell, with different staining properties, may be seen.These I interpret as spermatogonia, in the dorsal part of the testis; sperma-tocytes, in the ventral part; and spermatozoa in the vas deferens. Each vasdeferens leads from the testis to the region of the sixth abdominal segmentwhere it forms a loop and then runs forward in a ventro-lateral direction toopen through a penis laterally placed at the hind ventral border of the eighththoracic segment (fig. 4, A ; 6, E). In their backward course the vasa deferentialie on each side of the intestine immediately above the intestinal caeca, andfrom the level of the third or fourth thoracic segment until the loop forwardis made in the abdominal region, each duct is dilated into an ampulla (figs.4, A ; 8, E), which in the living specimen is often faintly pink in colour. NeitherMonod (1927 a, b, 1940) nor Siewing (1958) identify these ampullae as such,though they are clearly visible in Monod's figures, and Siewing mentionsthat the vasa deferentia are sometimes dilated. In his figure 38, a drawing ofa transverse section in the region of the penes, Siewing (1958) shows dilatedducts (here identified as ampullae); but his figure 1, A shows a vas deferens ofsmall uniform diameter throughout. I have never observed this conditionbut find that ampullae are always present, the degree of their dilatation varyingaccording to the amount of spermatozoa contained within them. There isreason to believe that most of the males at Siewing's disposal were immature(see p. 278).

On reaching the penis, each vas deferens runs centrally within the organ toopen by a pore at its tip; 2 muscle-strands arising from the body-wall runon each side of the duct enabling the penis to move backwards and forwardsin a sagittal plane. The spermatozoa are filiform, being short in the mostanterior portions of the vasa deferentia and very long in the ampullae and ondischarge. The seminal fluid in which they are bathed is often visible in fixedsectioned material in irregular droplet form not unlike yolk in appearance.It is clear from Stella's account (1959) that the male reproductive system ofM. argentarii is similar.

FIG. 4. Schematic diagrams of the male (A) and female (B to D) reproductive organs ofThermosbaena. E, the brood-pouch condition of the female as illustrated by a drawing ofa preserved specimen. The pouch contains 7 embryos at the caudal papilla stage of develop-

ment.


B

C

D

1 2

stis

apicalgerminal

zone

<

thoracic segments

3 4 5 6

/I/

7 8

oocyte

oi'O/y

ampul'/o

FE

T= j(

Pent

'MAi

M

vaginalpapilla

Tsiyi

!.£

duct

k.

1

o y

^^seminalreceptacle

1

Abdominal

2 3

segments

A

TURE

MAI

= a s

^Sem

os tfe

I"bryo

i 6

= « ^

ferer

IPULA

1s

TORY

brood-pouch

embryo

FIG. 4


Female reproductive system

Until Siewing's (1958) account of the female reproductive system it hadnot been described beyond my own brief reference to it (Barker, 1956) inwhich I incorrectly stated that the oviducts opened in the eighth thoracicsegment. The ovaries described and figured by Monod (1927a) and Absolon(1935) were in fact portions of the intestinal caeca, as later pointed out byMonod himself (1940).

I have seen the ovaries in living and in sectioned specimens. Thoughthere are variations in structure related to maturity, the female reproductivesystem (fig. 4, B-E) consists essentially of a pair of tubular ovaries contain-ing from 6 to 8 oocytes which are arranged in a single row and increase insize caudally. The ovaries are separate from each other and for most oftheir course lie on either side of the intestine, immediately above the intestinalcaeca. Anteriorly they lie closer together and are located slightly abovethe intestine, a gradual shift to the lateral position accompanying their pro-gression through the thorax. The ovaries extend from the region of the firstthoracic segment to a point which lies variously between the region of theseventh thoracic and fourth abdominal segments, depending upon theirdegree of maturity. Their anterior ends taper off and constitute apical germinalzones as described by Stella (1959) in M. argentarii; it is here that oogenesisoccurs, the rest of the ovary being occupied by growing and maturing oocytes.The oviducts lead from the posterior ends of the ovaries to open in the sixththoracic segment. Each opening consists of a well-formed vagina borne ona papilla on the posterior side of the base of the fifth peraeopod. The oviductis lined with a single layer of epithelial cells which have large nuclei, and thelumen of the duct enlarges prior to the vaginal opening, which is oval in shape.

