CHAPTER SUMMARY
44.1 THE ANIMAL TREE OF LIFE CONSISTS OF MORE THAN A MILLION SPECIES.
- Sponges lie on the deepest branch of the animal tree of life.
- All animals other than sponges are eumetazoans and fall into four major groups: cnidarians, ctenophores, placozoans, and bilaterians.
- Early biologists grouped animals on the basis of shared features of adult bodies, such as symmetry and type of body cavity.
- Bilaterians are characterized by bilateral symmetry, three germ layers, and well-developed organ systems. They include insects, worms, and mammals, and can be grouped as protostomes and deuterostomes.
- Embryology provided additional information on animal phylogeny, and, in the age of molecular biology, molecular sequence comparisons have provided still more information.
44.2 THE EARLY BRANCHES OF THE ANIMAL TREE INCLUDE SPONGES, CNIDARIANS, CTENOPHORES, AND PLACOZOANS.
- Sponges have a simple anatomical organization, feed by drawing water containing dissolved food particles into their interiors, and are widespread in the oceans.
- Cnidarians form true epithelial tissue, differentiate muscle and nerve cells, and act as predators by means of specialized cells called nematocysts.
- Ctenophores, or comb-jellies, resemble cnidarians but move by the beating of cilia, have an anal pore for waste excretion, and have a rudimentary mesoderm germ layer. They remain a difficult limb to locate on the animal tree of life.
- Placozoans have a very simple organization, but genomic data support the hypothesis that they are the sister group to all other eumetazoans.
44.3 BILATERIANS, INCLUDING PROTOSTOMES AND DEUTEROSTOMES, HAVE BILATERAL SYMMETRY AND DEVELOP FROM THREE GERM LAYERS.
- Protostomes are divided into lophotrochozoans and ecdysozoans.
- Lophotrochozoans consist of 17 phyla, including annelid worms and mollusks.
- Annelid worms include earthworms and leeches, but most species of annelids live in the oceans.
- Mollusks include gastropods (snails, slugs), cephalopods (squid, octopus), and bivalve mollusks (clams, oysters).
- Ecdysozoans include several phyla that molt their cuticle, notably the arthropods.
- Arthropods can be divided into four main groups—insects, chelicerates (spiders, scorpions), myriapods (centipedes, millipedes), and crustaceans (lobsters, shrimp).
- Deuterostomes include three main phyla—Chordata, Hemichordata, and Echinodermata.
- Well-known echinoderms are sea stars and sea lilies, which have outward fivefold symmetry.
- Chordates include vertebrates, cephalochordates, and tunicates.
44.4 VERTEBRATES HAVE A CRANIUM AND VERTEBRAL COLUMN, AND INCLUDE FISH, REPTILES, BIRDS, AND MAMMALS.
- The group commonly known as “fish” consists of four distinct groups of aquatic vertebrates: hagfish and lampreys, cartilaginous fish, bony fish, and fleshy-finned fish (lungfish).
- Lungfish are the closest relatives of tetrapods, which include amphibians, lizards, turtles, crocodilians, birds, and mammals.
- Amphibians have an aquatic larval form and a terrestrial adult form.
- Amniotes, such as lizards, snakes, crocodiles, birds, and mammals, have an amniotic egg, permitting movement into dry, terrestrial habitats.
- Mammals are covered with hair and feed their young milk from mammary glands.
44.5 ANIMALS EVOLVED MORE THAN 575 MILLION YEARS AGO IN THE OCEANS, AND BY 500 MILLION YEARS AGO THE MAJOR STRUCTURAL AND FUNCTIONAL BODY PLANS OF ANIMAL PHYLA WERE IN PLACE.
- Ediacaran fossils from 575 million years ago provide evidence of early animals.
- The Cambrian explosion was an interval of rapid diversification beginning 542 million years ago, during which time most of the animal body plans we see today first evolved.
- Chelicerates and insects were the first animals to colonize the land, sometime after 420 million years ago.
- Tetrapods first appear in the fossil record about 360 million years ago.
- Mammals originated at least 210 million years ago, but became dominant only after the extinction of the non-avian dinosaurs 65 million years ago.
- The fossil record of reef ecosystems reflects both evolutionary innovation and mass extinction.
Self-Assessment Question 1
Draw a simplified animal tree of life, indicating the relationships among sponges and the four other major groups of animals.
Show Model Answer
Model Answer:
Self-Assessment Question 2
Describe what kinds of trait are used to classify animals.
