Lecture III.5a. Animals – II.

Deuterostomes include echinoderms and chordates.

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Overview.

Deuterostomes:

1. Echinoderms - Pentam-eral (5-way) symmetry. Marine. Include sea lilies (crinoids - mostly extinct), sand dollars, sea urchins, starfish, etc.

2. Hemichordates – Ptero-

branch and acorn worms. The former traditionally in a separate group.

3. Chordata.

a. Urochordates – tuni-cates, sea squirts.

b. Cephalochordates –

lancelets.

c. Vertebrates – Jawless fish, sharks and rays, bony fish, tetrapods.

Traditional deuterostome phylogeny. The two major groups are the echinoderms and the chordates. Pentam-eral (here called "biradial") symmetry in echinoderms is a secondary character as in-dicated by the fact that echi-noderm larvae are bilaterally symmetric. Chordates include the vertebrates, i.e., the forms we colloquially refer to as "animals.”

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Deuterostome Origins Remain Obscure. 1. Fossil (Cambrian) Yunnano-

zoa. a. Originally considered a

“basal deuterostome”

b. More plausibly a hemi-chordate (see below).

2. Living Xenoturbella.

1. Small, worm-like, acoelo-mate.

2. Suggested affinities in-clude the following:

Degenerate mollusk – more likely a mollusk preda-tor, i.e., mollusk genes came from its food.

Basal bilaterian related to acoel flatworms. For discussion, see assigned reading.

A 4th deuterostome phylum most closely related to echinoderms and hemichordates.

Yunnanozoa. Top. Fossil.

Bottom. Reconstructed as a

hemichordate – see below.

From Shu et al. (1996).

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Above. Xenoturbella. Note the almost complete lack of differentiat-ed structure. Below. Deuterostome phylogeny according to Free-man. Compare with Boll’s proposed phylogeny (assigned reading).

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Echinoderms. 1. Internal skeletons com-

posed of calcareous plates lying just below the skin and superficial musculature.

2. Water vascular systems

composed of calcified, hy-draulic canals leading to ex-tensions called tube feet that function in a. Gas exchange

b. Locomotion

c. Feeding

3. Five-way symmetry a de-

rived character.

4. In sea cucumbers,

a. Adult symmetry partially & secondarily bilateral.

b. Five rows of tube feet retained in many.

Top. Adult manifesting five-fold symmetry. Bottom left. Transverse section of one of the five arms. Bottom right. Bilaterally symmetrical ciliat-ed larva. During metamor-phosis, ciliated arms re-sorbed & new arms develop.

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Life cycles in three groups of echinoderms. Abbreviation or complete elimination of the planktonic larval stage is observed in each. Bilateral symmetry in sea cucumbers is secondary. From Smith, A. B. 1997. Ann. Rev. Ecol. Syst. 28: 219-241.

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Hemichordates.

1. Pharyngeal gill slits1 used for gas exchange and / or filter feeding

2. Three-part body plan: pro-

boscis, collar, trunk. 3. Group I. Acorn worms.

a. Proboscis: Mucous cov-

ered burrowing organ; food capture.

b. Collar contains mouth.

c. Trunk contains numerous pharyngeal gill slits.

4. Group II. Pterobranchs.

a. Proboscis reduced.

b. Collar bears tentacles.

c. Gill slits reduced in num-ber or absent.

1 Long presumed homology to vertebrate gill slits recently supported by molecular stud-ies (Simakov et al. 2015. Nature. 527: 459-465).

Above. Acorn worm. Be-low. Pterobranch colony.

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Chordates.

Along with Hemichordates Chordates have pharyngeal gill slits. Synapomorphies as follows:

1. Hollow, dorsal nerve cord.

2. Dorsal supporting rod, the notochord.

3. Muscled tail that extends beyond the anus.

4. Endostyle or thyroid gland derived therefrom.

Gill slits specialized for filter-feeding.

Subphyla: Urochordata, Cephalochordata, Vertebrata.

Urochordates (Tunicata2). 1. Marine and mostly sessile.

2. Adult pharynx expanded into gill basket – traps food

particles with mucous secreted by the endostyle.

3. Motile larva with notochord, dorsal nerve cord & tail that are resorbed in most species during metamorphosis.

2 The name refers to the outer covering, which is called a tunic.

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A solitary tunicate. A. Free-swimming larva shown with dorsal nerve cord, notochord and a portion of the tail. B. The sessile adult. Water flows into the mouth and through the pharynx where food particles are trapped by mucous secreted by the endostyle. Both the pharynx and digestive tract empty (the pharynx via the gills) into a common space called the atrium.

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Tunicate metamorphosis. The free-swimming larva at-taches to the substrate via the so-called "adhesive or-gan." Thereafter, the swimming part degenerates while the visceral structures, particularly the gill basket, which strains food from the water, expands.

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Cephalochordates (lancelets). 1. Vertebrate similarities:

a. Segmented muscles; b. Notochord; c. Dorsal nerve cord; d. Pharynx; e. Gills and gill slits. f. Post-anal tail.

