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The Chapter in Review
Evolution of Animals

Animals are motile, multicellular heterotrophs that ingest their food.

Ancestry of Animals

The choanoflagellates are protists that most likely resemble the last unicellular ancestor of animals. A colony of flagellated cells could have led to a multicellular animal that formed tissues by invagination.

The Evolutionary Tree of Animals

The evolutionary tree of animals is chiefly based on molecular and developmental data. During the evolution of animals, multicellularity preceded true tissues and bilateral symmetry. Bilateral symmetrical animals either undergo protostome development or deuterostome development. Certain of the protostomes and all deuterostomes have a internal cavity called a coelom. Some of the coelomate animals are segmented.

Introducing the Invertebrates
  • Sponges are multicellular (lack tissues) and have various symmetries.

  • Cnidarian shave two tissue layers, are radially symmetrical, have a saclike digestive cavity, and possess stinging cells and nematocysts.

  • Flatworms have ectoderm, endoderm, and mesoderm but no coelom. They are bilaterally symmetrical and have a saclike digestive cavity.

  • Roundworms have a pseudocoelom and a complete digestive tract.

Protostomes and Deuterostomes Compared

On the basis of molecular and developmental data:

  • Flatworms, roundworms, molluscs, annelids, and arthropods are protostomes; the first embryonic opening becomes the mouth. In the coelomate protostomes, the coelom develops by a splitting of mesoderm.

  • Echinoderms and chordates are deuterostomes; the second embryonic opening becomes the mouth, and the coelom forms by outpocketing of the primitive gut.

Molluscs, Annelids, and Arthropods

These groups of animals are the coelomate protostomes.


The body of a mollusc typically contains a visceral mass, a mantle, and a foot.

  • Gastropods Snails, representatives of this group, have a flat foot, a one-part shell, and a mantle cavity that carries on gas exchange.

  • Cephalopods Octopuses and squids display marked cephalization, move rapidly by jet propulsion, and have a closed circulatory system.

  • Bivalves Bivalves such as clams have a hatchet foot and a two-part shell, and are filter feeders.

Annelids: Segmented Worms

Annelids are segmented worms; segmentation is seen both externally and internally.

  • Marine Worms Polychaetes are worms that have many setae. A clam worm is a predaceous marine worm with a defined head region.

  • Earthworms Earthworms are oligochaetes that scavenge for food in the soil and do not have a well-defined head region.

  • Leeches Leeches are annelids that feed by sucking blood.

Arthropods: Jointed Appendages

Arthropods are the most varied and numerous of animals. Their success is largely attributable to a flexible exoskeleton and specialized body regions.

  • Crustaceans have a head that bears compound eyes, antennae, and mouthparts. Five pairs of walking legs are present.

  • Arachnids include spiders, scorpions, ticks, mites, harvestmen and horseshoe crabs. Spiders live on land and spin silk, which they use to capture prey as well as for other purposes.

  • Insects have three pairs of legs attached to the thorax. Insects have adaptations to a terrestrial life, such as wings for flying.

Echinoderms and Chordates

Echinoderms have radial symmetry as adults (not as larvae) and endoskeletal spines. Typical echinoderms have tiny skin gills, a central nerve ring with branches, and a water vascular system for locomotion, as exemplified by the sea star.


Chordates (tunicates, lancelets, and vertebrates) have a notochord, a dorsal tubular nerve cord, pharyngeal pouches, and a postanal tail at some time in their life history.

  • Invertebrate Chordates Adult tunicates lack chordate characteristics except gill slits, but adult lancelets have the four chordate characteristics and show obvious segmentation.

  • Vertebrate Chordates Vertebrate chordates include the fishes, amphibians, reptiles, birds, and mammals.

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  • The first vertebrates, represented by hagfishes and lampreys, lacked jaws and fins.

  • Cartilaginous fishes, represented by sharks and rays, have jaws and a skeleton made of cartilage.

  • Bony fishes have jaws and fins supported by bony spikes; the bony fishes include those that are ray-finned and a few that are lobe-finned. Some of the lobe-finned fishes have lungs.

  • Amphibians (frogs, toads, newts, and salamanders) evolved from ancient lobe-finned fishes and have two pairs of jointed vertebrate limbs. Frogs usually return to the water to reproduce; and metamorphose into terrestrial adults.

