Organization of the Animal Body

Organization of the Animal Body

Animals are characteristically multicellular heterotrophs whose cells lack cell walls. At some point during their lives, animals are capable of movement. In the most commonly encountered animals, this stage is the adult, although some animals (corals) have sessile (nonmobile) adult phases and mobile juvenile forms. Animal and plant evolutionary history both show the development of multicellularity and the move from water to land (as well as secondary adaptation back to water).

Animals developed external or internal skeletons to provide support, skin to prevent or lessen water loss, muscles that allowed them to move in search of food, brains and nervous systems for integration of stimuli, and internal digestive systems.

Most animals have a life cycle with a preadult stage, a predominance of the diploid stage, and a series of embryonic developmental stages.
Evolution and Classification of Animals

Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals. Cells in primitive animals (sponges in particular) show similarities to collared choanoflagellates as well as pseudopod-producing amoeboid cells.

Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear nearly 700 million years ago, during the late precambrian time (the part of the Proterozoic era termed the Vendian). All known Vendian animal fossils had soft body parts: no shells or hard (and hence preservable as fossils) parts. Learn more about these early animal fossils at Learning About the Vendian Animals. Animals in numerous phyla appear at (or in many cases before) the beginning of the Cambrian Period, Multicellular protists appeared in the fossil record more than 600 million years ago near the very end of the precambrian. This time is referred to as the Vendian Period (650 to 544 million years ago), and is characterized by the appearance of soft-bodied animal fossils, some of which are. Multicellular animal fossils and burrows (presumably made by unknown, soft-bodied multicellular animals) first appear 700 million years ago, during the Vendian time. All known Proterozoic animal fossils had soft body parts: no shells or hard (and hence preservable as fossils) parts. There are some paleontologists who suspect that the Vendian faunas were reduced by an extinction event, possibly related to massive glaciation, at the close of the vendian time. In any event, many animals in the Vendian assemblages are quite unlike anything living today, while others can be traced to extant phyla.
The Cambrian: Animals with Hard Body Parts

Beginning 570 million years ago, early during the Cambrian time, animals with external skeletons appeared in great abundance. This sudden appearance of fossils was used to define the beginning of the Cambrian (named after Cambria, an ancient name for the country of Wales). External skeletons were hard enough to be more readily preserved, leading to the apparent explosion of animals early in the Cambrian. Soft-bodied animals had dominated early animal evolution during an earlier time just prior to the Cambrian, the Vendian.

Modern animals are classified into between 30 and 35 phyla: all major modern phyla were present at the beginning of the Cambrian, along with a great variety of now-extinct phyla recorded in the Burgess Shale (Cambrian) in Canada. Of the animal phyla, scientists consider nine major invertebrate phyla and the chordates to be of major importance in terms of biological diversity. While all major animal phyla are represented by Cambrian fossils, reconstructing fossil history is extremely difficult since earlier, soft-bodied animals did not fossilize well. Consequently, evolutionary relationships have been established for the most part on a studies of living (referred to as extant) animal anatomy.
Trends in Animal Evolution

Within the animal kingdom several evolutionary trends and advancements are seen. Note that not all animal groups have all of the organs and organ systems found in the "higher animals", a fact consistent with stepwise evolutionary history . Nor will their body plans necessarily conform to ours.
Body Plans

Most animals have a body plan best described as a "tube-within-a-tube". This plan calls for two openings: one for food to enter the body (mouth), one for wastes to leave the body (anus). The tube-within-a-tube plan allows specialization of parts along the tube, such as a stomach, intestine, etc. The sac-like body plan has only one opening for both food intake and waste removal. Sac-like body plan animals do not have tissue specialization or development of organs. Animals with the "tube-within-a-tube" plan are 10% more efficient at digesting and absorbing their food than animals with the sac-like body plan.
Triploblasty: Three Tissue Layers

Many, but not all, animals produce three embryonic tissue tissue layers (shown in Table 1) as they develop: the endoderm, mesoderm, and ectoderm. Some animals, most notably sponges, lack these tissue layers. Cnidarians (a group including coral and jellyfish) have only two of these layers, and are termed diploblastic. Flatworms, ribbon worms, humans, etc. have all three tissue layers, and are triploblastic.
Asymmetry and Symmetry