2. Differences: No skeleton; no

paired appendages.

3. Respiration through the skin; gills used only for feeding.

4. Despite fusiform shape, lancelets are sedentary.

Cephalochordates from the mid-Cambrian.

Lancelet in its burrow.

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Amphioxus, a cephalochordate, as seen through the transparent skin (top) and in sagittal section (bottom). The animal is a filter feeder, Mucous secreted by the endostyle (as in tunicates) is carried by cili-ary currents up the pharynx wall and over the gills. In the process, food particles adhere to the mucous, which is then passed to the in-testine. The gills thus function as a food-gathering system, whereas respiration, their principal function in vertebrates, is through the skin. As in vertebrates, swimming is accomplished by contraction of seg-mented muscles that run the length of the animal. Both the gonads and excratory nephridia (not shown) are also segmental, in which regard, they differ from all living vertebrates. Amphioxus-like fossils are known from the mid-Cambrian.

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Vertebrates.

1. Synapomorphies: vertebral column and skull enclosing a hollow brain.

2. Archetypal vertebrate free-swimming and fish-like.

3. Motion accomplished by contraction of segmented, V-

shaped (>>>) trunk muscles (tips point forward).

a. Muscles arranged in dorsal and ventral bundles called myomeres – one of each per vertebra.

b. Attach to vertebral column via sheets of tendon-like connective tissue.

c. Ribs develop within the sheets.

4. Support for the musculature provided by vertebral col-

umn (backbone).

a. Elements of the vertebral column called centra.

b. Notochord (stiff supporting rod) runs through the ver-tebral centra.

c. Hollow spinal cord passes through vertebral arches.

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Page facing. Archetypal vertebrate. Above. Sagittal sec-tion. Below. Transverse sections through the tail (A and C) and trunk (B and D).

The animal is a filter feeder, with water entering through the mouth and exiting through gill slits in the pharynge-al wall. Locomotion is via contraction of dorsal (epaxial) and ventral (hypaxial) trunk muscles that run segmental-ly the length of the animal.

The muscles are supported by ribs (dorsal and ventral) that attach to the vertebral centra, which surround the no-tochord. Mesenteries (membranes) supported by the ventral ribs encase the body cavity (coelom), which con-tains the internal organs.

Above the centra are vertebral arches that encase a hol-low, dorsal nerve cord (spinal cord) that expands anterior-ly to form the brain. Like the spinal cord, the brain is hol-low. It consists of three parts and is contained within a bony skull. Sense organs include paired eyes and ears, nose and pineal “eye” (not shown). Note the lung, which is represented as attaching to the back of the throat.

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5. Anteriorly, the spinal cord en-larges to form a three part, hol-low brain.

a. Brain enclosed in a bony or

cartilaginous cranium.

b. Sense organs access the environment through open-ings in the skull.

6. Evolutionary trends:

a. Forebrain enlargement

b. Transfer of midbrain / hind-

brain functions to forebrain.

7. Anterior to the anus, the muscle mass is dorsal to and encases the visceral organs.

8. Posterior to the anus, is a

post-anal tail comprised prin-cipally of muscles and support tissue.

Vertebrate Brain. Fore-brain receives / integrates olfactory, auditory and vis-ual input. Midbrain coor-dinates response to visual and auditory input. Hind-brain exercises reflex con-trol over tasks such as respiration and circulation.

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Vertebrate Segmentation.

Restricted to trunk muscles and associated structures: 1. Vertebrae.

2. Ribs.

3. Nerves and blood vessels.

4. Kidneys.

Contrast with annelid and ecdysozoan segmentation, which includes duplication of visceral structures.

Vertebrate segments develop sequentially from structures called somites. Somites

1. Develop from embryonic mesoderm.

2. Give rise to muscles, connective tissue and bone.

Somite development induced by signals from other embry-onic structures including the neural tube, which becomes the dorsal nerve cord), the notochord and surrounding tissues.

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Somite development during vertebrate embryogenesis is deter-mined by chemical signals produced by the neural tube and other tissues.

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Vertebrate Appendages.

Most vertebrates have paired appendages (pectoral / pelvic) on either side of the body.

First appear as fins in the line (jawed fish) leading to tetra-pods.

Fins later evolved into limbs.

Evolution of the tetrapod limb. Note progressive elongation of the ulna and differentiation of “fingers,” the number of which are re-duced from eight to five.

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Feeding and Respiration.

In primitive vertebrates, water 1. Entered through the mouth;

2. Passed over the gills; food particles strained out.

3. Exited through gill slits on the animal's side near the

head.

In Placoderms, gill function transitioned from filter feeding to respiration.

Placoderms: 1. Paraphyletic group.

2. First appear in late Silurian; abundant in Devonian.

3. Anterior gill arches modified to form jaws.

a. One known species with fish-like jaw bones. b. Rest had parrot-like jaws with boney plates substitut-

ing for teeth – next lecture.

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One view of skull and jaw evolution in vertebrates. With the discov-ery of Entelognathus (complete jaw), it now appears that jaws of the sort observed in bony fish evolved prior to the loss of a bony skeleton in sharks and rays. An alternative view is that sharks branched off before the evolution of bony skull.

