35.1. Radial Symmetry, Again (p. 614)
A. The Deuterostomes
1. All organisms in this chapter are deuterostomes---coelomate animals that have the following embryological characteristics.
2. Radial cleavage in embryonic cell division: the daughter cells sit on top of previous cells.
3. Fate of cells is indeterminate; if embryonic cells are separated, each one develops a complete organism.
4. The blastopore is associated with the anus, and the second embryonic opening is associated with the mouth.
5. The coelom (enterocoelom) is formed by outpocketing of the primitive gut.
6. The deuterostomes include two major phyla: phylum Echinodermata and phylum Chordata.
a. Echinoderms are radially symmetrical as adults, but larvae are bilaterally symmetrical.
B. Echinoderms Have a Spiny Skin
1. About 6,000 species of echinoderms are in the phylum Echinodermata. (Fig. 35.1)
2. Modern echinoderms are all marine.
3. They have an endoskeleton consisting of calcareous plates bearing spines.
4. The class Crinoidea includes about 600 species of crinoids.
a. This group includes the stalked sea lilies and the motile feather stars.
b. Their branched arms are used for filter-feeding and give the animals a flowerlike or plantlike appearance.
5. The class Holothuroidea has 1,500 species of sea cucumbers. (Fig. 35.1b)
a. They have a long leathery body.
b. They feed by tentacles about the mouth.
6. About 950 species of sea urchins and sand dollars are in the class Echinoidea. (Fig. 35.1c)
a. Both possess spines used for locomotion, defense, and burrowing; sea urchins have long, blunt spines.
b. Sand dollars are flattened with a five-part flowerlike pattern of pores for skin projections.
7. About 2,000 species of brittle stars are in the class Ophiuroidea.
a. Brittle stars possess a central disk from which long, flexible arms radiate.
b. Their arms allow them to move rapidly along.
8. The class Asteroidea contains about 1,500 species of sea stars (starfishes). (Fig. 35.1a) [transp. 186]
a. For most, the body tends to be dorsoventrally flattened.
b. They have a central disk from which five, or a multiple of five, sturdy arms extend.
9. Sea stars are common along rocky coasts where they feed on clams, oysters, and other bivalves.
10. The five-rayed body has an oral (mouth) and aboral (upper) side.
11. Various structures project through the body wall.
a. Spines project from the endoskeletal plate through the thin dermis.
b. Pincerlike pedicellarie keep the surface free from particles.
c. Gas exchange is conducted by skin gills.
d. On the oral surface, each arm has a groove lined with tube feet.
12. A sea star feeds by everting its stomach.
a. A sea star positions itself over a bivalve and attaches tube feet to each side of the shell.
b. By working tube feet alternatively, it pulls shell open; a small crack is sufficient to insert a cardiac stomach.
c. The stomach enzymes begin digestion of the bivalve while it is trying to close its shell.
d. Partially digested food is then taken into the pyloric stomach for full digestion.
e. A short intestine opens at the anus on the aboral side.
13. In each arm is a well-developed coelom containing a pair of digestive glands and male or female gonads.
14. The nervous system is a central ring with radial nerves in each arm.
15. A light-sensitive eyespot is at the end of each arm; the sea star is capable of coordinated but slow responses.
16. Locomotion depends upon the water vascular system. (Fig. 35.1)
a. Water enters on the aboral side through the sieve plate (madreporite).
b. Water passes through a stone canal to a ring canal and into the radial canals in each arm.
c. The radial canals feed into lateral canals extending into tube feet, each of which has an ampulla.
d. Contraction of an ampulla forces water into the tube foot, expanding it; when the foot touches a surface, the center withdraws forming a suction and adhering to surfaces.
17. Echinoderms lack a complex respiratory, excretory, or circulatory system.
Fluids within the coelomic cavity carry out the function of diffusing substances and gases.