  • Reptiles (snakes, lizards, turtles, crocodiles, and birds) lay a shelled egg, which contains extraembryonic membranes, including an amnion that allows them to reproduce on land.

  • Birds are feathered reptiles, which helps them maintain a constant body temperature. They are adapted for flight; their bones are hollow with air cavities; lungs form air sacs that allow one-way ventilation; and they have well-developed sense organs.

  • Mammals are amniotes that have hair and mammary glands. The former helps them maintain a constant body temperature, and the latter allows them to nurse their young.

  • Monotremes lay eggs.

  • Marsupials have a pouch in which the newborn matures.

  • Placental mammals, which are far more varied and numerous, retain offspring inside the uterus until birth.

Human Evolution

Arboreal primates are mammals adapted to living in trees. During the evolution of primates, various groups diverged in a particular sequence. Prosimians (tarsiers and lemurs) diverged first, followed by the monkeys, then the apes, and then humans. Molecular biologists tell us we are most closely related to the African apes, with which we share a common ancestor dated at about 7 MYA.

Evolution of Hominins

Human evolution occurred in Africa, where it has been possible to find the remains of several early hominins that date back to 7 MYA. The evolution of hominins includes these innovations:

  • The most famous australopithecine is Lucy (3.18 MYA) whose brain was small but who walked bipedally.

  • Homo habilis, present about 2 MYA, is certain to have made tools.

  • Homo erectus, with a brain capacity of 1,000 cc and a striding gait, was the first to migrate out of Africa.

Evolution of Modern Humans

Two contradicting hypotheses have been suggested about the origin of modern humans:

Key Terms
Testing Yourself

Choose the best answer for each question.

  1. Which of these protists is hypothesized to be ancestral to animals?

    1. green algal protist produced both the plants and animals.

    2. A choanoflagellate that moves by flagella.

    3. An amoeboid protist that moves by pseudopods.

    4. Animals unlike plants do not have a protist ancestor.

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  3. In the following diagram, label the major regions of the bodies of acoelomates, pseudocoelomates, and coelomates.

  4. Sponges are ancestors of

    1. cnidarians.

    2. protostomes.

    3. deuterostomes.

    4. None of these are correct.

For questions 4–10, identify the group(s) to which each feature belongs. Each answer may be used more than once. Each question may have more than one answer.


  1. flatworms

  2. roundworms

  3. molluscs

  4. annelids

  5. arthropods

  6. echinoderms

  7. chordates

  1. most have an endoskeleton

  2. typically bilaterally symmetrical as adults

  3. nonsegmented

  4. lack any kind of coelom

  5. chitin exoskeleton

  6. contain a mantle

  7. move by pumping water

  8. Insects have wings and three pairs of legs attached to the

    1. abdomen.

    2. thorax.

    3. head.

    4. midsection.

  9. Cartilaginous fishes detect their prey by sensing

    1. electrical currents.

    2. odors.

    3. pressure changes.

    4. More than one of these are correct.

    5. All of these are correct.

  10. Which of the following is not a feature of mammals?

    1. hair

    2. milk-producing glands

    3. ectothermic

    4. four-chambered heart

    5. diaphragm to help expand lungs

  11. This species was probably the first to have the use of fire.

    1. Homo habilis

    2. Homo erectus

    3. Homo sapiens

    4. Sahelanthropus tchadensis

    5. Australopithecus afarensis

  12. Which of these is matched correctly?

    1. Australopithecus afarensis—bipedal but small brain

    2. Homo habilis—made tools and migrated often

    3. Homo erectus—had fire and migrated out of Europe to Africa

    4. Homo sapiens—projecting face and had culture

    5. Both a and c are correct.

  13. Cnidarians are considered to be organized at the tissue level because they contain

    1. ectoderm and endoderm.

    2. ectoderm.

    3. ectoderm and mesoderm.

    4. endoderm and mesoderm.

    5. mesoderm.

  14. A cnidocyte is

    1. a digestive cell.

    2. a reproductive cell.

    3. a stinging cell.

    4. an excretory cell.

  15. Which of the following is not a feature of a coelomate?

    1. radial symmetry

    2. three germ layers

    3. complete digestive tract body plan

    4. organ level of organization

  16. A mollusc's shell is secreted by the

    1. foot.

    2. head.

    3. visceral mass.