Asymmetrical animals (sponges, shown in .e 4) have no general body plan or axis of symmetry that divides the body into mirror-image halves. Within the animal kingdom this appears to be a primitive condition. More advanced animals have symmetry. Radially symmetrical animals (such as coral and jelly fish, .e 4) have body parts organized about a central axis, like the spokes in a bicycle wheel, with multiple planes of symmetry. Radially symmetrical animals are often, for some part of their life, nonmotile (termed in animals as sessile), and live attached to a substrate. Radial symmetry allows animals, such as jellyfish, corals, and sea anemones, to reach out in all directions from one central point. Bilaterally symmetrical animals (such as humans, .e 3) have only a single plane of symmetry that produces mirror halves. Bilaterally symmetrical animals tend to be active and to move forward at an anterior end, which eventually led to concentration of sensory organs in the anterior end, or head (a trend known as cephalization).
Body Cavity and Development

Acoelomate animals (like flatworms and flukes,) do not have a coelom (or body cavity) produced during preadult development. Pseudocoelomate animals (such as roundworms) have a body cavity but it does not develop from splitting of the mesoderm embryonic tissue layer.Coelomate
animals (humans, fish, shrimp,) have a body cavity lined with mesoderm cells.
Protostome and Deuterostome

Coelomates fall into either protostomes or deuterostomes, depending on how their embryos develop, Protostomes (from the Greek meaning literally "first mouth") are coelomates whose embryonic development makes a blastopore (the first opening in the blastula) that later develops into a mouth. Deuterostomes ("second mouth") are coelomates whose embryonic development produces a blastopore that later forms an anus, with a second opening forming the mouth (hence the designation of "second mouth"). Vertebrates are deuterostomes.
Segmented Bodies

Some animals have their bodies divided into segments, Segmentation allows them to specialize certain segments, such as for antennae, eyes, claws, etc. Humans, insects, and earthworms are examples of segmented animals. The systematic value of segmentation has been downplayed, with most specialists favoring segmentation arising from convergent evolution. However, the genes controlling segmentation in each of these groups are the same, leading to a rethinking of the taxonomic value of segmentation.
Sponges: The Phylum Porifera

The phylum Porifera ("pore-bearing") consists of approximately 5,000 species of sponges. These asymmetrical animals have sac-like bodies that lack tissues, and are usually interpreted as representing the cellular level of evolution. Cells from fragmented sponges can reorganize/regenerate the sponge organism, something not possible with animals that have tissues. Most zoologists consider sponges as offshoots that represent an evolutionary dead-end., although others consider some groups of sponges as being related to other animal groups. Sponges are aquatic, largely marine, animals with a great diversity in size, shape, and color.

Modern sponges greatly resemble some fossil Cambrian sponges. Sponges may have evolved from a colonial protozoan, There are no true tissues in sponges: merely specialized cell layers. Epidermal cells in sponges line the outer surface. Collar cells line the inner cavity. Beating collar cells produce water currents that flow through pores in sponge wall into a central cavity and out through an osculum, the upper opening. A 10 cm tall sponge will filter as much as 100 liters of water a day. Amoeboid cells occupy the "inner" layer, along with hardened structures known as spicules.

Sponges feed by drawing water into the body through a network of pores (hence the name porifera, pore-bearer) and passing it out through the large opening (osculum) at one end of the body.

Sponges can reproduce asexually (by budding or from fragments) or sexually. Sponges produce eggs and sperm that are released into a central cavity of the sponge, in which the zygote develops into a ciliated larva. The larval stage is able to move about while the adult is stationary.

The fossil record of sponges has been at times quite good. The oldest sponges date from the precambrian. One early example of fossil sponges are the archaeocyathids, one of the first reef-building animals. Archaeocyathids evolved and went extinct before then end of the Cambrian Period. Cladistic analysis by J. Reitner in 1990 suggests archaeocyathids are properly placed in the Phylum Porifera instead of having their own phylum. Living sponges fall into three groups: the calcareous (an example of which is), glass, and demosponges, based on the chemical composition of spicules.
Tissues: Jellyfish, Corals and Sea Anemones

The phylum Cnidaria contains 10,000 species characterized by adult bodies having radial symmetry. Cnidarians are aquatic, mostly all marine. The cnidarian body has only the ectoderm and endoderm tissue layers, making this group diploblastic. Members of this phylum all have stinging cells that eject a barbed thread and possibly a toxin. Only cnidaria have these cnidocytes a specialized cell that contains a nematocyst, a fluid-filled capsule containing a long, spirally coiled hollow thread. When the trigger of the cnidocyte is touched, the nematocyst is discharged. Some threads merely trap a prey or predator, while others have spines that penetrate and inject paralyzing toxins. These toxins make some jellyfish (and a related group the box jellies) among the most poisonous of animals.

