Anatomy of Animals


Contents

Animals
Sponges
Cnidarians
Flatworms
Roundworms
Mollusks
Annelids
Arthropods
Echinoderms
Chordates - Fishes, Amphibians, Reptiles, Birds, Mammals, Primates

Animals

All animals digest their food, carry on gas exchange, excrete waste, circulate nutrient and waste products to and from the cells, coordinate their movements, protect themselves, and reproduce and disperse the species. The more complex animals have organ systems to carry out these functions; in simple animals, these functions sometimes are carried out by specialized tissues. All these functions can be found in the Human Organ Systems as shown in Table 10-01, Topic 10. Although endocrine system does not exist in invertebrates, neurosecretory systems are widespread. Such systems probably exist in all phyla. Work to date has demonstrated the important role of neurosecretion in growth and reproduction of many model systems.
Family Tree All phyla of animals had evolved by the beginning of the Paleozoic Era some 540 million years ago. The evolutionary tree of animals in Figure 01 indicates that animals are the descendants of protozoans - perhaps in a colonial form whose cells differentiated into various types of cells. The evolutionary tree of animals resembles a tree with 2 main branches. The animal phyla located on the main trunk of the tree are referred to as the primitive invertebrates, and the animals of the main 2 branches include the advanced invertebrates and the vertebrates. Invertebrates lack a dorsal backbone, while vertebrates have a backbone made up of vertebras. A study of the evolution of animals reveals that the most complex animals have the most advanced features as listed in Table 01 below. Classification of animals therefore is based on type of body plan, symmetry, number of

Figure 01 Evolutionary Tree of Animals [view large image]

germ layers, level of organization, type of body cavity, and presence or absence of segmentation.

Features Most Primitive Primitive Advanced Most Advanced
Body Plan None Sac plan Tube-within-tube plan + specialization of parts
Symmetry None Radial Bilateral + cephalization (head)
Germ layers None 2 3 3
Level of organization None Tissue Organ Organ system
Body cavity Diploblastic Acoelomate Pseudocoelom True coelom
Segmentation Nonsegmented Nonsegmented Segmented + specialization of parts

Table 01 Animal Features

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Sponges

Sponges are mainly marine animals that make up the phylum Porifera. They are the most primitive of all animals. Comparing gene sequences (with other animals) suggests that they may be very close to the "Animal Eve" called urmetazoan. Tracing the root further back, it is found that the choanoflagellates (the single-celled ancestors) have possessed many of the genes
Sponge Sponge Anatomy necessary for multicellularity already. Their evolutionary steps are clearly demonstrated from single-celled aquatic protists to colonies and then appear as the collar cells in sponges. A sponge does not have anything in their bodies that can be called tissues or organs. Instead each type of sponge cell is responsible for a different activity to keep the sponge alive. Some sponges grow on rocks and are brightly colored. Sponges often are shaped like vases with a central cavity (Figure 02). Figure 03 shows the anatomy of the sponge. The internal structures are described in the followings.

Figure 02 Sponge
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Figure 03 Sponge Anatomy [view large image]

Although the terminologies (and hence the functions) for the various systems are in close parallel to those for the human body, the composition and distribution are vastly different.

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Cnidarians

Cnidarians have radial symmetry, and there is typically a ring of tentacles surrounding the mouth region. Some cnidarians, referred to as hydroids or polyps, have a tubular shape, with the mouth region directed upward. Others, which have a bell
Cnidaria Hydra Anatomy shape with the mouth region directed downward, are called jellyfishes, or medusae. The polyp is adapted to a sessile life, while the medusa is adapted to a floating or free-swimming existence. At one time, both body forms may have been a part of the life cycle of all cnidarians, because today we see an alternation of generations life cycle of these two forms in certain cnidarians, such as members of the genus obelia. In such life cycle, the polyp stage produces medusae, and the medusae,

Figure 04 Cnidaria [view large image]

Figure 05 Hydra Anatomy [view large image]

which produce eggs and sperm, disperse the species. Cnidarians are quite diversified including the Portuguese man-of-war, sea anemones, hydra and many other species.