    4. mantle.

  17. The type of mollusc that has tentacles is a

    1. gastropod.

    2. bivalve.

    3. univalve.

    4. cephalopod.

  18. A feature of annelids is

    1. a segmented body.

    2. acoelomate.

    3. a sac body plan.

    4. radial symmetry.

  19. Which of the following is not a feature of an insect?

    1. compound eyes

    2. eight legs

    3. antennae

    4. an exoskeleton

    5. jointed legs

  20. Which of the following undergoes metamorphosis?

    1. insects

    2. mammals

    3. amphibians

    4. reptiles

    5. Both a and c are correct.

  21. Which of the following is not an arachnid?

    1. spider

    2. tick

    3. mite

    4. scorpion

    5. beetle

  22. Unlike bony fishes, amphibians have

    1. ears.

    2. jaws.

    3. a circulatory system.

    4. a heart.

  23. Which of the following is not an adaptation for flight in birds?

    1. air sacs

    2. modified forelimbs

    3. bones with air cavities

    4. acute vision

    5. well-developed bladder

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  25. Examples of monotremes include the

    1. spiny anteater and duckbill platypus.

    2. opossum and koala.

    3. badger and skunk.

    4. porcupine and armadillo.

  26. Mammals are distinguished based on

    1. size and hair type.

    2. mode of reproduction.

    3. number of limbs and method of caring for young.

    4. number of mammary glands and number of offspring.

  27. The first humanlike feature to evolve in the hominins was

    1. a large brain.

    2. massive jaws.

    3. a slender body.

    4. bipedal locomotion.

  28. In H. habilis, enlargement of the portions of the brain associated with speech probably led to

    1. cooperative hunting.

    2. the sharing of food.

    3. the development of culture.

    4. All of these are correct.

  29. Mitochondrial DNA data support which hypothesis for the evolution of humans?

    1. multiregional continuity

    2. replacement model (out-of-Africa)

  30. Which of the following is an anthropoid, but not a hominid?

    1. human

    2. gibbon

    3. orangutan

    4. chimpanzee

    5. gorilla

  31. Which of the following does not have cephalization?

    1. sponge

    2. planarian

    3. frog

    4. snail

    5. scorpion

  32. Which of the following is endothermic?

    1. roundworm

    2. octopus

    3. eagle

    4. opossum

    5. Both c and d are correct.

  33. Which of the following is a bilaterally symmetrical, hermaphroditic acoelomate?

    1. planarian

    2. sponge

    3. jellyfish

    4. earthworm

    5. sea star

Thinking Scientifically
  1. Recently, three fossil skulls of Homo sapiens, dating to about 160,000 years BP, were discovered in eastern Africa. These skulls fill a gap between the 100,000-year-old H. sapiens skulls found in Africa and Israel and 500,000-year-old skulls of archaic H. sapiens found in Ethiopia. Supporters of the replacement model (out-of-Africa hypothesis) argue that this discovery strengthens their position by documenting the succession of human ancestors from 6 MYA through this most recent group. Does this finding prove that the replacement model is correct? If not, what fossil evidence might yet be found to support the multiregional continuity hypothesis?

  2. Think of the animals in this chapter that are radially symmetrical (cnidarians, many adult echinoderms). How is their lifestyle different from that of bilaterally symmetrical animals? How does their body plan complement their lifestyle?

Bioethical Issue
Use of Animals in Research

People who approve of laboratory research involving animals point out that even today it would be difficult to develop new vaccines and medicines against infectious diseases, new surgical techniques for saving human lives, or new treatments for spinal cord injuries without the use of animals. Even so, most scientists favor what are now called the “three Rs”: (1) replacement of animals by in vitro, or test-tube, methods whenever possible; (2) reduction of the number of animals used in experiments; and (3) refinement of experiments to cause less suffering to animals.

F. Barbara Orlans of the Kennedy Institute of Ethics at Georgetown University says, “It is possible to be both pro research and pro reform.” She feels that animal activists need to accept that sometimes animal research is beneficial to humans, and all scientists need to consider the ethical dilemmas that arise when animals are used for laboratory research. Do you approve of this compromise?

Essentials of Biology Website

The companion website for Essentials of Biology provides a wealth of information organized and integrated by chapter. You will find practice tests, animations, videos, and much more that will complement your learning and understanding of general biology.