Gas exchange occurs across the skin gills and tube feet.
a. Nitrogenous wastes diffuse through coelomic fluid and across the body wall.
b. Cilia on the peritoneum lining the coelom keep the coelomic fluid moving.
18. Sea stars reproduce both sexually and asexually.
a. If the body is fragmented, each fragment can regenerate a whole animal.
b. Sea stars spawn and release either eggs or sperm at the same time.
c. The bilateral larvae undergoes a metamorphosis to become a radially symmetrical adult.
35.2. Chordate Characteristics Evolve (p. 616)
A. The Chordates
1. The chordates include about 45,000 species in the phylum Chordata.
2. All chordates at sometime during their life history have a notochord, a dorsal hollow nerve cord, and pharyngeal gill pouches.
a. A notochord is a dorsal supporting rod located dorsally just below the nerve cord; it provides support and is replaced by the vertebral column in vertebrates.
b. A dorsal hollow nerve cord is where the chord contains a fluid-filled canal; spinal cord is protected by vertebrae.
c. Pharyngeal gill slits are openings that function in feeding, gas exchange, or both.
1) They are seen only during embryonic development in most vertebrates.
2) In invertebrate chordates, fish, and amphibian larvae, they become functioning gills.
3) In terrestrial vertebrates, the pouches are modified for various purposes; in humans, the first pouches become the auditory tubes, the second become tonsils, and the third and fourth pairs become the thymus and parathyroid glands.
3. Most chordates have an internal skeleton against which muscles work.
4. Most have a postanal tail that extends beyond the anus; in some, this may only appear in embryos.
5. The phylum Hemichordata includes about 90 species of hemichordates.
a. Some acorn worms living on tidal mud flats have a proboscis, a collar, and a trunk. (Fig. 35.2)
b. The dorsal nerve cord in the collar and trunk resembles the nerve cord of chordates.
c. The pharynx below the collar has gill slits.
d. Larva resembles the larva of echinoderms; some phylogeneticists think echinoderms and hemichordates share a common ancestor and hemichordates and chordates are related by a common ancestor.
B. Invertebrate Chordates
1. The notochord persists and is never replaced by the vertebral column.
2. The subphylum Urochordata contains 1,250 species of tunuicates that have gill slits. (Fig 35.3)
3. Adults have a body composed of an outer tunic with an incurrent and excurrent siphon; they squirt water out when disturbed.
4. Water passes into a pharynx and out numerous gill slits, the only chordate characteristic that remains in adults.
5. Microscopic particles adhere to a mucous secretion in the pharynx and are eaten.
6. The larvae are bilaterally symmetrical and have the three chordate characteristics.
7. The larva undergoes metamorphosis to develop into the sessile adult.
8. Beating of numerous cilia lining the inside of the pharynx creates a current to move water through a tunicate.
9. Some suggest larvae became sexually mature without developing tunicate characteristics; thus, a urochordate larva was ancestral to vertebrates; or a cephalochordate larva may have been ancestral to vertebrates.
10. The lancelets have three chordate characteristics (Fig. 35.4) [transp. 187]
11. The 23 species of lancelets are in the genus Branchiostoma in the subphylum Cephalochordata.
12. Their elongated, lance-shaped body resembles the lancelet, a two-edged surgical knife.
13. They inhabit shallow coastal waters; they lie partly buried in sandy substrates and filter feed.
14. They feed on microscopic particles filtered from the constant stream of water that enters the mouth and exits through the gill slits into an atrium that opens at the atriopore.
15. Lancelets retain the three chordate characteristics as an adult.
16. The notochord extends from head to tail, accounting for the name "Cephalochordata."
17. They possess segmented muscles and the dorsal hollow nerve cord has periodic branches.
35.3. The Vertebrate Body Plan Evolves (p. 619)
A. Subphylum Vertebrata
1. Vertebrates make up about 43,700 species and are in the subphylum Vertebrata.
2. Vertebrates have all three chordate characteristics at some point during their lives.
3. The embryonic notochord is by a vertebral column.
4. Vertebral column is made of individual hard segments (vertebrae) that surrounds the dorsal hollow nerve cord.
5. The vertebral column, part of a flexible but strong endoskeleton, is evidence that vertebrates are segmented.
6. Vertebrate skeleton is living tissue (either cartilage or bone) that grows with the animal.
7. Together, the endoskeleton and muscles form a system that permits rapid and efficient movement.
8. Pectoral and pelvic fins of fishes evolved into jointed appendages that allowed vertebrates to move onto land.
9. The skull is the most anterior component of the main axis of the vertebrate endoskeleton; it encases the brain.
10. The high degree of cephalization in vertebrates is accompanied by complex sense organs.
a. Eyes developed as outgrowths of the brain.
b. Ears were equilibrium devices in aquatic vertebrates; function as sound-wave receivers in land vertebrates.
11. They possess a complete digestive system and a large coelom.
12. Circulatory system is closed, with respiratory pigments contained within blood vessels.
13. Gas exchange is efficiently accomplished by gills or lungs.
14. Kidneys are efficient in excretion of nitrogenous waste and regulation of water.
15. Reproduction is usually sexual with separate sexes.
16. The first vertebrates were fishlike. (Fig. 35.5) [transp. 188]
17. Fishes are aquatic, gill-breathing vertebrates that usually have fins and skin covered with scales.
18. The larval form of a modern-day lamprey, which looks like a lancelet, may resemble the first vertebrates.
a. It not only has the three chordate characteristics like the tunicate larva, it also has a two-chambered heart, a three-part brain, and other internal organs that are like those of vertebrates.
b. The small, jawless, and finless ostracoderms are the earliest vertebrate fossils.
c. They were filter feeders, but probably were also able to move water through their gills by muscular action.
d. Ostracoderms are dated from Cambrian and as late as the Devonian period, but then they became extinct.
e. Although extant jawless fishes lack protection, many early jawless fishes had large defensive head shields.
B. Lampreys and Hagfishes
1. Jawless fishes are called agnathans, and 63 species belong to the superclass Agnatha.
2. Lampreys and hagfishes are modern jawless fishes; they lack a bony skeleton. (Fig. 35.6)
3. They have smooth nonscaly skin; have cylindrical bodies, and are up to a meter long.
4. Hagfishes are scavengers feeding on soft-bodied invertebrates and dead fishes.
5. Many lampreys are filter feeders; others are parasitic with a round muscular mouth equipped with teeth that enables them to attach themselves to a fish and suck nutrients from the host's circulatory system. (Fig. 35.6)
6. Marine parasitic lampreys entered the Great Lakes and devastated the trout population in the 1950s.
C. Jaws Evolve
1. Animals beyond this point are gnathostomates, animals that have jaws, tooth-bearing bones of the head.
2. Jaws are believed to have evolved from the first pair of gill arches of agnathans.
3. Placoderms are extinct jawed fishes of the Devonian period.
a. They were armored with heavy plates and had strong jaws. (Fig. 35.5)
b. Like extant fishes, they had paired pectoral and pelvic fins. (Figs. 35.5 and 35.7)
c. Paired fins allow a fish to balance and to maneuver well in water, which facilitate predation.
D. Cartilaginous Fishes Include Sharks
1. About 850 species of sharks, rays, and skates are in the class Chondrichthyes, the cartilaginous fishes.
2. They have a cartilaginous skeleton rather than bone.
3. There are five to seven gill slits on both sides of the pharynx; they lack the gill covers of bony fish.
4. The body is covered epidermal placoid (toothlike) scales; the teeth of sharks are enlarged scales.
5. Three senses detect electric currents in water, detect pressure (a lateral line system), and smell.
6. The largest sharks are filter feeders, not predators; basking and whale sharks eat tons of crustacea.
7. Most sharks are fast-swimming, open-sea predators; a great white shark feeds on dolphins, sea lions and seals.
8. Rays and skates live on the ocean floor; their pectoral fins are enlarged into winglike fins; they swim slowly.
9. Stingrays have a venomous spine.
10. The electric ray family can feed on fish that have been stunned with electric shock of over 300 volts.
11. Sawfish rays have a large anterior "saw" that they use to slash through schools of fish.
E. Bony Fishes Are Most Numerous of Vertebrates
1. About 20,000 species of bony fishes are in the class Osteichthyes.
2. Bony fishes have a bony skeleton; most are ray-finned with thin, bony rays supporting the fins.
3. A few fishes are lobe-finned and related to ancestors of amphibians.
4. The ray-finned fishes include familiar fishes.
a. They are the most successful and diverse of the vertebrates.
b. They vary from filter feeders to insectivores to predaceous carnivores.
c. Their skin is covered by scales formed of bone.
d. Their gills do not open separately and instead are covered by an operculum.
e. A swim bladder is a gas-filled sac whose pressure can be altered to regulate buoyancy and depth.
f. Salmon, trout, and eels can migrate from fresh water to salt water, but must adjust kidney and gill function.
g. Bony fishes depend on color vision to detect both rivals and mates.
h. Sperm and eggs are shed into the water.
i. For most, fertilization and embryonic development occur outside the female's body.
5. The lobe-finned fishes include six species of lungfishes and one species of coelacanth. (Fig. 35.8)
a. They possess fleshy fins that are supported by central bones.
b. Lungfishes live in stagnant water or ponds that dry up; found in Africa, South America and Australia.
c. Coelacanths live in deep oceans; once considered extinct, more than 200 have been captured since 1938.
d. From mitochondrial DNA analysis, lungfishes are probably the closest living relatives of amphibians.
F. Limbs Evolve
1. All other animals from this point have four limbs and are called tetrapods.
2. Lobe-finned fishes of Devonian period are ancestral to the amphibians. (Figs. 35.5 and 35.9)
[transp. 188 and 189]
3. Animals that live on land use limbs to support the body, especially since air is less buoyant than water.
4. Lobe-finned fishes and early amphibians also had lungs and internal nares to respire air.
5. Two hypotheses describe evolution of amphibians from lobe-finned fishes.
a. Lobe-finned fishes capable of moving from pond-to-pond had an advantage over those that could not.
b. The supply of food on land, and the absence of predators, promoted adaptation to land.
6. The first amphibians diversified during Carboniferous period (Age of the Amphibians). (Fig. 35.5)
H. Amphibians
1. About 3,900 species of amphibians belong to class Amphibia; this includes frogs, toads, salamanders, and neuts.
2. Amphibians have many features not seen in bony fishes.
a. Limbs with girdles are adapted for walking on land.
b. A tongue is used for catching prey.
c. Eyelids keep eyes moist.
d. Ears are adapted for detecting sound waves.
e. A larynx is adapted for vocalization.
f. Their brain is larger than that of fishes, and the cerebral cortex is more developed.
g. Their skin is thin, smooth, and nonscaly, and contains numerous mucous glands; the skin plays an active role in osmotic balance and respiration.
h. Amphibians usually have lungs; usually they are supplemented by exchange of gases across a porous skin.
i. The single-loop path of fish has been replaced by a closed double-loop circulatory system.
(Fig. 35.10)
j. They possess a three-chambered heart that pumps mixed blood before and after it has gone to the lungs. (Fig. 35.10b) [transp. 190]
k. Reproduction involves a return to the water; "amphibian" refers to two life styles.
1) They shed eggs into the water for external fertilization.
2) Generally, eggs are protected by a coat of jelly but not by a shell.
3) Young hatch into aquatic larvae with gills (tadpoles).
