Lecture III.5a. Animals II. Deuterostomes include echinoderms and chordates.

Lecture III.5a. Animals II. Deuterostomes include echinoderms and chordates.

Overview. Deuterostomes: 1. Echinoderms - Pentameral (5-way) symmetry. Marine. Include sea lilies (crinoids - mostly extinct), sand dollars, sea urchins, starfish, etc. 2. Hemichordates Pterobranch and acorn worms. The former traditionally in a separate group. 3. Chordata. a. Urochordates tunicates, sea squirts. b. Cephalochordates lancelets. Traditional deuterostome phylogeny. The two major groups are the echinoderms and the chordates. Pentameral (here called "biradial") symmetry in echinoderms is a secondary character as indicated by the fact that echinoderm larvae are bilaterally symmetric. Chordates include the vertebrates, i.e., the forms we colloquially refer to as "animals. c. Vertebrates Jawless fish, sharks and rays, bony fish, tetrapods. 2

Deuterostome Origins Remain Obscure. 1. Fossil (Cambrian) Yunnanozoa. a. Originally considered a basal deuterostome b. More plausibly a hemichordate (see below). 2. Living Xenoturbella. a. Small, worm-like, acoelomate. b. Suggested affinities include the following: Yunnanozoa. Top. Fossil. Bottom. Reconstructed as a hemichordate see below. From Shu et al. (1996). Degenerate mollusk more likely a mollusk predator, i.e., mollusk genes came from its food. Basal Bilaterian related to Acoel flatworms. For discussion, see assigned reading. A 4 th deuterostome phylum most closely related to echinoderms and hemichordates. 3

Above. Xenoturbella. Note the almost complete lack of differentiated structure. Below. Deuterostome phylogeny according to Freeman. Compare with Boll s proposed phylogeny (assigned reading). 4

Echinoderms. 1. Internal skeletons composed of calcareous plates lying just below the skin and superficial musculature. 2. Water vascular systems composed of calcified, hydraulic canals leading to extensions called tube feet that function in a. Gas exchange b. Locomotion c. Feeding 3. Five-way symmetry a derived character as evidenced by larval bilateral symmetry. 4. In some sea urchins, five-fold symmetry in the adult is replaced by partial secondary bilateral symmetry adaption for burrowing. Top. Adult manifesting fivefold symmetry. Bottom left. Transverse section of one of the five arms. Bottom right. Bilaterally symmetrical ciliated larva. During metamorphosis, the ciliated arms are resorbed. New arms develop in the adult. 5

Hemichordates. 1. Pharyngeal gill slits gas exchange and/or filter feeding 1 2. Three-part body plan: proboscis, collar, trunk. 3. Group I. Acorn worms. a. Proboscis: Mucous covered 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 number or absent. Top. Acorn worm. Bottom. Colonial pterobranchs. 1 Recent studies suggest alternation between filter and deposit feeding. 6

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 (Tunicata 2 ). 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 in most species are resorbed during metamorphosis. 2 The name refers to the outer covering, which is called a tunic. 7

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. 8

Neural complex of a tunicate. Localization of the known pituitary peptides, [GnRH, PRL, etc.], and related substances are depicted. The ganglion is hatched; the dorsal strand [nerve cord] is dotted. CS, cloacal siphon; OS, oral siphon. Picture and caption from Kawamura et. al. (2002). Gen. Comp. Endocrinology.126: 130-135. 9

Tunicate metamorphosis. The free-swimming larva attaches to the substrate via the so-called "adhesive organ." Thereafter, the swimming part degenerates while the visceral structures, particularly the gill basket, which strains food from the water, expands. 10

Neural complex of a tunicate. Localization of the known pituitary peptides [GnRH, PRL, etc.], and related substances are depicted. The ganglion is hatched; the dorsal strand [nerve cord] is dotted. CS, cloacal siphon; OS, oral siphon. Picture and caption from Kawamura et. al. (2002). Gen. Comp. Endocrinology.126: 130-135. 11

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. Lancelet in its burrow. 3. Respiration through the skin; gills used only for feeding. 4. Despite fusiform shape, lancelets are sedentary. Cephalochordates from the mid-cambrian. 12

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 ciliary currents up the pharynx wall and over the gills. In the process, food particles adhere to the mucous, which is then passed to the intestine. 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 segmented muscles that run the length of the animal. Both the gonads and nephridia (excretory organs) are also segmental, in which regard, they differ from all living vertebrates. Amphioxus-like fossils are known from the mid-cambrian. 13