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Flatworms

Tapeworm Flatworms are nonsegmented, lack a coelom, and have the sac body plan with only one opening. Therefore, if we analyze them according to Table 01, they have a combination of primitive and advanced features. There are three classes of flatworms: one is free living and two are parasitic. The free-living specimen, the planarian, best exemplifies the characteristics of the phylum. Tapeworms and flukes are parasitic with structure reflecting the modifications that occur in parasitic animals. Concomitant with the loss of predation, there is an absence of cephalization; the anterior end notably carries hooks and/or suckers for attachment to the host. The parasite acquires nutrient molecules from the host, and the digestive system is reduced. It is

Figure 06 Tapeworm Life Cycle [view large image]

covered by a specialized body wall resistant to host digestive juices. The extensive development of the reproductive system, with the production of millions of eggs, may be associated with difficulties in dispersing the species. Figure 06 shows the life cycle of the tapeworm.

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Roundworms

Roundworms (phylum Nematoda - meaning thread in Greek), as their name implies, are round rather than flat. They have a smooth outside body wall (called cuticle), indicating that they are nonsegmented. These worms, which are generally colorless and less than 5 cm long, occur almost anywhere and in great variety (15000 known species). There are many thousands of individual nematodes in even a single handful of garden soil. For the simple body plan and genetic sequence as well as the fast life cycle, the C. Elegans become indispensable in the biological research laboratory. Many nematodes are able to suspend their life processes completely when conditions become unfavorable; in these resistant states they can survive extreme dryness, heat, or cold, and then return to life when favorable conditions return. This is known as cryptobiosis - a feature shares
Roundworm Anatomy with the rotifers. Roundworms possess two anatomical features not seen in more primitive animals: a tube-within-a-tube body plan and a body cavity. The body cavity is a pseudocoelom, or a cavity incompletely lined with mesoderm. This fluid-filled pseudocoelom provides space for the development of organs, and serves as a type of skeleton. When roundworms are analyzed according to Table 01, they are seen to have features associated with advanced animals except that they are nonsegmented.

Figure 08 Roundworm Anatomy [view large image]

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Mollusks

Coelom All the advanced invertebrate phyla have a true coelom. Nevertheless, they can be divided into two groups on the basis of embryological evidence (Figure 09). In mollusks, annelids, and arthropods, the coelom forms by splitting of the mesoderm. Therefore, they are called the schizocoelomates. In echinoderms and chordates, the coelom forms by outpocketing of the primitive gut. It is thus called enterocoelomates. Note the interchange of mouth and anus in these two different types of development.

Figure 09 Coelom Formation
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    Mollusks are a very large and diversified group containing many thousands of living and extinct forms. However, all forms of mollusks have a body composed of at three distinct parts:

  1. Visceral mass: the soft-bodied portion that contains internal organs.
  2. Foot: the strong, muscular portion used for locomotion.
  3. Mantle: the membranous or sometimes muscular covering that envelops but does not completely enclose the visceral mass. The mantle cavity is the space between the two folds of the mantle. The mantle may secrete a shell.
In addition to these three parts, many mollusks show cephalization and have a head region with eyes and other sense organs.

The division of the body into distinct areas seems to have allowed diversification to occur because there are many different types of mollusks adapted to various ways of life. The molluskan groups can be distinguished by the modification of their foot. In bivalves, such as clams, the foot is laterally compressed and is sometimes called pelecypods ("hatchet-footed"). In the gastropods ("belly-footed"), such as snail, the foot is ventrally flattened, and the animal moves by muscle contractions that pass along the foot. In cephalopods ("head-footed"), such as octopuses, the foot has evolved into tentacles about the head. The tentacles are used to seize prey.

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Annelids

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Arthropods

Arthropods Segmented animals have repeating units. This has led to specialization of parts over evolutionary time because the different segments could become specialized for different purposes. The ancestral arthropods probably had many segments with a pair of appendages on each segment. These animals belong to the classes Chilopoda (see Figure 15C) and Diplopoda (not shown, 2 segments fused - 2 pairs of legs for each segment). The segments of arthropods today (classes Arachnida and Insecta, Figure 15B and D) are often fused into regions such as the head, thorax, and abdomen. The head and thorax may be fused to form a cephalothorax (class Crustacea, Figure 15A). Since insects comprise one of the largest animal

Figure 15 Arthropods
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groups - both in number of species (about 1 million) and in number of individuals, the grasshopper will be used as the specific example.