It is necessary to distinguish between 4 conditions of the adult femalereproductive system which differ according to the maturity of the individual;they may for convenience be referred to as the immature, mature, copulatory,and brood-pouch conditions. In the immature condition (fig. 4, B) the ovariesextend as far as the sixth or seventh thoracic segment where the oviducts leadoff from their hind ends. Each duct makes a ventro-lateral turn before em-barking on its forward course, which consists of a dorsal ascent accompaniedby a shift out into a more lateral position. On reaching the sixth thoracicsegment, the duct makes an abrupt turn and vertical descent before openingto the exterior. Following on the apical germinal zone, the first oocyte to fillthe lumen of the ovary measures 25-30 ju. in diameter in Bouin-fixed material.This is succeeded by from 5 to 7 oocytes increasing in size posteriorly to attaina diameter of 65-75 M- Each oocyte possesses a large nucleus with a prominentnucleolus surrounded by a coarse granular cytoplasm containing small drop-lets of yolk (fig. 8, B). The oocytes are oval, those in the middle region of theovary having dimensions typically of the order of 35 /u. diameter and 50 /u.long. The ovaries are slightly constricted at intervals along their length, be-tween neighbouring oocytes. At these constrictions the ovarian epithelium


extends across the lumen of the ovary so as to enclose each oocyte in a follicle.Such follicles tend to disappear towards the posterior end of the ovary as theoocytes become larger.

In the mature condition the ovaries extend to the level of the first abdominalsegment (fig. 4, c). The hind end of each tube bends downwards ventro-laterally doubling back upon itself to run forward again for a short distance,so that a transverse section in this region shows a double ovary on each sidelike a figure 8 (fig. 8, c). The lower forward extension of each tube leads intothe oviduct (fig. 8, D), which thereafter traces a course to open in the sixththoracic segment. In this condition the first and most anterior oocytes measureon the average 50-60 JJ. wide, n o /x long, and 45 /x deep, as compared withthose at the posterior ends of the ovaries which measure 100-140 /x wide, 190 JU,long, and 80 fi deep. As will be evident from these measurements the oocytesare now ellipsoid in shape, being somewhat flattened dorso-ventrally. Thelast and largest oocyte of the series is contained within the bend and lowerforward extension of each ovary. The oocytes are now typically centrolecithalwith large nuclei surrounded by closely packed droplets of yolk. The apicalgerminal zones are less evident, and the walls of the ovaries, though stillconstricted at intervals, no longer form ovarian follicles.

seminal fluidprecipitate

8 th 1stthoracic abdominalsegment segment ovary 4-celled embryo

peroeopod^Z aginal spermpapilla

seminalreceptacle

0'2

envelope .'. .r b/astomere

FIG. 5. The left oviduct, seminal receptacle, and vagina of a copulatory female Thermosbaena.Reconstruction from the camera lucida drawings of 23 8-M serial sagittal sections amalgamatedso as to present an internal sagittal view of the right half of the receptacle and oviduct. Theblastomeres of the 4-celled embryo are shown projected on to one plane, and the yolk massesare indicated schematically. The main body of the ovary lies in a more medial plane and is

indicated in broken outline.

In the copulatory condition the ovaries attain their farthest caudal extension,reaching the level of the third or fourth abdominal segments (fig. 4, D). Thedoubling forwards of the hind end of each ovary persists, but each oviductnow leads to its vaginal opening by way of a seminal receptacle (figs. 5; 8, A).This structure is a dilatation of the oviduct extending from the seventh


thoracic segment to the first abdominal. The cells and nuclei of the epitheliumforming the walls of the receptacle are about 4 times larger than those liningthe rest of the duct. The anterior end of the receptacle leads directly intothe abrupt turn and descent of the oviduct in the sixth thoracic segment. Thatthis structure serves as a seminal receptacle is indicated by the fact thatsections show it to contain spermatozoa and the fixed precipitate of seminalfluid, and both sperm and fluid may be seen in the oviduct leading from thevagina to the receptacle. Moreover, in a few preparations the most posterioreggs in the ovarian tubes have clearly been fertilized and are undergoing thefirst stages of segmentation. The receptacle is a temporary structure de-veloped for copulation, and also possibly for assistance in expelling theembryos from the body.

The female evidently achieves the brood-pouch condition (figs. 4, E; 6, A)by successive moults: sloughed off exoskeletons were occasionally sifted outof the sludge at El Hamma, and female specimens were observed in which adistinct backward growth of the carapace indicated a brood-pouch in courseof formation. In the copulatory condition the carapace extends back only asfar as the sixth thoracic segment, so the formation of the pouch for the re-ception of the young embryos presumably takes place rapidly thereafter.When the pouch is fully formed and full of embryos, the ovaries are empty,and the hind border of the carapace, which normally extends to the fourththoracic segment, now extends to the first or second abdominal segment.As compared with the normal carapace, that of the embryo-carrying femaleis approximately doubled in length, breadth, and depth, so that whereas itnormally measures o-8 mm long and o-6 mm wide, it enlarges to approxi-mately i*7 mm long and 1-2 mm wide. The ventral border of the brood-pouch lies above the bases of the thoracic appendages and is folded inwards,while communication with the exterior at the hind end is confined to a fairlynarrow opening above the dorsal body-surface. The vascular lacunae in thewall of the brood-pouch are much more extensive than in the normal carapace,particularly in the posterio-lateral regions where they form large sinuses. Inpreserved specimens these regions sometimes appear as opaque patches(fig. 6, A). Presumably these vascular modifications are associated with theincreased respiratory requirements of the breeding female.

Judging by Siewing's description and figures (1958) of the female repro-ductive system of Thermosbaena, it is apparent that he has not observed thefully mature and copulatory conditions. He does not describe the seminalreceptacles or the characteristic doubling forwards of the hind ends of themature and copulatory ovaries. Siewing had at his disposal the specimenscollected by Bruun in May, 1938; this month marks the presumed start of thebreeding season (see p. 279), and it is probable that most of the adults collectedwere immature. Stella (1959) is also of the opinion that Siewing worked withimmature specimens. This would also have rendered it difficult for him toobserve the vaginal papillae, which only become obvious in the matureand copulatory conditions. Siewing maintains that the oviducts open on the

brood-pouchwith 2 embryos

embryos removed frompouch

caudalpapilla

pieopod H

peraeopod ^ B penisendopodite dactylus

FIG. 6D. BARKER


anterior side of the bases of the fifth peraeopods; the posterior location ofthese openings, however, and the presence of distinct vaginal papillae, areclearly evident in mature and copulatory specimens (figs. 4, c, D; 5; 8, A), andthis location is more probable if the mechanics of copulation and embryoextrusion are considered.

Stella's account (1959) of the female reproductive system in M. argentariishows that there is a close similarity with Thermosbaena, though she does notdescribe a copulatory condition. It was previously maintained (Taramelli,1954; Stella, 1955) that in addition to vaginal ducts for the entry of sperm,there were oviducts leading from the posterior ends of the ovaries to opendorsally through the body-wall into the brood-pouch. Stella (1959) has sincewithdrawn this claim and is now of the opinion that the eggs are ejected fromthe distal ends of the ovaries (which lose their ducts) and pass into the brood-pouch from the body-cavity by breaking through the dorsal body-wall. Theproblem of how the embryos may be transferred to the brood-pouch inThermosbaena is dealt with below (p. 280); in the meantime it may be notedthat the oviducts of the mature ovaries in Thermosbaena do not disappear butremain to develop seminal receptacles.

Reproduction and developmentIt is not clear in which month Seurat collected Thermosbaena, but Omer-

Cooper and Hill visited El Hamma on 28 March 1925; Absolon on 26 April1927; and Bruun on 18 May 1938. Seurat's visits in 1923 and 1925 may alsohave been made in the spring, for visitors tend to avoid the oasis in the extremeheat of the summer. Breeding females were absent from all these collections.Our own collection was made over a period of 23 days in September 1950, andcontained 50 females with brood-pouches. These facts may indicate that thebreeding season of Thermosbaena extends from May to September as Stella(1953, 1955, 1959) has found in M. argentarii. The presence of juvenilespecimens in Bruun's collection suggests that the breeding season had startedat the time of his visit even though many of the adults obtained were apparentlyimmature His failure to catch breeding female specimens may be attributedto their relative inactivity, which renders them scarce in dip-net collections,e.g. 50 out of 714 specimens in our collection; 12 out of 275 specimens of M.argentarii collected by Stella (1955). The brevity of Bruun's visit, and therelatively small number (158) of specimens collected by him, have also to beborne in mind.

FIG. 6 (plate). Photographs of preserved specimens of Thermosbaena.A, female with brood-pouch. Seven embryos have been removed from the pouch, and 2

left inside; all are at the caudal-papilla stage of development.B, an embryo at the stage when limb-buds first appear (compare fig. 7, A) together with

two embryos at the caudal-papilla stage.c, an embryo at the stage shown in fig. 7, B; peraeopods biramous, carapace appearing.D, an embryo at the stage immediately prior to escape from the pouch (compare fig. 7, c).E, male specimen, abd 1, first abdominal segment; th 8, eighth thoracic segment.2421.2 U


Copulation presumably takes place with the ventral parts of the sexesapposed in a fashion probably similar to that described by Nair (1939) in themysid Mesopodopsis. The mobility of the penes would clearly be of assistancefor introducing the spermatozoa by way of the vaginae into the seminal recep-tacles, from whence they proceed to fertilize the eggs in the ovarian tubes.It is here that segmentation begins, to be continued further, together with therest of the development, in the brood-pouch. Before describing development,however, the problem of how the embryos reach the brood-pouch must beconsidered.

Females carrying embryos appear to have difficulty in swimming, doing so,for the most part, upside-down as if encumbered by the heavy swollen carapace.The brood-pouch carries an average of 10 embryos, which are constantlyagitated by the inhalant respiratory current; those in the most anterior region,as already noted (p. 270), tend to be further disturbed by coming into contactwith the movements of the maxillipede epipodites. I can find no evidence ofembryos leaving the hind ends of the ovaries and reaching the brood-pouchby extrusion through the dorsal body-wall, as is suggested by Stella (1959) tooccur in the case of the ova of M. argentarii. Moreover, in Thermosbaena thereis an alternative method whereby embryo transfer could more simply beachieved. If we assume that the embryos are discharged while the animal isstationary and upside-down, we may imagine that their expulsion from theposteriorly directed vaginae over the sides of the abdomen is followed bytheir suction into the brood-pouch through its posterior opening by theinhalant respiratory current. Expulsion may well be assisted by the seminalreceptacles acting as muscular pumps. Once in the brood-pouch, exit canonly be achieved by swimming out of the posterior opening against the inflowof water; exit anteriorly is barred by the maxillipede epipodites and thenarrowness of the channels leading to the branchial chambers: and exitventrally is prevented by infolds of the carapace wall. Thus for particles ofgrit as large as gastrulating embryos there is no escape, and their presence inbrood-pouches is not uncommon.

The embryos are transferred to the brood-pouch during the early stages ofsegmentation; 4-celled embryos have been observed in the ovaries of copula-tory females (fig. 5), and the earliest stage found in the pouch was 12-celled.The blastomeres are first formed within the body of the yolk and subsequentlyrise to the surface, the segmentation thus resembling that oiHemimysis (Manton,1928) and Mesopodopsis (Nair, 1939). In the 4-celled stage all the blastomeresare located within the yolk, while a reconstruction made of a 12-celled stageshowed that 6 had attained a superficial position. The blastomeres areirregular stellate masses, and a thin layer of cytoplasm envelops the surfaceof the yolk. A vitelline membrane becomes obvious in the later stages ofsegmentation, but I have been unable to detect its presence around embryosprior to expulsion. The yolk loses its fairly regular droplet form as evident inmature oocytes, and in the 4-celled stage is more evenly distributed in largermasses which break down as segmentation proceeds. The embryo is neces-


sarily ellipsoid in shape while developing in the ovarian tube, but on transferto the brood-pouch it becomes spherical, and prior to gastrulation measuresapproximately 0-26 mm in diameter.

I have been unable to follow the process of gastrulation in detail, primarilyowing to the poor qualities of Bouin's fluid as an embryological fixative.Formol bichromate, as used in the embryological studies of Manton (1928),Hickman (1937), and Nair (1939) is essential for such critical work, and it isa matter of regret that this was not appreciated at the time of the visit toEl Hamma. Thus restricted to following the gross changes rather than thefiner details of development, the first change observed was the appearanceof the caudal papilla in embryos measuring 0-3 to 0-4 mm in diameter. Thisoutgrowth of small cells is initially bifid, and sections at this stage show thatmuch of the free yolk has been absorbed by large vacuolated vitellophagesor yolk cells. The embryo is now pear-shaped (figs. 4, E ; 6 A, B), and furtherdevelopment of the papilla is accompanied by differentiation of limb-budsand the beginning of cephalo-caudal elongation. With further elongation ofthe body, the thoracic and abdominal regions become apparent and the caudalpapilla gives rise to a rudimentary telson and uropod limb-buds. The vitel-line membrane is lost, and the yolk and yolk cells are contained within a yolksac placed so as to form a central core occupying the space later filled by themid-gut and intestinal caeca (figs. 6, B ; 7, A). The yolk cells continue to absorbyolk, and, as in Hemimysis (Manton, 1928), begin to aggregrate into second-ary yolk pyramids. Limb-buds are present for all the appendages except thepleopods, the antennules being the most advanced of the series with rudi-mentary twin branches. Differentiation of the cerebrum and the segmentalnerve-ganglia has begun, and this, together with the limb-buds, enablesclear identification of the body segments except at the posterior end of theabdomen. At this stage the embryo measures 0-4 to 0-5 mm long and a cuticleis present.

The appearance of the stomodaeal and proctodaeal invaginations, andfurther differentiation of the limbs and nervous system, results in the embryoincreasing in length by o-i to 0-2 mm (figs. 6, c; 7, B). The mouthparts andperaeopods begin to develop their individual characteristics, the peraeopodsbecoming biramous. The pleopod limb-buds appear, and the carapace beginsto take shape. The yolk cells are further organized into secondary yolkpyramids, and a regular epithelium begins to form at each end of the centralcore. Further differentiation of the nervous system occurs, the cerebrum andsegmental ganglia being more distinct. There appear to be 6 abdominalganglia only; special efforts failed to reveal the transitory presence of a seventhganglion as occurs in Hemimysis (Manton, 1928) and Anaspides (Hickman,1937). In section, rudiments of the intestinal caeca may be seen stretchingback from behind the stomodaeum to the level of the second thoracic segment,situated ventro-laterally above the nerve-cord and ganglia to either side ofthe yolk sac.

Further development leads to the gradual reduction of the yolk sac to

282 Barker—Thermosbaena mirdbilis and its reproduction

within the limits of the thoracic region; its enclosure by epithelium of themid-gut; and the fusion of this epithelium with the stomodaeal and procto-daeal invaginations to form a complete alimentary canal. The intestinal caecagrow farther back, and in the living embryo they can be seen in the hindthoracic and anterior abdominal regions. The carapace becomes more distinct,the limbs continue to differentiate, and sections reveal the presence of endo-skeletal elements and striated musculature. Finally, a length of o-8 to 0-9 mmis attained immediately prior to leaving the brood-pouch. In this condition(figs. 6, D; 7, c) the embryo differs little from the free-swimming juvenile.The carapace has yet to grow back and cover the second and third thoracicsegments, and the appendages, though now fully individuated and sproutingsetae, have yet to attain full development. In respect of other features, how-ever, such as the musculature, endoskeleton, nervous system, alimentarycanal, and intestinal caeca, the morphology is that of a juvenile. The defi-nitive adult form may be said to be reached after the juvenile, freshly escapedfrom the brood-pouch, has developed gonads and approximately doubled insize so as to measure 2 mm long.

Stella has recently (1959) amplified her earlier brief accounts (1953, 1955)of the development of M. argentarii. Apart from the absence of pleopods,the stage of development attained by M. argentarii immediately prior toescape from the brood-pouch would appear to be similar in all respects to thatattained by Thermosbaena. However, while the post-marsupial developmentof Thermosbaena consists simply of increase in size and further individuationof structures already present, in M. argentarii 3 post-marsupial stages arerecognized by Stella during which the pleopods and the last 2 pairs of peraeo-pods (VI and VII) are added. The fact that the embryos of both Monodellaand Bathynella escape without the full adult complement of peraeopods isagain urged by Stella (1959) as a strong point in favour of claiming a syncaridanaffinity for the Thermosbaenacea. This view is evaluated elsewhere (Barker,1959), but it may be observed here that the nature of the segmentation of theThermosbaena embryo provides further evidence of peracaridan rather thansyncaridan affinity, since it is superficial, as in the majority of the Peracarida,and not total, as in Anaspides (Hickman, 1937). Stella does not describe thesegmentation of the embryo of M. argentarii, and the nature of the segmen-tation in Bathynella has not been described.

FIG. 7. Camera lucida drawings of paracarmine-stained whole mounts of Thermosbaenaembryos. To avoid confusion, the left-hand series of appendages has been omitted. A, shortlyafter the loss of the vitelline membrane. Limb-buds present for all appendages except pleo-pods; rudiments of nervous system developed (compare fig. 6, B). B, a later stage showingstomadaeal and proctodaeal invaginations; differentiation of the appendages and appear-ance of pleopod limb-buds; further differentiation of the nervous system; and the beginningof carapace formation (compare fig. 6, c). c, advanced stage of development shortly beforeescaping from brood-pouch. Left antennule and antenna included; musculature omitted(compare fig. 6, D). abd I, abd 6, first and sixth abdominal segments; abd 5 gn, fifth abdo-minal ganglion; th 8, eighth thoracic segment; th 8 gn, eighth thoracic ganglion. Peraeopods

indicated by roman numerals.

Barker—Thermosbaena mirabilis and its reproduction

yolk cell .secondary

yolk pyramid

283

>> maxilla uropodantennule \ \ \max///u/e

mandibleantenna

carapace

0-2 mm

proctodaeum

IFpleopod

rnaxilhpede i uropodabd 5 gn

stomach

antenna carapace

uropod

la brummandibularpalp mouth 0-2 mm

FIG. 7


With regard to the apparent absence of a seventh abdominal segment, itshould be noted that in both Hemimysis and Anaspides, the sixth abdominalsegment, composed of the fused sixth and seventh somites, is distinct fromthe telson, whereas in Thermosbaena the telson and the sixth are fused. Itis possible, therefore, that in this fusion the seventh fails to develop at all.In Monodella, where the telson and the sixth are separate, the transitorypresence of a seventh may well occur in development, but Stella's study (1959)offers no information on this point.