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, veeshaped (>>>) 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 column (backbone). a. Elements of the vertebral column called centra. b. Notochord (stiff supporting rod) runs through the vertebral centra. c. Hollow spinal cord passes through vertebral arches. 14

Next Page. Archetypal vertebrate. Above. Sagittal section. 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 pharyngeal wall. Locomotion is via contraction of dorsal (epaxial) and ventral (hypaxial) trunk muscles that run segmentally the length of the animal. The muscles are supported by ribs (dorsal and ventral) that attach to the vertebral centra, which surround the notochord. Mesenteries (membranes) supported by the ventral ribs encase the body cavity (coelom), which contains the internal organs. Above the centra are vertebral arches that encase a hollow, dorsal nerve cord (spinal cord) that expands anteriorly to form the brain. Like the spinal cord, the brain is hollow. 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. 15

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5. Anteriorly, the spinal cord enlarges to form a three part, hollow brain. a. Brain enclosed in a bony or cartilaginous cranium. b. Sense organs access the environment through openings in the skull. 6. Evolutionary trends: a. Enlargement of forebrain b. Transfer of midbrain / hindbrain functions thereto. 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 principally of muscles and support tissue. Vertebrate Brain. Forebrain receives / integrates olfactory, auditory and visual input. Midbrain coordinates response to visual and auditory input. Hindbrain exercises reflex control over tasks such as respiration and circulation. 17

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Vertebrate Segmentation. Restricted to trunk muscles and associated structures: 1. Vertebrae. 2. Ribs. 3. Nerves and blood vessels. 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 embryonic structures including the neural tube (becomes the dorsal nerve cord), the notochord and surrounding tissues. 19

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Somite development during vertebrate embryogenesis is determined by chemical signals produced by the neural tube and other tissues. 21

Vertebrate Appendages. Most vertebrates have paired appendages (pectoral / pelvic) on either side of the body. First appeared as fins in jawed fish in the line leading to tetrapods. 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 reduced from eight to five. 22

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 substituting for teeth. 23

One view of skull and jaw evolution in vertebrates. With the discovery of Entelognathus, 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. 24

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 muscles. 25

Water flow over and blood flow through fish gills. Waste products, including CO 2 and NH 4 + (by-product of nitrogen metabolism) are exchanged for oxygen. This is an example of countercurrent exchange. 26

Countercurrent exchange (left) increases extraction of oxygen from water flowing over the gills by maintaining a high concentration gradient 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. 27

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. 28

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. Countercurrent heat exchange in the legs of wading birds. 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. Thorax temperature in hawkmoths. Aside: When ambient temperatures are high, honey bees also regurgitate fluid to evaporatively cool head and thorax. 29

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 develop 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 recapitulates 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. 30

Rube Goldberg cartoon. 31

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. 32

c. Tunicate larva theory recently supported apparent presence notochord and gill slits in Vetulicolia. i. Small (5 cm.) worm-like animals previously believed to legless arthropods. ii. Expanded pharynx at the front of the animal. iii. Segmented tail at the back. Photographs and interpretation of Cambrian Vetulicolian. From Garcia-Bellido (2014). 33

2. Bilaterian Ancestry Theory. a. Urochordate ancestors bilaterally symmetric; b. Tunicate adult form an evolutionary add-on. In either case, an important theme of early vertebrate evolution was the progressive integration of viscera (digestive organs and related) and soma (trunk muscles) see figure previous page. Questions. 1. Suppose that echinoderms were originally bilaterally symmetric. How would that change the synapomorphies in the cladogram on page 2? 2. If echinoderms were originally bilaterally symmetric, what factor(s) might have selected for the evolution of radial, i.e. pentameral symmetry in adults? 3. How do tube feet work? See assigned readings (here and here). 4. How does a starfish (sea star) eat a clam? 34

5. Crossopterygian fishes crawled out of Devonian streams and ponds some 370 million years ago. 3 Give two reasons why they might have done so. 6. Redraw the scenario on the page 22 assuming a motile, bilaterian ancestor. 7. Tetrapods and insects both obtain oxygen from the air. Compare gas exchange and transport in these two groups. Relate to insect size past and present. (Requires outside reading). 3 Only the adults were terrestrial. Reproduction occurred in water necessitating an aquatic larval stage. 35