Download Lecture III.5a. Animals II.

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.
Traditional
deuterostome
phylogeny. The two major
groups are the echinoderms
and the chordates. Pentameral (here called "biradial")
symmetry in echinoderms is
3. Chordata.
a secondary character as ina. Urochordates – tuni- dicated by the fact that echinoderm larvae are bilaterally
cates, sea squirts.
symmetric. Chordates include
the vertebrates, i.e., the
b. Cephalochordates
– forms we colloquially refer to
lancelets.
as "animals.”
c. Vertebrates – Jawless fish, sharks and rays, bony
fish, tetrapods.
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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, acoe- Yunnanozoa. Top. Fossil.
lomate.
Bottom. Reconstructed as a
hemichordate – see below.
b. Suggested affinities in- From Shu et al. (1996).
clude the following:

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 4th deuterostome phylum most closely related to
echinoderms and hemichordates.
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Above. Xenoturbella. Note the almost complete lack of differentiated structure. Below. Deuterostome phylogeny according to Freeman. Compare with traditional phylogeny on page 2.
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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.
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.
4. In some sea urchins, five-fold
symmetry in the adult is replaced by partial secondary
bilateral symmetry – adaption for burrowing.
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Hemichordates.
1. Pharyngeal gill slits – gas exchange and/or filter feeding1
2. Three-part body plan: proboscis, collar, trunk.
3. Acorn worms.
a. Proboscis: Mucous covered burrowing organ;
food capture.
b. Collar contains mouth.
c. Trunk contains numerous
pharyngeal gill slits.
4. Pterobranchs.
a. Proboscis reduced.
b. Collar bears tentacles.
Hemichordates.
Top.
c. Gill slits reduced in num- Acorn worm. B. Bottom.
ber or absent.
Pterobranch worms (colonial).
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Recent studies suggest alternation between filter and deposit feeding.
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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 (tunicates).
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 during metamorphosis in most.
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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 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.
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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.
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.
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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 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.
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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, 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.
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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 and ventral (epaxial and 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.
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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.
Vertebrate Brain. Fore-
7. Anterior to the anus, the brain receives / integrates
muscle mass is dorsal to and olfactory, auditory and visencases the visceral organs.
ual input. Midbrain coor-
dinates response to visual
8. Posterior to the anus, is a and auditory input. Hindpost-anal tail comprised prin- brain exercises reflex concipally of muscles and support trol over tasks such as
respiration and circulation.
tissue.
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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.
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Somite development during vertebrate embryogenesis is determined 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 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.
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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 substituting for teeth.
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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.
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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 muscles.
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Water flow over and blood flow through fish gills. Waste products, including 𝑪𝑶𝟐 and 𝑵𝑯𝟒 + (by-product of nitrogen metabolism) are exchanged 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 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.
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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 Countercurrent heat exthe water in which they change in the legs of wading
birds.
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.
Thorax temperature in hawk When ambient temperatures moths.
are high, honey bees also
regurgitate fluid to evaporatively cool head and thorax.
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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 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.
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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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c. Tunicate larva theory recently supported apparent presence notochord and gill slits in Vetulicolea.
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).
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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. (2 pts) Suppose that echinoderms were originally bilaterally symmetric. How would that change the synapomorphies in the cladogram on page 2?
2. (2 pts) If echinoderms were originally bilaterally symmetric, what factor(s) might have selected for the evolution of
“radial,” i.e. pentameral symmetry in adults?
3. (8 pts) How do tube feet work? (Requires outside reading. Be sure to cite references)
4. (2 pts) How does a starfish (sea star) eat a clam?
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5. (2 pts) Crossopterygian fishes crawled out of Devonian
streams and ponds some 370 million years ago. 2 Give
two reasons why they might have done so.
6. (2 pts) Redraw the scenario on the page 22 assuming a
motile, bilaterian ancestor.
7. (8 pts) 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).
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Only the adults were terrestrial. Reproduction occurred in water necessitating an aquatic larval stage.
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