4) The aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult. (Fig. 35.11)
l. Amphibians are ectothermic; they depend upon external heat to regulate body temperatures.
m. If the environmental temperature becomes too low, ectotherms become inactive (enter torpor).
n. Salamanders more likely resemble earliest amphibians due to their S-shaped movements.
o. Salamanders practice internal fertilization; males produce a spermatophore that females pick up.
p. Frogs and toads are tailless as adults; the hind limbs are specialized for jumping.
35.4. The Amniote Egg Evolves (p. 624)
A. Amniote Egg Is a Land Adaptation
1. The amniote egg is a fertilized egg that contains extraembryonic membranes.
2. The reptiles were the first vertebrates to practice internal fertilization through copulation and to lay eggs that are protected by a leathery shell. (Fig. 35.12) [transp. 191]
3. Extraembryonic membranes are not part of embryo itself and are disposed of after development completes.
4. They protect the embryo, remove nitrogenous wastes, and provide the embryo with oxygen, food, and water.
5. The amnion is one of the extraembryonic membranes of the amniote egg; creates a sac that fills with fluid and provides a watery environment in which the embryo develops.
6. This permits land animals to reproduce without a dependence on water.
7. Reptiles evolved from amphibian ancestors sometime during the Carboniferous period. (Fig. 35.13)
8. The first reptiles (stem reptiles) gave rise to several other lineages; each adapted to a different way of life.
a. Pelycosaurs (sail lizards) are related to therapsids, mammal-like reptiles ancestral to mammals.
b. Some lines returned to aquatic environments; ichthyosaurs were fishlike and plesiosaurs had a long neck.
c. Thecodonts were reptiles that gave rise to most of the reptiles, living and extinct.
d. Pterosaurs of the Mesozoic era had a keel for attachment of flight muscles and air spaces in bones to reduce weight.
9. In dinosaurs and mammal-like reptiles' limbs supported the body from underneath, providing increased agility.
10. Reptiles dominated the earth for about 170 million years during the Mesozoic era and then most died out.
11 A hypothesis by Luis and Walter Alvarez and several others proposes the impact of a large meteorite at the end of the Cretaceous period set off earthquakes and fires, raising enough dust and smoke to block out the sun.
B. Reptiles
1. About 6,000 species of reptiles are in the phylum Reptilia. (Figs. 35.12- 35.14)
2. Most live in tropics or subtropics; lizards and snakes live on soil; turtles and alligators live in water.
3. Reptiles have a thick, scaly skin that is keratinized and impermeable to water; keratin is protein found also in hair, fingernails, and feathers; this protective skin prevents water loss but requires several molts a year.
4. Reptilian lungs are more developed than those of amphibians; air rhythmically moves in and out of the lungs due to the presence of an expandable rib cage (except in turtles).
5. Most reptiles have a nearly four-chambered heart, except the crocodile is completely four-chambered; therefore, oxygenated blood is more fully separated from deoxygenated. (Fig. 35.10c) [transp. 190]
6. The well-developed kidneys excrete uric acid; less water is lost in excretion.
7. Reptiles are ectothermic; they require a fraction of the food per body weight of birds and mammals, but are behaviorally adapted to warm their body temperature by sunbathing.
8. About 6,000 species of snakes and lizards live mainly in the tropics and desert.
a. Lizards have four clawed legs and are carnivorous; marine iguanas on the Galapagos are adapted to spend time in the sea; frilled lizards have a collar to scare predators, and blind worm lizards live underground.
b. Snakes evolved from lizards and lost legs as an adaptation to burrowing; their jaws can readily dislocate to engulf large food; their tongue collects airborne molecules and transfers them to the Jacobson's organ for tasting; a minority of snakes that are poisonous have special fangs.
c. Turtles have a heavy shell fused to the ribs and thoracic vertebrae; they lack teeth but use a sharp beak; sea turtles must leave the ocean to lay eggs onshore.
d. Crocodiles and alligators are largely aquatic, feeding on fishes and other animals; muscular tail is a paddle to swim and a weapon; male crocodiles bellow to attract mates and some species protect eggs and young.