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Echinoderms

The echinoderms include only marine animals -- starfishes, sea urchins, sea cucumbers, and sand dollars (see Figure 17). The starfish will be studied as the representative for this group. Their unique feature is the water vascular system, which is used as a means of locomotion. They also have a carbonaceous endoskeleton, whose projecting spines give the phylum its name - "spiny skin" in Greek. The echinoderms seem the most unpromising of all as potential ancestors of the vertebrates. They are
Echinoderms radially symmetrical, in contrast to vertebrates; they have no internal skeleton, no trace of any of the three major chordate characters of notochord, nerve cord, or gill slits, and they have many peculiar and complicated organs of their own. But the embryology sheds an unexpected gleam of light. The early embryo of the echinoderm is a tiny creature, which floats freely in the sea water. Unlike the adult, the larva is bilaterally symmetrical, suggesting that the radial symmetry of the starfish is a secondary affair, assumed when the ancestors of these forms look up a sedentary existence. Then, too, the type of development of certain of the body cavities is identical with that found in the embryos of some primitive vertebrates. It is believed that the bilateral larva developed types which retained the original symmetry,

Figure 17 Echinoderms
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and gradually evolved into the chordates and, finally, the true vertebrates.


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Chordates

    Among the chordates are those animals with which we are most familiar, including human beings. Figure 19a shows all the animal classes with indicators about added features. All members of this phylum are observed to have the following three basic characteristics at some time in their life history.

  1. A dorsal supporting rod called a notochord, which is replaced by the vertebral column in the adult vertebrates.
  2. A dorsal hollow nerve cord, in contrast to invertebrates, which have a ventral solid nerve cord. By hollow, it is meant that the cord contains a canal that is filled with fluid.
  3. Pharangeal pouches or gill clefts (slits), which are seen only during embryological development in most vertebrate groups, although they persist in adult fishes. Water passing into the mouth and the pharynx goes through the gill slits, which are supported by gill bars and used for gas exchange.
The tunicates and lancelets sometimes are called the protochordates (Figure 19b) because they possess all three typical chordate structures in either the larval and/or adult forms, as did the first chordates to evolve. These two groups of animals link the vertebrates to the rest of the invertebrates and show how modestly the chordates most likely began. A tunicate, or sea squirt, appears to be a thick-walled, squat sac with two openings. Inside the central cavity of the animal are numerous gill slits,
Vertebrates Protochordates the only chordate feature retained by the adult. The larva of the tunicate, however, has a tadpole shape and possesses the three chordate characteristics. It has been suggested that such a larva may have become sexually mature without developing the other adult tunicate characteristics, and may have evolved into a fishlike vertebrate similar to the lancelet, which is a chordate that shows the three chordate characteristics as an adult.

Figure 19a Vertebrates
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Figure 19b Protochordates
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Fishes

    There are three classes of fishes (Figure 21):

  1. Jawless fishes - They are cylindrical, up to a meter long, with smooth, scaleless skin and no jaws or paired fins. There are two families of jawless fishes, e.g., the hagfishes are scavengers, feeding mainly on dead fishes, while some lampreys are parasitic.
  2. Cartilaginous fishes - They are the sharks, the rays, and the skates, which have skeletons of cartilage instead of bone.
  3. Bony fishes - They are by far the most numerous and varied of the fishes. Most of these fishes, such the grouper in
    Fishes Figure 21, are a type of bony fish called ray-finned fishes. They have a swim bladder that aids them in changing their depth in the water. Species of fish that do not possess a swim bladder sink to the bottom if they stop swimming. "Ray-finned" refers to the fact that the fins are thin and are supported by bony rays. Another type of bony fish, called the lob-finned fishes, evolved into the amphibians. These fishes not only have fleshy appendages that could be adapted to land locomotion, they also have a lung that is used for respiration. The coelacanth (Figure 21), which exists today, is the only "living fossil" among the fishes.

    Figure 21 Fishes
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    Figure 22 shows the internal anatomy of a common fish.


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Amphibians

The living amphibians include frogs, toads, newts, and salamanders (see Figure 23). The frog will be used for the study of amphibian anatomy (Figure 24) below.

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Reptiles

The reptiles living today are turtles, alligators, snakes, and lizards (see Figure 25). Reptiles with limbs, such as lizards, are able to lift their body off the ground, and the body is covered with hard, horny scales that protect the animal from desication and from predators. Both of these features are adaptations to life on land. The anatomy of lizard is illustrated in Figure 26.

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Birds

Birds Birds are characterized by the presence of feathers, which are actually modified reptilian scales. There are many orders of birds, including birds that are flightless (ostrich), web footed (penguin), divers (loons), fish eaters (pelicans), waders (flamingos), broad billed (ducks), birds of prey (hawks), vegetarians (fowl), shorebirds (sandpipers), nocturnal (owl), small (hummingbirds), and songbirds, the most familiar of the birds. Some of them are showed in Figure 25.

Figure 25 Birds
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Nearly every anatomical feature of a bird can be related to its ability to fly. Figure 26 shows the anatomy of a common bird.

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Mammals

Mammals The chief characteristics of mammals are hair and mammary glands that produce milk to nourish the young. Human mammary glands are called breasts. Mammals are classified according to their means of reproduction: there are egg-laying mammals called monotremes such as the duck-billed platypus; mammals with pouches for immature embryos are the marsupials such as the kangaroos; while the placental mammals are the majority of living mammals. Figure 27 shows just a few of these animals. Table 02 lists the twelve orders of placental mammals. They are classified largely according to the mode of locomotion and how they get their food. Figure 28 illustrates a cat's anatomy,

Figure 27 Mammals
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which is very similar to the human's.


Order Examples Characteristics
Insectivora Moles, shrews Primitive; small, sharp-pointed teeth
Edentata anteaters, armadillos, sloths Primitive terrestrial mammal; few or no teeth; well developed claws
Pholidota Pangolin Medium size; large, plate-like scales; lack teeth, use powerful front claws and long tongues to reach ants or termite
Chiroptera Bats Digits support membranous wings
Carnivora Dogs, bears, cats, sea lions Long canine teeth; pointed teeth
Rodentia Mice, rats, squirrels, beavers, porcupines Incisor teeth grow continuously
Lagomorpha Rabbits, hares, pikas Chisel-like incisors; hind legs longer than front legs; herbivorous
Perissodactyla Horses, zebras, tapirs, rhinoceroses Large, long-legged, one or 3 toes, each with hoof; grinding teeth
Artiodactyla Pigs, camels, buffalos, giraffes Medium to large; 2/4 toes, each with hoof; many with antlers/horns
Cetacea Whales, porpoises Medium to very large; paddlelike forelimbs; hind limbs absent
Primates Lemurs, monkeys, gibbons, chimpanzees, gorillas Mostly tree dwelling; head freely movable on neck; 5 digits, usually with nails; thumbs and/or large toes usually opposable
Subungulate Elephants, sirenians, hyraxes, aardvarks Medium to very large; lack a clavicle, and have nails or hooves instead of claws.

Table 02 The Twelve Orders of Placental Mammals

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Primates

Primates Humans are mammals in the order Primates. The first primates may have resembled today's tree shrews, rat-size animals with a snout, claws, and sharp front teeth. By 50 million years ago, however, primates had evolved characteristics suitable to move freely through the trees. The first primates were prosimians (meaning "premonkeys"). They are represented today by several types of animals, including the lemurs. Monkeys, along with apes and humans, are anthropoids. Monkeys evolved from the prosimians about 38 million years ago, when the weather was warm and vegetation was like that of a tropical rain forest. There are two types of monkeys: the New World monkeys such as the spider monkeys, which have long grasping tails and flat noses, and the Old World monkeys such as the baboons, which are now ground dwellers and lack such tails. Ape (gibbon, gorilla, and chimpanzee) evolved later. The human lineage split from that of the apes occurred about 5 - 10 million years ago in Africa. Figure 29 shows some of the primates. Figure 30 illustrates the chimpanzee anatomy, which is virtually identical to the human's.

Figure 29 Primates
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