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Blood Flow and Respiration. 1. Deoxygenated blood pumped anteriorly by the heart

and then over the gills through aortic arches.

2. Re-oxygenated blood flows posteriorly to the tissues.

3. The gills supported by cartilaginous / bony gill arches.

Blood flow in a hypothetical vertebrate ancestor, Direction of flow indicated by arrows: anterior from the ventral heart; up and over the gills via aortic arches where the blood is oxygenated; and posterior, i.e., toward the tail, through the dorsal aorta. The result is delivery of oxygenated blood to the trunk (swimming) muscles.

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Water flow over and blood flow through fish gills. Waste products, in-

cluding 𝑪𝑶𝟐 and 𝑵𝑯𝟒+ (by-product of nitrogen metabolism) are ex-

changed for oxygen. This is an example of countercurrent exchange.

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Countercurrent exchange (left) increases extraction of oxygen from water flowing over the gills by maintaining a high concentration gra-dient between water and blood. Compare with concurrent exchange (right). Medial (throat side) and lateral (skin side) refer to the ends of blood vessels that penetrate the gill lamellae – see previous diagram.

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Fick’s Law of Diffusion.

Solutes diffuse from regions of higher concentration to lower.

Approximately, the rate of diffusion (flux) as given above.

1. Increases with concentration gradient (P2 – P1), surface area (A).

2. Decreases with membrane thickness.

Applies to heat conduction as well as to fluid flow.

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Countercurrent Exchange a Common Device.

"Hot" fish keep swimming muscles warm by placing blood vessels leading to and away from the core in close proximity.

Wading birds have a similar arrangement in the legs to prevent loss of body heat to the water in which they stand.

Facilitates waste product concentration in kidney.

Some insects (bumblebees, honey bees, hawk moths) have a similar arrangement to keep heat generated by flight muscles in the thorax.

Aside: When ambient tem-peratures are high, honey bees also regurgitate fluid to evaporatively cool head and thorax.

Countercurrent heat ex-change in the legs of wading birds.

Thorax temperature in hawk-moths.

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Evolution and Embryology.

Many steps in vertebrate evolution recapitulated during development – e.g.,

4. Gill arches become jaws;

5. Upper jaw fuses to braincase to become the tetrapod

skull;

6. Reptilian jaw hinge becomes mammalian ear bones.

Structures start out as one thing, "change course" and de-velop into something else – recall “Chickenosaurus” video.

Famous example: Two of the three mammalian middle ear bones begin life as pieces of the embryonic skull and jaw and then migrate to new positions.

1. Illustrates the proposition that development recapitu-

lates embryonic states of ancestral forms - modern version of Haeckel's "Ontogenetic Law."

2. Consequent to the fact that mutations less likely to be deleterious if their action occurs later in development.

3. ⇒ "Rube Goldberg" character of evolutionary change.

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Rube Goldberg cartoon.

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Vertebrate Origins.

Whence cometh vertebrate ancestors?

1. Tunicate Larva Theory. a. Amphioxus-like chordates derive from the larvae of

sessile urochordates such as living tunicates.

b. Larvae became sexually competent (paedogenesis) with the accompanying loss of the sessile adult phase.

Tunicate larva scenario of vertebrate evolution.

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2. Bilaterian Ancestry Theory (BAT).

a. Urochordate ancestors bilaterally symmetric;

b. Tunicate adult form an evolutionary “add-on”.

Bilaterian ancestry theory of vertebrate evolution. (Answer to Practice Problem 6.)

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BAT requires inversion of dorso-ventral (D-V) pattern-ing during development if acorn worm-like animals the common ancestor of vertebrates and tunicates.

1. Vertebrates as upside-down

invertebrates first promoted by Étienne Geoffroy Saint- Hilaire in 1822 – doctrine of “unity of type”.

2. Long ridiculed. Now support-ed by molecular biology of development.

3. Proposed corollary: Three-part vertebrate brain (fore-brain, midbrain, hindbrain) reflects acorn worm three-part body plan (prosome, mesosome, metasome).

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Tunicate-vertebrate relation supported by fossil Vetulico-lians.

1. Small worm-like animals.

2. Previously believed to be

legless arthropods.

3. Enlarged anterior pharynx with gill slits.

4. Posterior segmented tail.

Annelids have also been proposed as vertebrate ances-tors. Would also require D-V inversion.

Regardless of vertebrate origins, progressive integration of viscera (digestive organs and related) and soma (trunk muscles) – an important theme of early vertebrate evolu-tion. See previous scenarios.

Cambrian chordates and allies.

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Early Cambrian vetucolian fossil (a, b, d) and reconstruction (c) from South Australia. Labels as follows: dk = dorsal keel; eb = epibionts; ism = intersegmental membrane; lg = lateral groove; nc = notochord; om = oral margin; vk = ventral keel; S1–S7 = posterior body region segment number; scale bars, 5 mm. From Garcia-Bellido (2014).