Show Model Answer
Model Answer:
Several criteria can be used to classify animals, including morphological characteristics and molecular sequences. Symmetry (radial vs. bilaterial) and blastopore fate (deuterostomes vs. protostomes) are two traits used to classify animals at a large scale. More specific phyla are often characterized by distinct traits. For example, these include: (1) the appearance of a mantle in mollusks; (2) jointed legs and a chitin skeleton for arthropods; and (3) a cranium and jointed skeleton in vertebrates. Comparisons of molecular sequences for specific proteins can help to solidify relationships between organisms, and facilitate groupings into phyla.
Self-Assessment Question 3
Compare and contrast the anatomical organization and feeding patterns of sponges, cnidarians, comb-jellies, and placozoans.
Show Model Answer
Model Answer:
In a sense, sponges, cnidarians, ctenophores (comb-jellies), and placozoans are all composed of three different layers of tissue or fluid with a specific organization. However, the organization of these layers and the associated feeding behaviors of these four organisms differ greatly.
Placozoans are formed by two layers of tissue separated by a third layer of fluid. These animals do not possess a distinct cavity (or mouth) into which food or water is drawn. Rather, these animals envelop a food source and feed by endocytosis performed by individual cells.
Sponges are also made up of three distinct layers: a porous outer layer of tissue, followed by a layer of mesohyl, and finally by an innermost layer composed of (in part) choanocytes. Unlike placozoans, sponges do possess a central cavity into which food-rich water is funneled. The cells within this central cavity, rather than secreting digestive enzyme into this environment, internalize food particles via endocytosis.
Cnidarians are similarly composed of three different layers: an outer epidermis, a middle mesoglea layer, and an innermost endoderm that lines a distinct gastric cavity. Unlike sponges and/or placozoans, cnidarians demonstrate radial symmetry and possess a distinctive mouth into which food is drawn by tentacles. Interestingly, the cnidarian mouth acts as both the entryway for food and exit for waste. Once through the mouth, food enters the gastric cavity and is digested by enzymes.
Comb-jellies or ctenophores, similar to cnidarians, possess an epithelium, a “gelatinous” middle layer, and an internalized endoderm. These animals are also radially symmetric. However, in contrast to cnidarians, comb-jellies possess both a mouth and anal pore; waste doesn’t exit from the same hole that food enters.
Self-Assessment Question 4
Describe feeding habits that do not require a head.
Show Model Answer
Model Answer:
Many feeding methods don’t require a distinct head or mouth, and instead make use of endocytosis. Sponges don’t possess definitive heads, but funnel food-rich water into a centralized cavity. Cells within this cavity subsequently take up food particles via endocytosis. Placozoans also do not possess heads. To feed, these organisms envelop a food source and release digestive enzymes. Through endocytosis, single placozoan cells will then take up digested material.
Self-Assessment Question 5
Name three features that account at least in part for the success of insects.
Show Model Answer
Model Answer:
Insects possess several unique traits, many of which are shared with other arthropods. These include jointed legs, a chitin exoskeleton, and eggs that can resist desiccation. Additional characteristics of some insects include the process of metamorphosis and the development of wings. Collectively, these traits have allowed insects to colonize various environments, and have made them remarkably successful organisms.
Self-Assessment Question 6
Draw a phylogeny of deuterostomes, indicating the most likely relationships among chordates, hemichordates, and echinoderms, with common examples of each group.
Show Model Answer
Model Answer:
Self-Assessment Question 7
List features common to fish, amphibians, amniotes, and mammals.
Show Model Answer
Model Answer:
Amphibians, amniotes and mammals all fall under the broader category of vertebrates. Being vertebrates, all of these organisms demonstrate a jointed and mineralized skeleton, a cranium-encased brain, and a vertebral column. Animals classified as amniotes, amphibians, and mammals demonstrate additional characteristics unique to the phyla. For example, amniotes—much like insects—are characterized by desiccation-resistant eggs. Mammals are classified by the appearance of mammary glands and hair, whereas amphibians demonstrate a gill-based larval stage and lung-focused adult stage.
Self-Assessment Question 8
Describe the evolutionary significance of the Cambrian explosion.
Show Model Answer
Model Answer:
The Cambrian Explosion holds great evolutionary significance, as it brought with it the development of a diverse array of novel animal body plans (e.g., the combination of new characters in a single animal) reminiscent of those seen in modern-day organisms. In earlier Ediacaran fossils, such familiar characteristics are not observed.