Cnidarians have two body forms that may occur: a mobile medusa and a sessile (fancy term for not mobile) polyp, both of which are shown in .e 10. Both body forms have tentacles arranged around an opening into the two-layered sac-like body. The inner tissue layer (derived from endoderm) secretes digestive juices into the gastrovascular cavity, which digests food and circulates nutrients (doing the job our circulatory AND digestive systems do). Muscle fibers occur at the base of the epidermal and gastrodermal cells, making this the first group of muscled animals. Nerve cells located below epidermis near the mesoglea interconnect and form a nerve net throughout the body. Cnidarians have both muscle fibers and nerve fibers, making these animals capable of directional movement. The nerve net allows transmission of messages in more than one direction, possibly an advantage in a radially symmetrical animal, while contraction of muscle fibers (under control of the nerve fibers) allows for movement. While they have a nerve net, brains are not present.

Cells are organized into tissues. The adult in most species of cnidarian is radially symmetrical. The typical cnidarian life cycle involves both sexual and asexual reproduction. A bilaterally symmetrical larva known as a planula , develops from a zygote. The planula moves around and eventually settles down in an appropriate location and grows into the adult polyp. The polyp grows and may eventually reproduce asexually to form medusae. Each medusa develops gonads and uses meiosis to form gametes.

The phylum Cnidaria is usually subdivided into three taxonomic classes: class Anthozoa, class Hydrozoa, and class Schyphozoa.
Class Anthozoa: Sea Anemones and Corals

Sea anemones, are solitary polyps 5-100 mm in height and 5-200 mm in diameter or larger. They are often brightly colored and look like flowers (specifically anemones) on the seafloor. You might remember them from the film Finding Nemo. The anemone's thick, heavy body rests on a pedal disk and supports an upward-turned mouth surrounded by hollow tentacles. Sea anemones feed on various invertebrates and fish. They attach to a variety of substrates, or may be mutualistic with hermit crabs, living attached to crab's shell.

Corals may be solitary but most today are colonial. The majority of corals occur in warm shallow waters; the accumulation of their calcium-carbonate remains builds reefs. Some corals occur in colder waters, so the mere presence of coral does not necessarily indicate a tropical environment. Modern scleractinian coral, dominant reef-builders since the Triassic period (some 230 million years ago), have symbiotic photosynthetic dinoflagellates living within the coral body. These dinoflagellates are in the genus Symbiodinium, and are termed collectively zooxanthellae, shown in .e 13a. .e 13b illustrates several living coral tyypes.
Class Hydrozoa: Hydras and Portuguese Man-of-war (Physalia)

The polyp stage is dominant in members of this taxonomic class. The Portuguese man-of-war is a colony of polyps, with the original polyp becoming a gas-filled float. Other polyps specialize for feeding or reproduction. The Portuguese man-of-war can cause serious injury to swimmers since each tentacle (in reality a string of individual organisms) has numerous nematocysts.

Hydra, are solitary, freshwater hydrozoan polyps. The body is a small tube about one-quarter inch long, best observed with a dissecting microscope. Four to six tentacles surround the mouth, the only opening at into the body. Hydra can move from one location by gliding or even somersaulting. Hydras have both muscular and nerve fibers, and respond to touch. Epidermal cells are termed epitheliomuscular cells and contain muscle fibers. Cnidocytes and sensory cells are also present in the epidermis. Interstitial cells can produce an ovary or testis, and may assist regeneration. Gland cells secrete digestive juices into the gastrovascular cavity into which tentacles have stuffed captured prey. Digestion is completed within food vacuoles of nutritive-muscular cells. Nutrients diffuse to the rest of the body. Hydras reproduce both sexually asexually (by budding). In sexual reproduction, sperm from a testis swim to an egg within an ovary. Following early development within the ovary, a protective shell forms and allows the embryo to survive until conditions are optimum. Hydra are commonly utilized animals in biology labs.
Class Schyphozoa: True Jellyfishes (Aurelia)

The medusal stage is dominant in jellyfish and other members of this taxonomic class. The polyp remains small and inconspicuous. Jellyfishes also serve as food for larger marine animals.
The Fossil Record of Cnidarians

The fossil record of cnidarians is very good for hard-part containing corals, but usually not as good for soft-bodied forms like jellyfish. Corals become dominant reef-building animals during the Ordovician, and continue their importance today. Corals, which had appeared possibly as early as the late Proterozoic (precambrian, more than 540 million years ago), diversified into a number of groups during the Silurian times. Tabulate corals and rugose corals were major components of the new, larger reefs built during the Silurian through Permian (the Permian ended 250 million years ago). Rugose corals included the horn corals, while tabulate corals were colonial. Both the rugose and tabulate corals went extinct at the close of the Permian period. .e 16 shows a coral collected in central Arizona.

Near the end of the Devonian a mass extinction occurred. This one was more severe on marine creatures than on the newly established terrestrial forms. The corals were quite seriously decimated, and the return of extensive reef building did not occur until the Triassic with the evolution of a new group of reef-building corals, the scleractinians (shown in .e 17).

Corals were much restricted after the Devonian crisis and the large reefs of the Devonian were replaced with smaller reefs known as patch reefs. The role of corals in these new reefs was much reduced from what it had been in earlier times.

Coral reefs, which had been decimated by the Carboniferous extinction returned to prominence with the evolution of new groups of reef-building animals and algae.
Bilateral Symmetry and Cephalization: Phylum Platyhelminthes

The phylum Platyhelminthes contains about 13,000 species of flatworms subdivided into three classes: two parasitic and one free-living. The planaria and relatives are freshwater animals placed in the class Turbellaria. Flukes are external or internal parasites belonging to the the Class Trematoda. Tapeworms are internal parasites and form the Class Cestoda. The phylum as a whole has adult bilateral symmetry and cephalization (the development of a head with sensory organs, in most members).

Flatworms have three tissue layers: ectoderm, mesoderm and endoderm and a body plan that is acoelomate and sac-like with a single opening. The mesoderm layer gives rise to muscles and reproductive organs. Free living forms have muscles, a nerve cord, and digestive organs, but lack both the respiratory and circulatory systems common to the so-called "higher" animals (in other words like ourselves). Flatworms, have a branched gastrovascular cavity that is the site of extracellular digestion and which distributes nutrients throughout the body. Gas exchange occurs by diffusion through the skin. Platyhelminths have an excretory system that also functions as an osmotic-regulating system. Flatworms have a ladder-style nervous system composed of paired ganglia that form a brain connected via nerve cells to sensory cells in the body wall.

Parasitic members of this phylum, such as flukes and tapeworms, are characterized by these modifications:

1. loss of cephalization producing a head bearing hooks and suckers to attach to the host as opposed to the sensory organs of free-living forms
2. extensive development of the reproductive system coinciding with the loss of other systems (what do they do but gain food from the host's digestion and reproduce, anyway?)
3. lack of a well-developed nervous and gastrovascular system (the live in a fairly stable environment and the host has already digested their food)
4. development of a tegument that protects them from host digestive juices

Both flukes and tapeworms use secondary or intermediate hosts to transport the species from primary host to primary host. The primary host is infected with the sexually mature adult while the secondary host contains the larval stage(s).
Class Turbellaria

The class Turbellaria includes freshwater planaria such as Dugesia that feed on small organisms or the remains of small creatures, as well as often colorful marine forms. Their small size and ease of care also make Dugesia a common animals in introductory biology labs. The planarian head is normally arrow-shaped, with side extensions that are sensory organs for detection of food and the presence of other organisms. Flatworms have two light-sensitive eyespots that have pigmentation making them look cross-eyed. The presence of three muscle layers facilitates varied movement. Gland cells secrete a mucous material upon which the animal slides or glides.

The animal captures food by wrapping itself around its prey, entangling it in slime, and pinning it down. The pharynx is a muscular tube that extrudes from the mouth and through which food is ingested. Often in biology labs the "prey" can be liver and students can watch the pharynx extend out of the worm's body.

The flame cell system functions in excretion and consists of a series of interconnecting canals that run length of the body on either side of the longitudinal axis and side branches of the canals, each ending in a flame cell. The flame cell is a bulb-shaped cell containing a tuft of cilia within the hollow interior of the bulb. The cilia move back and forth, bringing water into the canals that empty through pores at the body surface. This flame-cell system functions in both water excretion and osmotic regulation in a typical freshwater, free-living flatworm.

Planaria can reproduce both sexually and asexually. They can constrict beneath the pharynx and each half will grow into a whole animal by the process of regeneration. Planaria are hermaphroditic, possessing both male and female sex organs, and can cross-fertilize each other. Fertilized eggs are enclosed in a cocoon and hatch in two to three weeks.
Class Trematoda

The class Trematoda includes flukes. Flukes, such as blood, liver, and lung flukes are named after the organs they inhabit. Fluke bodies tend to be oval and elongate. They lack a definite head but have an oral sucker surrounded by sensory papillae. Flukes have reduced digestive, nervous, and excretory systems. Reproductive systems are well developed and usually hermaphroditic.

The blood fluke, shown in . causes schistosomiasis, a disease found predominantly in tropical Africa and South America. Unlike most flukes, blood flukes are male or female. Flukes deposit eggs in blood vessels around the host's intestine. The eggs migrate to the intestine and are passed out with feces. Larvae hatch in water and swim about until they detect and enter a particular species of snail. The larvae reproduce asexually and eventually leave the snail. