The Conus Arteriosus in Fishes.

By

C. W. Parsons, B.A.Department of Zoology, University of Glasgow.

With 10 Text-figures.

INTRODUCTION.

THE structure intervening between the ventricle of the heartin fishes and its prolongation beyond the borders of the peri-cardiac cavity has been described under a number of differentnames. In literature of the early nineteenth century it occursas the ' branchial artery ' (cf. Home, 10). Other terms such asbulbus arteriosus, bulbus aortae, and bulbus cordis were alsocommonly applied to it, however, and these have passed intogeneral use.

In a note published in 1842 (15) and again two years laterin his important paper on the morphology and definition of theGanoidei (16, p. 127), Joh. Miiller emphasized the presence ofa muscular covering over the bulbus arteriosus in the so-calledganoid fishes and in certain of the Selachii. He was followed,1866, by Gegenbaur (6), who was so deeply impressed with theapparent distinctions, both structural and functional, betweenthe bulbus arteriosus in Selachii and Teleostei that he intro-duced the special term for the former, ' conus arteriosus'.(' Ich bezeichne diesen Theil als Conus arteriosus, da er demgleichnamigen Abschnitte der Kammer der hoheren Wirbel-thiere entspricht.')

Gegenbaur's nomenclature was adopted. Without questionhis opinion was supported in the strongest possible manner bythe evidence he brought together from the comparative anatomyof the heart in adult fishes. When however the more recentfacts of the early development of this organ are considered, it

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becomes questionable whether the distinction is in reality a truemorphological one. The heart is now regarded as a speciallymodified part of the primitive subintestinal blood-vessel, whosecontractile tissues are localized by the boundaries of a compara-tively small coelomic space—the pericardiac cavity. The head-ward or anterior boundary of this cavity is fixed early in de-velopment, and with the completion of the posterior wall thedeveloping cardiac tube becomes entirely enclosed. There aresome exceptions to this. In certain Selachii and Chondrostei, forexample, the pericardiac cavity is never completely shut offfrom the general body-cavity, and in Myxinoidea the two spacesremain in communication throughout life by a wide lateralopening. As a rule, however, the forerunner of the heart finishesits development within a completely sealed cavity.

It is a commonplace that the growth in length of the heartrudiment and that of the pericardiac space do not keep pacewith one another. The heart rudiment grows much the fasterand, being fixed at each end by the pericardiac wall, it is throwninto folds. The amount of folding is obviously controlled at anyone time by the size of the pericardiac cavity, and it is nottherefore surprising to find that the different orders of fishesshow much variation in this respect. It is probably most ex-aggerated in the development of the heart in Dipnoi. Eobertson(20, Text-fig. 8, pp. 82, 86), for instance, showed how inL e p i d o s i r e n the heart rudiment suffers a severe twist indevelopment which is not corrected until later enlargement ofthe pericardiac space antero-posteriorly releases some of thetension at the extremities of the heart tube.

A fundamental relationship between the anterior and pos-terior boundaries of the pericardiac cavity and the developingheart is clearly established. Indeed, in a number of the lowerGnathostomata they form indisputably the limits of the adultheart also. There is strong support therefore for the opinionthat every part of the cardiac tube within the boundaries of thepericardiac space is to be regarded as morphologically a part ofthe heart, and it is the point of view adopted here. In conflictwith this view is the hypothesis that the bulbus arteriosus in

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Teleostei is a part of the ventral aorta which has secondarilyextended backwards so as to replace part of the cardiac tube. Thisattractive theory, illustrated by Boas's well-known figure (1,fig. 352, p. 418), receives very general acceptance. But thefacts that may be brought forward to show that the bulbusarteriosus in Teleostei is simply part of the original conus arter-iosus, i. e. the pre-ventricular portion of the primitive cardiactube, indicate that the theory ought to be abandoned, andthat the explanation of the contrast between the structuresevolved should be sought in the substitution of an automati-cally contractile apparatus in this part of the heart of higherfishes in place of one which had originally independent powers ofcontraction.

The term conus arteriosus is accordingly used in the followingpages in the widest sense to include the whole of the ' morpho-logically anterior portion of the primitive cardiac tube ' (cf. 11,p. 38).

E l a s m o b r a n c h i i .

The conus arteriosus in Elasmobranchii is a well formedstraight and important chamber of the heart. It has fairly thickwalls and a wide lumen. The presence of a muscular covering—the so-called myocardiac coat—over it has already been in-dicated (Miiller 15 and 16), and correlated with this is the well-known fact that the conus ' contracts rhythmically like the restof the heart' (cf. Clark 4, p. 23). The myocardiac fibres con-tinue apparently without interruption into the ventricle, wherethey mingle with similar fibres belonging to the ventricle itself.

The lumen of the conus is occupied to a greater or lessextent, according to the species, by a notable development ofthe endocardiac lining. The tissue expands into loose, almostgelatinous sheets (cf. Gegenbaur 7, p. 352), which are throwninto ridges extending from end to end of the conus. Gegenbaur'sfigure of a longitudinal section of the conus in a 5 cm. embryoA can t hi as (8, fig. 6, p. 605), for example, shows two of thewide continuous endocardiac ridges. A similar longitudinalsection through the conus of a young specimen 22 cm. in length

1 2

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shows the ridges segmented into valves. It will be noted that thevalves in each longitudinal row are not equally well developed.It will also be clear from the transverse sections 1 a and 1 b,

TEXT-FIGS. 1, la, lb.

Fig. 1.—Horizontal section of the conus arteriosus of a youngA c a n t h i a s , 22 cm. in length, a., region of the transversesection, Text-fig, l a ; &., region of the transverse section, Text-fig.1 b ; e., endocardiac lining of the conus ; I., lumen of the conus ;m., myocardiac coat in the conus wall; p., pericardiac wall;r., ridge of endocardiac tissue between valve circles 1 and 2 ;v., pocket valve.

Figs. 1 a, 1 b.—Transverse sections of the above in the regions a.and b.—D., R., L., morphologically dorsal, right, and left valvesrespectively.

that the number of valves in a circle at the base of the conusis four, and that at the headward end, i. e. the point fixed in

CONUS ARTERIOSU3 149

position by the anterior wall of the pericardiac cavity, it isreduced to three.

The convention followed in describing the endocardiac ridgesof the conus arteriosus and their derivatives, the valves, maybe explained with advantage here. It is a method due toGraham Kerr (12, p. 378) of considering the morphological posi-tion of these structures at the headward end of the conusarteriosus and of naming them accordingly. Thus four valvesoccupying these positions at the fixed headward end are namedEight, Dorsal, Left, and Ventral respectively. Abbreviationto E., D., L., and V. is clearly convenient for this particularcircle of valves.

Applying this convention to the disposition of the chief conusvalves in A c a n t h i a s (Squalus):

1. The valves are in four circles.2. The right, dorsal, and left endocardiac ridges are present

throughout the adult conus arteriosus ; the ventralridge is reduced and appears only in the hinder sectionof the conus, i. e. nearest to the ventricle.

3. The circles of valves :Circle 1.—E., D., and L. well-developed pocket valves.Circle 2.—Eeduced valves, or plain ridges, occur in the

right, dorsal, and left axes only.Circles 3 and 4.—Pocket valves in the right, dorsal, and

left axes; a smaller pocket valve, or solid strip of endo-cardiac tissue appears in the ventral axis.

Gegenbaur (6, p. 366) showed that the only valves consis-tently well developed in the conus of adult A c a n t h i a s weresituated in the foremost and hindmost circles, i. e. the first andfourth. Stohr (26, p. 212) recognized among the interveningstructures two types of valves, namely (1) typical pocket valves,and (2) a form of valve without cavity to which he applied theterm tongue valve. No doubt the latter are represented by thestrips of endocardiac tissue mentioned above. It is interestingto note that Stohr found he could distinguish four differentgrades of endocardiac valve among the Selachii and Ganoidei,but the. intermediate types which he particularized do not

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appear to be constant either in position or form, and the dis-tinctions between them are not therefore specially stressedin this paper.

O'Donoghue and Abbott (18, p. 828) describe ' very tinyaccessory flaps ' in the conus of A c a n t h i a s . These super-numerary valves are not always present, but when found theywill be seen in circles 3 and 4 on one side or other of the dorsalrow of valves. These authors do not find units in the ventral axisof the conus and therefore describe the position of the pre-dominant rows of valves as one dorsal and two ventro-lateralrows. A conspicuous feature of the conus arteriosus in Acan-t h i a s is the wide space between circles 1 and 2 (cf. Gegenbaur,loc. cit., 6, p. 367). It is bridged by thickened endocardiacridges (cf. Text-fig. 1, r.). An important detail to notice alsois the presence of the myocardiac coat over the whole of theconus from its base to the pericardiac boundary.

The conus of the heart in the commoner species of E a i aresembles that of A c a n t h i a s in having a fully developedmyocardiac covering. There is a difference in the arrangement ofthe endocardiac valves. In E a i a there is no wide space betweenthe first and second circles of valves. The number of circles ofvalves may vary within the genus between four and five more-over (cf. Joh. Miiller, loc. cit., 15, and Pettigrew, 19). Thelatter observed in R. b a t i s ' three pyramidal rows of fiveeach ', i. e. five circles of valves in three longitudinal rows (cf.19, p. 779). With this computation Stohr disagreed. He foundonly four circles of valves in both R. b a t i s and R. c l a v a t a(loc. cit., 26, p. 219). Stohr drew attention to a distinctionbetween these two however, in the size of the valves of thefirst circle as compared with those that follow in the second. InR. c l a v a t a the valves of the foremost circle are not con-spicuous, whereas in R. b a t i s they are distinctly larger thanthose that occur in any of the other circles. Transverse sectionsthrough the conus arteriosus of R. b a t i s at its apex and baseshowed no trace of the ventral endocardiac ridge in eithersituation, and the condition of the endocardiac apparatus inthe two species may therefore be summarized as follows :

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E a i a batis.—Valves in five circles. Pocket valves welldeveloped in every circle.

Circle 1, E., D., L., larger than the remaining valves.Circles 2-5, three valves in each circle lying on three

endocardiac ridges in the right, dorsal, and left axes.E a i a clavata.—Valves in four circles. Pocket valves in

each circle about equal in size and their positions identicalwith those of the corresponding valves in E . b a t i s .

In Scy l l i um c a n i c u l a the muscular covering of the conusin the adult heart does not as a rule extend as far as the peri-cardiac boundary. It falls short of this fixed point by anamount that appears to vary in different specimens. At itsmaximum about one-third of the conus may be without itsmyocardiac coat and the place of it is taken by tissue of thesame nature as that which comprises the wall of the ventralaorta beyond the pericardiac boundary. Though compara-tively short, this section of the conus has only unstriped musclein its wall therefore,- and if Gegenbaur's distinction betweenconus and bulbus arteriosus were to be applied strictly, it wouldbe necessary to recognize that both are represented in the conusof this Elasmobranch. The distinction is not only an externalone. The development of the endocardiac tissue, and thereforeof the valves within the lumen of the conus, closely correspondswith the amount of reduction in the outer myocardiaccovering. Thus, three circles of complete pocket valves maybe found in the conuS of Scy l l i um when the myocardiac coatis at its maximum length (cf. Text-fig. 2 b). In cases where thismuscular layer is much reduced, however, only two circles ofeonus valves may be separately identified. Probably it was forthis reason that Meckel found only two circles of valves inScy l l i um c a n i c u l a , and Stohr described three (loc. cit., 26,p. 214). A series of transverse sections of the conus do notshow more than three complete valves in each circle. Thoseon the right and left sides of the conus are larger than the valvesin the dorsal position. The former tend, especially at the baseof the conus, to enclose narrow areas close to the attachmentswhich they make with the conus wall. Outward appearances

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then suggest the presence of incipient pocket valves. Thishowever is not the case, for the cavity of any one of the appar-ently new valves can be shown to be continuous with the cavityof the valve from which it has been derived.

TEXT-FIGS. 2 a, 2 b, 2 C.

'KoV2a 2b 2c

Kg. 2a.—Horizontal section of the conus arteriosus of Ba iac lava ta .

Fig. 2 b.—Horizontal section of the conus arteriosus of Scy Iliumcanicula .

Fig. 2 c.—Horizontal section of the conus arteriosus of P r i s t i u rusmelanostomus.

Showing the reduction in extent of the myocardiac coat m. Otherstructures as in Text-fig. 1.

The position in Scy l l ium is therefore:Valves in two to three circles. Pocket valves clearly differen-

tiated in every circle. They occur only in the right, dorsal,and left axes.

This formula is true also of the heart of P r i s t i u r u s m e l a n o -

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s torn us except that the number of circles of valves is notgreater than two. In P r i s t i u r u s moreover, the myocardiaccoat in the conus wall is invariably short. As a rule it doesnot extend over more than half the distance between the baseof the conus and the pericardiac boundary. The result isthat the base of the first circle of valves is half-way down theconus, and there is a considerable region in front, entirely freefrom valves, which presumably cannot be rhythmically con-tractile (cf. Text-fig. 2 c).

The four Elasmobranchii that have been under discussionthus far clearly show a tendency to reduce the muscular cover-ing of the conus arteriosus. It is notable that hand in hand withthis process has gone the virtual elimination of the ventral endo-cardiac ridge, a reduction in the number of valves in each of thelongitudinal rows remaining—from a maximum of five to aminimum of two—and, the commencement in Scy l l ium ofa process which resulted in the enlargement of the valves of themorphologically right- and left-hand sides with atrophy of thetraces of the dorsal endocardiac ridge also.

C r o s s o p t e r y g i i .

Turning to some of the fishes which occupy a ratherisolated zoological position, it will be convenient to considerfirst the condition of the conus arteriosus in P o l y p t e r u s .The figure shows that the proportions of this chamber in com-parison with those of the remainder of the heart are far frommean. The myocardiac coat extends forwards over the conusto the pericardiac boundary and the six endocardiae ridges aresplit up into a very large number of valves. Boas (2, p. 324)recognized three chief longitudinal rows of valves, each com-prising nine units, and three lesser rows in between them, alsocomposed of nine small units, i. e. nine circles of valves arrangedin six longitudinal rows each of which is made up of threelarge and three small units. Gegenbaur, on the other hand (loc.cit., 7, p. 356), stated that the number of valves in the fore-most circle—circle 1—may vary between two and five. Henoted eight further circles, comprising three valves each, and

TEXT-FIGS. 3, 3 a.

3a

Fig. 3.—Diagram of a ventral dissection of the conus arteriosusand ventricle o f P o l y p t e r u s . R., pocket valve in the morpho-logically right axis. D., pocket valve in the morphologically dorsalaxis. L., pocket valve in the morphologically left axis. V., pocketvalve in the morphologically ventral axis, a., region of the trans-verse section, Text-fig. 3a ; p., pericardiac wall; At., atrium;Ve., ventricle ; e., endocardiac valves.

Fig. 3a.—Transverse section of the conus of P o l y p t e r u s i n theregion a. Lettering as in Text-fig. 3 ; TO., myocardiac coat.

CONUS ABTEEIOSUS 155

mentioned that rows of smaller valves occurred in betweenthem. Of the variability in the number of endocardiac valvesin this species there can be no doubt—eight circles of valves in-stead of nine, for example, are shown in Text-fig. 3. There are sixlongitudinal rows of valves of which only three extend through-out the whole length of the conus. With regard to the circles :

Circle 1 comprises three large pocket valves, R., L., V.,and a smaller pocket valve D. The rim of the latter doesnot reach to the same level as the rims of the other valvesin the circle.

Circles 2-7 comprise six valves each, many of which haveno cavity. The valves in the central areas of the conusshow by their serrated margins a clear tendency to dividefurther, and the axes they occupy are right, dorsal, left,ventral, and intermediately, between the ventral and rightaxis and the ventral and left axis.

Circle 8 comprises four pocket valves, three of which liein the left, right, and ventral axes of the conus and thefourth between the left and ventral axes.

The reduction of the morphologically dorsal valve (D.) at theheadward end of the conus in the specimen illustrated and thestrong development of the unit in the ventral axis (V.) isbrought out by the transverse section, Text-fig. 3 a. In thisdetail is a marked contrast to the arrangement noted in theElasmobranchii, where the ventral axis in the correspondingpart of the conus bears no trace of endocardiac tissue.

It is also clear that while i n P o l y p t e r u s endocardiac ridgesare well developed in each of the four main morphological axes,right, dorsal, left, and ventral, the dorsal ridge and its valvederivatives give signs of decreased usefulness and therefore ofapproaching elimination.

H o l o s t e i .

Very similar to the conus of P o l y p t e r u s is that of Lep i -d o s t e u s o s s e u s . In this species, however, there are typicallyeight circles of endocardiac valves set in eight longitudinalrows. Four of the latter are developed more strongly than the

TEXT-FIGS. 4, 4 a.

Kg. 4.—Diagram of a ventral dissection of the conus arteriosus andventricle of L e p i d o s t e u s . a., region of transverse section,Text-fig. 4 a ; other structures as in Text-fig. 3.

Pig. 4 a.—Transverse section of the above in the region a.

CONUS ARTERIOSUS 157

remainder and are remarkable because the positions theyoccupy are intermediate between the right, dorsal, left, andventral axes.

The arrangement of the circles is shown diagrammatically inText-fig. 4, thus :

Circle 1 comprises four equally well-developed pocket valves.Circles 2 and 3 comprise seven valves. Pour of these are

in line with the intermediate pocket valves of circle 1.The remainder, in the right, dorsal, and left axes of theconus, are small valves with their cavities poorly developedor quite obliterated.

Circles 4 and 5 resemble the preceding circle with theaddition of an extra unit in the ventral axis. The totalnumber of valves in these circles is thus made up to eight.

Circles 6 and 8 comprise five valves. Units are omittedfrom the right, dorsal, and left axes, but one is present inthe ventral axis.

Circle 7 comprises six valves. Five arranged as in thecircles 6 and 8, and an extra unit in the right axis.

Variation in the number and position of the minor valves ineach circle relative to the dominant valves is to be expected.Stohr (loc. cit., 26, p. 224) pointed out that they become fewerbeyond the fourth circle, and Boas (loc. cit., 2, p. 323) noticedthat the small rows of valves do not extend uniformly through-out the conus, &c. Many of the valve units are reduced to solidknots of endocardiac tissue which contrast strongly with theprominent pocket valves of circle 1.

In both L e p i d o s t e u s and P o l y p t e r u s the individualvalves are joined to one another by fibrous connecting strandsof tissue. A more strongly marked central ridge of this tissueruns forward from the base of each valve and probably repre-sents the original endocardiac ridge. There are clearly eight ofthese in L e p i d o s t e u s and six in P o l y p t e r u s .

C h o n d r o s t e i .

The clear-cut, well-formed pocket valves in the conusarteriosus of an adult Ac ipenser do not present a very close

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resemblance to their homologues in P o l y p t e r u s and L e p i -d o s t e u s . Stohr (loc. cit., 26, p. 221) described the variationin the number of these valves in different specimens of Aci-pense r s t u r i o . The number of circles of valves (three) was

TEXT-FIGS. 5, 5 a, 5 b.

Fig. 5.—Sagittal section of the heart in Ac ipense r ru thenus . At.,p., e., as in Text-fig. 3; m., myocardiac coat in the conus wall; s.,peripheral blood sinuses ; a. and b., region of transverse sections.

Kg. 5 a.—Transverse section through the conus arteriosus of A c i -penser in the region a. D., L., R., dorsal, left, and right endo-cardiac valves of circle 1.

Fig. 5 b.—Transverse section through the conus arteriosus of A c i -pense r in the region b. 1-6, the six pocket valves of circle 2.

identical in all cases ; but the number of longitudinal rows wasindeterminable owing to the fact that the number of valvesin each circle was not constant.

Circle 1 comprised four valves, one of which is said to havebeen frail or rudimentary.

Circle 2 comprised five to six valves.Circle 3 comprised four to five valves.

With this formula the arrangement of the corresponding valvesin the sterlet A c i p e n s e r r u t h e n u s and in the 'shovelnosed Sturgeon' S c a p h i r y n c h u s p l a t y r y n c h u s wasfound to agree very well. In the former, three valves onlyoccurred in circle 1, lying in the axes D., E., and L. (cf. Text-fig. 5 a).

CONUS ARTERIOSUS 159

There is a long interval between circles 1 and 2 (cf. Text-fig. 5).

Circle 2 comprises six valves (of. Text-fig. 5 b).Circle 3 comprises only four valves.

The enlargement of the morphologically right-hand valve (E.)at the headward end of the conus, and the consequent deflexionof the dorsal valve (D.) to the left-hand side, is depicted in theText-fig. 5 a.

In S c a p h i r y n c h u s :Circle 1 comprises three well-developed pocket valves and

a small solid strip of endocardiac tissue in the morpho-logically ventral position. The pocket valves, E., D., andL., are so large that they practically occlude the conuslumen by themselves. Partly because of their size andpartly because they are set farther from the pericardiacboundary than are the corresponding valves in circle 1 of thesterlet, the interval between these valves and the valves ofcircle 2 is considerably less than the corresponding intervalin the conus arteriosus of the sterlet.

Circle 2 comprises six pocket valves.Circle 3 comprises four well-developed pocket valves and

one solid rudiment.The external appearance of the heart in both Ac ipense r

and S c a p h i r y n c h u s is striking because the wall of theventricle is enclosed by a loose covering of connective tissuewhich bears a profuse development of peripheral blood sinuses.This covering spreads over the conus so completely that it givesto it an appearance of being telescoped into the ventricle (cf.Text-fig. 5).

In P o l y o d o n , according to Danforth (5), the number ofcircles of valves may vary between three and four. The numberof prominent longitudinal rows, on the other hand, is four.

The myocardiac covering of the conus arteriosus in Po ly-p t e r u s , L e p i d o s t e u s , and Acipenser meets theboundary of the pericardiac space. In S c a p h i r y n c h u s it isset back from this fixed point by a small amount, and in Ami athe process has advanced a great deal farther.

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A mi a—usually classified with the Holostei—stands in anisolated position where the conus arteriosus is concerned. Forrather more than half its length the conus wall is built uplargely of striped muscle, but the remaining headward portionis composed entirely of smooth, unstriped muscle, without anytrace of myocardiac covering. The construction of the wholerecalls that of the conus arteriosus in P r i s t i u r u s althoughthe endocardiac apparatus is not so closely paralleled.

The valves are arranged in Ami a, as Stohr (loc. cit., 26,p. 225) and Boas (loc. cit., 2, p. 324) pointed out, in threecircles and four longitudinal rows. They are all functionalpocket valves but are not all of the same size. The rows ofvalves in the right and left morphological axes predominate ineach circle, and though quite definite, the valves in the dorsaland ventral axes are by comparison very small indeed (cf.Text-fig. 6 a, valves no. 1 and 3).

The biggest pair of all the valves are situated at the headwardend of the conus, in circle 1. They are especially notable becausea large number of fine fibres, which are fixed to the inner wall ofthe conus just behind the pericardiac boundary, run backwardsto their attachments on the valve margins and traverse thelumen of the conus in doing so (cf. Text-fig. 6, t). The impor-tance of this feature lies in the fact that it may be interpretedas evidence of the former forward extension of the part of theconus that has striped muscle in its wall. The valves in questionlie immediately beneath this muscular part of the conus wall,whereas the fibres extend well into the region that has no myo-cardiac covering. If the view which regards such fibres as dueto the destruction of the valve walls is reasonable, their forwardattachments are clear indications of the former boundaries ofthe valves themselves. The association between the foremostcircle of endocardiac valves and the headward limit of themyocardiac coat is close in this and other species, and prob-ability seems therefore to favour the view that the endocardiacridges extended farther forwards in pre-existing forms, and thatthere went with this, a greater forward extension of the muscularcovering.

CONUS AETEEIOSUS 161

A m i a is not unique in the possession of fibrous connectivesbetween the valves of circle 1 and the conus farther forwards.Boas drew a comparison between A m i a in this respect and oneof the Teleostean fishes (3, p. 529).

TEXT-FIGS. 6, 6 a.

Fig. 6.—Longitudinal section a little towards the right side of theconus arteriosus of A mi a. t., thread attaching the rim of theright-hand valve in circle 1 to the conus wall. Other lettering as inprevious Text-figures.

Fig. 6 a.—Transverse section through the conus arteriosus of A mi ain the region a. Note four pocket valves 1—4, and the junctionbetween the myocardiac coat m. and the conus wall IF. of theheadward section of the conus.

At this point attention ought also to be drawn to the factthat the development and degree of importance of the pair ofpocket valves at the headward end of the conus in A mi a isparalleled by a corresponding though much less marked develop-

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ment of valves in this situation, as compared with the others,in the conus of P o l y p t e r u s , L e p i d o s t e u s , Acipenser ,and S c a p h i r y n c h u s .

Dipnoi .

The conus arteriosus in Dipnoi has been the subject of in-vestigation, notably, byLankester (14, p. 493), Boas (loc. cit.,2),and Robertson (loc. cit., 20). Lankester and Boas were bothconcerned with Cera todus and P r o t o p t e r u s , Robertsonwith Lep idos i r en . The whole position has been summarizedfrom the point of view adopted in this paper by Graham Kerr(loc. cit., 11, pp. 40 et seq.; loc. cit., 12, pp. 877-8 ; 13, p. 383),so that only a few of the more important features of theseinteresting hearts need comment here.

It is commonly stated in both the older and more recentliterature that the conus arteriosus in Dipnoi is twisted. Theassertion is made, for example, of the adult heart of Cerat-odus (cf. Lankester, loc. cit., 14, p. 496, and Nierstrasz,17, p. 662). The evidence already quoted of the developingheart rudiment in Lep idos i r en (Robertson, 20, p. 86)shows that this is not so. The characteristic shape of the conusin its final form is to be interpreted rather as a fold or doubleflexure (cf. 11, p. 41). In Cera todus and P r o t o p t e r u sa similar flexure occurs.

With regard to its musculature, the conus in Dipnoi has thethird nearest to the ventricle covered with a rnyocardiac coatwhich is histologically identical with the muscle of the ventri-cular wall. At the apex of the first flexure, however, the myo-cardiac coat thins out perceptibly and continues as a very thinsheet of tissue to the anterior pericardiac boundary. Internalto this remnant of the former muscular wall there lies a layer oftissue similar in every respect to that which forms the wall of theventral aorta.

There is ample evidence for the opinion that endocardiacvalves in the Dipnoi have tended to revert to the primitivecondition of endocardiac ridges. The process in this directionhas advanced least in the conus arteriosus of C e r a t o d u s .

CONUS ARTERIOSUS 163

If this is laid open (cf. Goodrich, 9, fig. 219, p. 250) a display ofvalves will be seen at the headward and posterior ends of theconus flexure that recalls the arrangement of valves in the conusof L e p i d o s t e u s . The valves at the headward end are dis-posed in two circles of four units each, those at the base of theconus are in more than four circles; but their number is variable.Boas (loc. cit., 2, p. 329), for instance, calculated a circle of

TEXT-FIG. 7.

aff

Diagram of a ventral dissection of the conus arteriosus in Cera t -o d u s. aff., afferent branchial vessels ; /., longitudinal fold—Ridge R. ; v., valves at the base of the longitudinal fold; if.,p., e., lettered as in previous Text-figures.

ten units in one case. By far the most arresting feature of theconus in C e r a t o d u s is the great development that has takenplace in one of the longitudinal rows of endocardiac valves. Ithas gone on to the exclusion of all the other endocardiac valvesin the central region of the conus. This is of course the regionchiefly concerned in the actual flexure. The individual valvesin the row in question are distinguishable only at the ventri-cular end of the conus (cf. Text-fig. 7, v.). There are three or

M2

164 C. W. PARSONS

four valves closely bound to one another in this part of the row,and they occupy a ventral position in the adult heart. Beyondthe margin of the foremost of these units the row extends as acontinuous and strongly developed ridge or fold. The ridge hasreceived various names, e.g. 'spiral valve', ' conus valve',and ' longitudinal fold '. It seems best to adhere to the latter,the name originally employed by Boas. The longitudinal fold,then, reaches across the floor of the conus and round the doubleflexure. Arriving in the headward section of the conus, it stopsshort just behind the valve in circle 2 which lies in the morpho-logically right-hand axis. The conditions in Elasmobranchiiclearly show that each longitudinal ridge in the conus wasprimitively continuous from end to end. It is thereforejustifiable to regard the break in the longitudinal fold inC e r a t o d u s as secondary, and to name the fold ridgeE. for thewhole of its extent. The apparent spiral form of this fold isresponsible largely for the impression that the conus of the adultheart is twisted. Eobertson showed, however, that the similar,though more profound, spiral form of the ' bulbus ' valve inL e p i d o s i r e n was due in development to ' the process ofkinking and asymmetrical expansion of an elongated but originallystraight bulbus and not to any twisting or counter-twisting ofthat segment of the heart' (loc. eit., 20, p. 104). It is interestingto note also that the longitudinal fold arises in L e p i d o s i r e nin two portions discontinuously. One of these belongs to theright wall of the conus, the other to the ventral wall. The formergives rise to the longitudinal fold of the middle and headwardportions of the conus, and the latter to the longitudinal fold ofthe base of the conus, i.e. nearest to the ventricle. There istherefore agreement between C e r a t o d u s and L e p i d o s i r e non the position of the headward end of this fold although, ashas been pointed out, in C e r a t o d u s the fold stops short inthe headward portion of the conus. Reasons for regarding thefold in C e r a t o d u s as representing ridge E. throughout itslength have been given, and there is therefore every supportfor the opinion that the same is true of the longitudinal fold inL e p i d o s i r e n . In both cases the movement of the fold

CONUS ARTERIOSUS 165

from the right to the apparent ventral axis must be due to theconus flexure.

In P r o t o p t e r u s Lankester discovered minute traces ofvalves far down at the base of the conus, but he failed to findany similar traces in the headward portion (loc. cit., 14, pp. 501,502). The longitudinal fold is prolonged as it is in L e p i d o -sir en, and it fuses, in the headward end of the conus, with analmost equally well-developed sheet of endocardiac tissue thatlies on the opposite wall in the morphological axis L. (cf. 9,fig. 218, p. 249). The two sheets of endocardiac tissue form amost effective septum which extends backwards to the begin-ning of the conus flexure.

In L e p i d o s i r e n again the longitudinal fold fuses at thepericardiac border with a sheet of endocardiac tissue on theopposite Avail of the conus, but the septum is not so stronglydeveloped as it is in P r o t o p t e r u s . Three persistent rowsof vestigial pocket valves in the basal portion of the conusof L e p i d o s i r e n were noted by Bobertson (loc. cit., 20, p. 57).

T e l e o s t e i .At the time when he proposed the name conus arteriosus for

the contractile anterior segment of the heart in Elasmobranchii,Gegenbaur commented upon the main points in which it differsfrom the corresponding part of the heart in Teleostei (loc. cit.,6, pp. 378-4). The latter is distinguished by the nature of itswalls and by the fact that it does not contract rhythmically.A myocardiac covering is entirely absent from the conus in thevast majority of Teleostei, and some differences of opinion havebeen expressed on the manner in which this came about. Thereare two explanations. The myocardiac coat has been lost eitherby recession, i. e. intussusception, into the ventricle, or byovergrowth of the mass of tissue in front of it. Both explana-tions are said to be valid. Smith (24, p. 70) noted that the situa-tion of the valves in relation to the ventricle in some casesfavoured one explanation, in others, it favoured the other.

The conus wall in Teleostei is built up characteristically oftough connective tissue, plain muscle-fibres, and elastic fibres

166 C. W. PAESONS

which together form a mass varying greatly in size among thedifferent genera. The lumen of the conus also presents a variableappearance. The common Wrasse, the Tarpon, and Sym-b r a n c h u s resemble one another, for example, in the posses-sion of a conus with the inner walls ridged by a number ofconspicuous plain muscle-strands or trabeculae, which .extendfrom one end of the conus to the other. At the opposite extremeare forms such as the Cod, whose inner conns wall is remarkablysmooth. Gegenbaur expressed the opinion that variation of theTeleostei on this point indicated a trend from the ridgedtowards the smooth condition. Intermediate types are not lack-ing. The inner wall of the conus of the Salmon, for example, isdivided by flat trabeculae at the headward end, but at its basethe lining is perfectly smooth. The contrast between thesmooth and ridged types of lumen is unmistakable if the figures,Text-figs. 9 a and 9 c, are compared.

Another very variable feature of the conus arteriosus inTeleostei is the degree of cohesion between the muscular andelastic fibres that form its wall. In trabeculate forms such asS y m b r a n c h u s (Text-fig. 9c) the conus wall is very compact.In the common Dab, on the other hand, the conus wall is com-posed of a loose spongework through which are innumerableinterstices (cf. transverse sections 8 b and 8 c).

The external appearance of the teleostean. conus is alsovariable. It may be very short and swollen as it is in the Dab,or long and narrow as it is in the Gurnard and S y m b r a n c h u s .The heart of Gymnarchusisremarkable because the surfaceof the conus is moulded into four or more large swellingssimilar to those shown in Text-fig. 8, cs. But, in spite of therelative differences in external diameter, the nature of the conuswall is constant, i. e. in forms that have an elongated conus thespongework is merely more compressed and its interstices arefewer and smaller.

The endocardiac tissue in Teleostei has a plain surface onwhich a single circle of valves are typically developed. Theseare situated at the ventricular end of the conus and may besunk deeply into the mass of the ventricle itself. There are

CONUS ABTERIOSUS 167

some exceptions. Boas (loc. cit., 3, p. 528) reckoned two circlesof valves in B u t i r i n u s (Albula) comprising six units in all.

Circle 1 comprises two valves.Circle 2 comprises four valves; two large valves situated

on the right and left wall respectively at the base of theconus, the other two intermediate in position and small.

TEXT-PIGS. 8, 8 a, 8 B, 8 b, 8 C, 8 c.

Fig. 8.—Diagram of the heart o fGymnarchus excluding the sinusvenosus. At., atrium; Ve., ventricle; c, conus; cs., conusswelling ; p., pericardiac wall; a., region of transverse section,Text-fig. 8 a.

Kg. 8 a.—Transverse section of the above in the region a. L, lumenof the conus ; cs., conus swelling.

Fig. 8B.—Diagram of the heart of the common dab (Hippo -g lossoides l imandoides) . 6., region of transverse section,Text-fig. 8 b ; other lettering as in Text-fig. 8.

Pig. 8 b.—Transverse section of the conus arteriosus of the dab.I., lumen of the conus.

Fig. 8 C.—Diagram of the heart of S y m b r a n c h u s . Lettering asin Text-fig. 8.

Fig. 8 c.—Transverse section of the conus arteriosus of Sym-b r a n c h u s . I., lumen of the conus; t., trabeculae.

Senior (21, p. 148) described four valves in two circles in theTarpon, Megalops a t l a n t i c u s , and (22, p. 379) inMegalops c y p r i n o i d e s . The valves are close to the ven-tricle and are arranged in two longitudinal rows on the rightand left sides of the conus (cf. Text-fig. 9 b).

168 C. W. PARSONS

In P t e r o t h r i s s u s , again, Senior found two transversetiers of valves, i. e. valves in two circles (23, p. 84).

The single circle of valves in Teleostei may comprise morethan two units. Gegenbaur pointed out, for instance, that whilethere is only one circle of valves in the conus of the sword-fishX i p h i a s , it is comprised of four units (loc. cit., 6, p. 372).

TEXT-FIOS. 9 a, 9 b, 9 C.

Fig. 9 a.—Diagram of a ventral dissection of the conus arteriosusand ventricle of the common cod (Gadus morrhua) . w., thickconus wall; c, conus ; p., pericardiac wall; Ve., ventricle ;v., pocket valve.

Kg. 9 b.—Diagram of a ventral dissection of the conus arteriosusand ventricle of the tarpon (Megalops a t l a n t i c u s ) . I., trabe-culae ; other lettering as in Text-fig. 9 a.

Fig. 9 c.—Diagram of a ventral dissection of the conua arteriosusand ventricle of S y m b r a n c h u s . Lettering as in Text-fig. 9 b.

Two of these units are again large valves attached to thelateral conus wall and the other two are small and intermediatein position. Pettigrew, describing a dissection of the heart ofa sun-fish, O r t h a g o r i s c u s , mentions three valves in thecircle (loc. cit., 19, p. 779), and a similar number is figured (loc.cit., 9, fig. 69, p. 110) in the conus of Salrno s a l a r .

Indications of the occurrence of more than two valves in the

CONUS ARTEBIOSUS 169

conus of Teleostei in the past are not lacking therefore. Theexisting condition in X i p h i a s makes it at least possible thatthere were originally four, developed of course from four endo-cardiac ridges.

Special peculiarities to individual valve systems are not un-

TEXT-FIG. 10.

-aff

Diagram of a ventral dissection of the conus arteriosus and ventricleof Loph ius p i s c a t o r i u s . aff., afferent branchial vessels;p., pericardiac wall; Ye.., ventricle; v., pocket valve; I., lobe onthe right endocardiac valve at the base of the conus.

common, and in this category no doubt there falls a peculiarityof the conus valves of Loph ius p i s c a t o r i u s . The wall ofthe right-hand pocket valve bears a conspicuous solid outgrowthwhich protrudes as a lobe dorsally, and has no doubt the effectof closing off the cavities of the ventricle and conus more per-fectly than would otherwise be the case (cf. Text-fig. 10,1.).

C y c l o s t o m a t a .

The conus arteriosus is frequently stated to be absent inCyclostomata (cf. Kierstrasz, loc. cit., 17, p. 658). It is clearly

170 C. W. PARSONS

represented in the sense in which the term is applied herehowever, the ventricle of the heart passing anteriorly into astructure homologous with the conus arteriosus of other fishes.A pair of semilunar valves may be found at the base of thisstructure, and they are developed as in the teleostean fisheson the right and left sides of the cardiac tube in this region.

The walls of the conus are not specially thickened, but theypossess, according to Clark (loc. cit., 4, p. 22), considerableautomaticity, and they contract on stimulation.

In Myxinoidea the pericardiac cavity remains in wide com-munication with the main body-cavity, and in Petromyzontidaeit also remains open during a large part of the larval existence.The completion of the pericardo-peritoneal septum subsequentlyseals off the pericardiac cavity in P e t r o m y z o n however,and it becomes increasingly difficult to delimit the pericardiacboundaries because they are gradually enveloped in cartilagederived from the branchial basket.

The partial development of the pericardiac wall in the earlystages of the development of the heart in Cyclostomata musthave a considerable effect on the folding of the growing cardiactube. But the latter remains fixed anteriorly, so that growthin length causes folding and constriction of the tube as it doesin other fishes. In comparison the amount of folding is lesspronounced, of course, and in the adult, conus, ventricle, andatrium succeed one another in practically a straight line.

In recent work, particularly that of Stensio (25), the relation-ship between Myxinoidea and Petromyzontidae is held to beextremely remote. Apart from the difference in respect of thepericardiac cavity which has just been discussed, there isnothing to distinguish between the hearts of present-dayrepresentatives of the two groups.

D i s c u s s i o n .

One of the first problems that arises in considering the fore-going description as a whole concerns the number of primitiveendocardiac ridges. Only two ridges are present in the Cyclo-stomata, a group of fishes that are in many respects archaic.

CONUS ARTBRIOSUS 171

But as'this group can claim no close relationship to the Elasmo-branchii, and as the number two is characteristic also ofTeleostei where it is clearly secondary, there can be no justifica-tion for supposing that two was the primitive number of theseridges.

The largest number of endocardiac ridges that have beendetailed appear in L e p i d o s t e u s , where there are eight.In P o l y p t e r u s there are six, and it is suggestive that whenthe functionally important valves at the headward end of theconus are considered, they should belong in the two cases toeight different morphological axes. In P o l y p t e r u s they arein the positions R., D., L., and V., but the representative of thedorsal axis is considerably reduced. In L e p i d o s t e u s thechief valves of circle 1 occupy the four axes intermediate be-tween E. and D., D. and L., L. and V., and V. and E. Themorphologically dorsal ridge is very strongly developed inElasmobranchii, and it must be counted in trying to estimatethe largest number of axes of functional significance. Thepossibility that the primitive number of ridges may have beenat least as high as eight cannot therefore be passed over.Gegenbaur (loc. cit., 8, p. 610) was inclined to admit it in spiteof his own evidence of only four endocardiac ridges in anembryo A c a n t h i a s (loc. cit., 8, p. 605). Confirmatory evi-dence from the embryos of the fishes cited is still wanting,however, and without it nothing more positive can be said infavour of the view that the primitive number of endocardiacridges in the conus may have been so high.

On the other hand, O'Donoghue and Abbot (18, p. 829) haveshown that four longitudinal rows of valves are common in theadult heart of the least specialized Elasmobranchii—includingNotidanidae. The evidence from Gegenbaur's embryo Acan-t h i a s may be interpreted as support for the view that theprimitive number of ridges was four, and there seems no doubtthat ' four was the number present in primitive Tetrapoda '(loc. cit., 12, p. 388). Strong reasons exist, therefore, for suppos-ing that the primitive number of these ridges in fishes was four.This is the number indicated by the presence of four longitu-

172 C. W. PARSONS

dinal rows of valves in such various forms as Po lyodon ,A mi a, and X i p h i a s , and if the view is correct, the largernumber of ridges characteristic of the conus in L e p i d o s t e u sand P o l y p t e r u s must have arisen secondarily.

The evolutionary importance of conus valves has been thesubject of much discussion. They differ much in different typesand are valuable links in phylogeny.

Starting with Elasmobranchii as the most primitive group,it is clear that the evolution of the conus has followed twodifferent paths.

1. Those forms which have a fully developed muscular conuswall point to a tendency to multiply the number of conus valveswhilst retaining full powers of contractility. In contrast to this,

2. Other Elasmobranchii—such as Pristiurus—show theexistence of the opposite tendency, namely, to localize the con-tractility of the conus and at the same time to reduce the valveapparatus it contains.

The first route obviously leads to greater complexity and thesecond to greater simplicity of structure. The additional valvesthat have been shown to occur in either of the circles nearestthe ventricle in Acipenser make of this conus an interestinglink between the condition described in P o l y p t e r u s withsix longitudinal rows of valves and Lep idos t eus with eight.The myocardiac coat in all three genera extends forwards overthe conus as far as the pericardiac boundary, and in thischaracteristic the conus in Dipnoi is similar. The dipnoancondition probably evolved from a complex form of conus likethat of L e p i d o s t e u s . It has been shown that the charac-teristic longitudinal fold of the dipnoan conus is a compositestructure, and the traces of more than four valves at the baseof the conus in Cera todus may be taken as evidence pointingto ancestry of a kind that was complex in the sense of havingmany endocardiac ridges.

The sequence may be advantageously expressed thus :Lep idos t eus

Acipense r or D i p n o iP o l y p t e r u s

CONUS ARTERIOSUS 173

and thence to terrestrial vertebrates.1 At each step the conusarteriosus becomes fundamentally more complex.

The commencement of variation in the opposite direction isindicated in Elasmobranchii by Scy I l i um and P r i s t i u r u s .In A mi a the outward sign of this tendency, i.e. restriction ofthe myocardiac covering, is particularly marked. Gegenbaur,Boas, and others, recognized that upon the available evidencethe only possible position for Amia , as far as the conus isconcerned, lies between Elasmobranchii and Teleostei. If thisis its true place, the single pair of valves in the conus of Teleosteimust be equivalent to the well-developed pair of them at theheadward end of the conus in Amia . The fibrous connexionsof the latter with the wall of the conus in front of them hasalready been emphasized as evidence for the assertion that thecontractile part of the conus formerly extended farther for-wards, and this assertion must be equally true of the conus inTeleostei. The sequence E lasmobranch—Amia—Te 1 eos tis less abrupt than appears at first because of the few exceptionalteleostean fishes that have more than a single pair of conusvalves. It is also a frequent experience to find the conus ofa teleostean fish no more muscular apparently than that ofAmia .

In view of the very great differences in the muscular develop-ment of the conus arteriosus in Teleostei, the question of itsfunction becomes a matter of interest. Ingenious suggestionswere made on the matter long ago, by Home (loc. cit., 10, p. 235).He related the muscularity of the conus in a particular fish withthe depth of water in which it habitually swims. The firstconsideration, however, must be the maintenance of an efficientcirculation through the finer blood-vessels of the gills. Ob-viously the blood should course through these fine vessels withan even pressure, and it seems probable that the advantagegained by the Teleostei in substituting an elastic apparatus forthe contractile type of conus has to do with the attainment of

1 The generic names are used to indicate phases in cardiac evolution,i.e. there is no suggestion that genera such as Lepidosteus or Poly-pterus are descended from Acipenser.

174 C. W. PARSONS

a measure of independence of the heart beat. The function ofthe so-called carotid gland in Amphibia has a similar signifi-cance. It acts apparently as a buffer to the blood-pressure inthe artery on the heart side and maintains a steady flow ofblood to the head.

The problem of the occurrence of a single pair of conus valvesin Cyclostomata has yet to be approached embryologically. Ifthe thesis here maintained of a primitive arrangement of fourendocardiac ridges is sound, it must be presumed that a parallelevolution to that of the conus in Teleostei is responsible for thereduction of this number to two in both cases.

SUMMARY.

1. The term ' conus arteriosus ' is used to define the whole ofthe headward portion of the heart in fishes which intervenesbetween the ventricle and the anterior boundary of the peri-cardiac space.

2. A brief description of the conus arteriosus in a number ofdifferent forms is given and attention is directed particularly(a) to the musculature of the conus wall, (b) to the number ofendocardiac ridges or valves which it contains, and (c) to theposition of these ridges in relation to the headward boundaryof the pericardiac cavity. The latter is taken as a fixed pointmorphologically and therefore as giving true indications of thepositions of the ridges (right, dorsal, left, and ventral). Thus,in Dipnoi, the conspicuous longitudinal ridge or fold whichforms the conus septum is morphologically right in position.The hearts examined include those of A c a n t h i a s , E a i a ,Scy l l ium, and P r i s t i u r u s , P o l y p t e r u s and Lep id-o s t e u s , Amia, the Dipnoi, and various Teleostei. Amongthe latter are Megalops , Gyrnnarchus , and Sym-b r a n c h u s .

3. The tendency to reduce the contractile function of theconus and to restrict its valves to the ventricular end is notedeven within the Elasmobranehii. The process has advanced sofar in Teleostei that the entrance to the lumen of the conus is

CONUS ARTBBIOSUS 175

protected by only one pair of valves as a rule, and the conushas lost its myocardiac covering.

Megalops is one of a small number of exceptional Teleosteiin which the endocardiac valves are more numerous.

4. The number and position of endocardiac valves in Cyclo-stomata is similar to their number and position in Teleostei.

5. Discussion is confined (1) to the primitive number ofendocardiac ridges ; (2) to the evolutionary significance of theconus arteriosus in fishes ; (3) to the function of the conusarteriosus in Teleostei.

ACKNOWLEDGEMENTS.

I desire to acknowledge my great indebtedness to ProfessorGraham Kerr for his advice in directing this work, his kindnessin reading the manuscript, and constant encouragement. I amalso very grateful for material received from the Superintendentof the Scottish Marine Biological Association, Mr. MichaelGraham, and Dr. G. S. Carter.

LlTBEATUBE.

1. Boas, J. E. V.—' Lehrbuch der Zoologie.' Jena, 1901.2. " tJber Herz und Arterienbogen bei Ceratodus und Protopterus ",

' Morph. Jahrb.', vol. vi, 1880.3. " tJber den Conus arteriosus bei Butirinus und bei anderen

Knoehenfischen ", ibid.4. Clark, A. J.—' Comparative Physiology of the Heart.' Cambridge,

1927.5. Danforth, C. H.—" The Heart and Arteries of Polyodon ", ' Journ.

Morph.', vol. 23, 1912.6. Gegenbaur, C.—" Zur vergleiohenden Anatomie des Herzens.", ' Jena.

Z. f. Naturw.', vol. ii, 1866.7. ' Vergleiohende Anatomie der Wirbelthiere', vol. ii. Leipzig,

1901.8. " t)ber den Conus arteriosus der Fische", ' Morph. Jahrb.',

vol. xvii, 1891.9. Goodrich, E. S.—" Vertebrata Craniata, 1st fasc, Cyolostomes and

Fishes", ' Treatise on Zoology', ed. by Lankester, Part IX.London, 1909.

10. Home, Sir E.—" Additions to an account of the anatomy of Squalusmaximus ", ' Phil. Trans. Roy. Soo.', vol. cii, 1813.

176 C. W. PARSONS

11. Kerc, J. Graham.—" Note on the morphology of the heart of Verte-brates " , ' Proc. Roy. Phys. Soc. Edinb.', vol. xix, 1913.

12. ' Text-book of Embryology,' vol. 2. London, 1919.13. ' Zoology for Medical Students.' London, 1921.14. Lankester, Sir E. Ray.—" On the hearts of Ceratodus, Protopterus, and

Chimaera " , ' Trans. Zool. Soc. Lond.', vol. x, 1879.15. Muller, J.—" Bemerkungen uber eigenthiimliche Herzen des Arterien-

und Venensystems " , ' Arch. Anat. Physiol. (Berlin)', Jahrg. 1842.16. " tJber den Bau und die Grenzen der Ganoiden " , ' Abh. der

k. Akad. Wiss. Berlin ', 1844.17. Nierstrasz, H. F.—' Vergleichende Anatomie der Wirbeltiere', von

Ihle, Van Kampen, Nierstrasz, und Versluys. Translated G. Chr.Hirsch, Berlin, 1927, from the Dutch.

18. O'Donoghue, Charles H., and Abbott, Eileen.—" The Blood VascularSystem of the Spiny Dogfish " , ' Trans. Roy. Soc. Edinb.', vol. lv,1928.

19. Pettigrew, James Bell.—" On the Relations, Structure, and Functionof the Valves of the Vascular System in Vertebrata " , ibid., vol. xxiii,1864.

20. Robertson, Jane I.—" The Development of the Heart and VascularSystem of Lepidosiren paradoxa", ' Quart. Journ. Micr. Sci.',vol. 59, 1913.

21. Senior, H. D.—" The Conus Arteriosus in Tarpon atlanticus " , ' Biol.Bull. Woods Holl', vol. 12, 1906-7.

22. " Note on the Conus Arteriosus of Megalops cyprinoides " , ibid.23. " Teleosts with a Conus Arteriosus having more than one row

of valves ", ' Anat. Rec.', vol. 1, 1907.24. Smith, Wilbur C.—" On the process of disappearance of the Conus

Arteriosus in Teleosts " , ibid., vol. 15, 1918.25. Stensio, Erik A.—" The Downtonian and Devonian Vertebrates of

Spitsbergen " , ' Skrifter om Svalbard. og Nordishavet', vol. 12,1927.

26.—Stohr, Phil.—" Ueber den Klappenapparat im Conus arteriosus derSelachier und Ganoiden " , ' Morph. Jahrb.', vol. ii, 1876.