The absence of a distinct telson, taken together with a number of otherfeatures, leads me to the view that Thermosbaena may be paedomorphic. Theembryos of both M. argentarii and Thermosbaena show a ventral flexion of thebody, and while this is largely lost in the adult Monodella, it is a characteristicfeature of the adult Thermosbaena. Paedomorphosis would account for thereduced number of peraeopods and the shorter and stouter body as comparedwith Monodella. That a distinct telson does not develop could be regardedas part of the same retardation which affects peraeopod formation and body-form. It seems plausible that neoteny may have played such a role in theadaptation required of Thermosbaena in colonizing a thermal brackish habitat,and that the easier conditions of the habitats colonized by Monodella have ledto the preservation of more primitive features. In this connexion the obser-vations of Baid (1959) on the occurrence of neoteny in Artemia may berelevant in that he shows there to be a clear relationship between degree ofsalinity and abdominal segmentation and body size.

FIG. 8 (plate). Photomicrographs of 8-fi paraffin sections of Thermosbaena stained byMallory's method.

A, part of sagittal section of copulatory female specimen showing seminal receptacle andvaginal papilla.

B, part of sagittal section of immature female specimen showing two young oocytes.c, part of transverse section of mature female at the level of the hind ends of the ovaries

showing the characteristic figure-eight pattern produced by the ovary doubling back uponitself to run forward again for a short distance. Note nucleus of the oocyte in the lower for-ward extension of the ovary.

D, part of transverse section from the same series as in c but 31 sections farther forward.The same right ovary is shown, but the lower forward extension has now led into the oviduct.

E, part of a transverse section of a male specimen at the level of the eighth thoracic segmentshowing the ampulla of the right vas deferens. The posterior aorta is also shown.

F, transverse section of a male specimen at the level of the first thoracic segment, selectedto illustrate the channel formed by the apposed concave surfaces of the maxillipedes, and themaxillipede epipodite.

G, part of the same section as in F, showing the channel between the bases of the maxillipedesfull of detritus.

H, horizontal longitudinal section of a left-hand maxillipede showing the 'skin' gland and itspresumed ampulla, amp, presumed ampulla of 'skin' gland; crp, carapace; dtr, detritus;^ gl Pi possible simple valve leading from 'skin' gland ampulla to exterior; h, heart; int,intestine; mxp, maxillipede; mxp cxl end, coxal endite of maxillipede; mxp ep, maxillipedeepipodite; mxp ex, maxillipede exopodite; n, nucleus of oocyte; ov, ovary; ovd, oviduct;perivis sp, perivisceral space; per V, base of peraeopod V; post ao, posterior aorta; sent rec,seminal receptacle; s gl, 'skin' gland; th 8, eighth thoracic segment; vag pap, vaginal papilla;vd, vas deferens; vd amp, ampulla of vas deferens; y, yolk droplet.

100JJ

FIG. 8D. BARKER


ConclusionsThe present study strengthens a conviction already expressed (Barker, 1959,

i960) that there are insufficient grounds for establishing a new malacostracandivision Pancarida (Siewing, 1958) to receive the order Thermosbaenacea.So far as Thermosbaena is concerned, the only 'pancaridan' characteristics arean apparent lack of excretory organs, and a dorsal brood-pouch, for withrespect to maxillipede endopodites in the male, and a 'manca stage' in develop-ment, it fails to qualify. As Stella (1959) has suggested, the use of the peraeo-pod endopodites in feeding may well have precluded the formation of a ventralbrood-pouch. While the use of either oostegites or carapace to form a brood-pouch is clearly a feature of taxonomic importance, the dorsal pouch is not soradical a departure as to exclude from the Peracarida organisms so typicallyperacaridan in other respects. The nature of embryo segmentation is furtherevidence of peracaridan affinity.

Stella (1959) is of the opinion that Thermosbaena is more primitive thanMonodella on the grounds that the 'manca stage' of development in Monodella,where the embryo has only 5 pairs of peraeopods, recapitulates the normalnumber of peraeopods in Thermosbaena. I suggest that the reverse is moreprobable, the reduced number of peraeopods in Thermosbaena representinga paedomorphic modification of the primitive condition where 7 pairs arepresent. Both Karaman (1953) and Slewing (1958) regard Monodella as themore primitive form; the free telson and structure of the male maxillipedein this genus, taken together with the higher degree of specialization requiredof Thermosbaena in adapting to a thermal brackish habitat, leave little doubtthat this is so. Whether internal fertilization and early segmentation ofthe embryo in the ovary are common to both genera, or specialized featurespeculiar to Thermosbaena, remains to be seen.

I wish to express my thanks to the following for the assistance they havekindly rendered me in this work: Sir Alister Hardy, for suggesting the in-vestigation in the first instance and for his comments on an earlier draft;Dr. S. M. McGee-Russell for his help and enthusiasm in collecting andobserving Thermosbaena at El Hamma; Miss B. T. Chiu and Mr. A. T.Marshall for botanical and histochemical assistance respectively; Dr. S. M.Manton, Dr. Isabella Gordon, and Dr. I. W. B. Thornton for much helpfuladvice; and the late Earl of Verulam, Mr. Basil Wright, and Dr. S. M. Mantonfor their financial assistance without which the visit to El Hamma could nothave been undertaken.

ReferencesABSOLON, K., 1935. Pfiroda, 28 (i), i.BAID, I. C., 1959. Nature, Lond., 184, 73.BARKER, D., 1956. Proc. XIV Int. Zool. Congr. 1953, Copenhagen, 503.

1959. Hydrobiologia, 13 (3), 209.i960. Proc. Centenary and Bicentenary Congr. Biol. 1958, Singapore, 253.

BRUUN, A. F., 1939. Vidensk. Medd. fra naturh. Foren., Copenhagen, 103, 493.


CANNON, H. G., and MANTON, S. M., 1927. Trans, roy. Soc. Edin., 55 (1), 219.DENNELL, R., 1934. Ibid., 58 (1), 125.

1937. Ibid., 59 (i), 57.HICKMAN, V. V., 1936. Pap. and Proc. roy. Soc. Tasmania, 1.KARAMAN, S. L., 1953. Acta Adriatica, 5 (3), 1.MANTON, S. M., 1928. Phil. Trans. B, 216, 363.MONOD, T., 1924a. Proc. Linn. Soc. Lond., Session 136, 63.

19246. Bull. Soc. zool. Fr., 49, 58.1927a. Faune Colon, franc., 1 (2), 29.1927ft. Bull. Soc. zool. Fr., 52, 196.194°. Thermosbaenacea. H. G. Bronns Klassen und Ordnungen des Tierreichs, Bd. 5,

Abt. 1, Buch 4. Leipzig (Becker und Erler).NAIR, K. B., 1939. Proc. Indian Acad. Sci., 9, 175.OMER-COOPER, J., 1928. The Vasculum, 14, 43.RUFFO, S., 1949a. Arch. Zool. Ital., 34, 31.

19496. Hydrobiologia, 2, 56.SIEWING, R., 1958. Anatomie und Histologie von Thermosbaena mirabilis. Ein Beitrag zur

Phylogenie der Reihe Pancarida (Thermosbaenacea). Akad. der Wiss. und der Lit. Mainz,Abh. Math.-nat. KJ., No. 7, 1957.

SMITH, G. M., 1950. The Fresh-water algae of the United States, 2nd ed. New York (McGraw-Hill).

STELLA, E., 195m. Arch. Zool. Ital., 36, 1.19516. Boll. Zool. Torino, 18 (4-6), 227.I9S3- Hydrobiologia, 5 (1-2), 226.1955. Proc. Int. Ass. Limnol., 12, 464.1959. Riv. di Biol., 51 (1), 121.

TARAMELLI, E., 1954. Monitore zool. ital. Firenze, 62 (i), 9.

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A study of Thermosbaena mirabilis (Malacostraca, …...261 A study of Thermosbaena mirabilis (Malacostraca, Peracarida) and its reproduction By D. BARKE R (From the Department of Zoology,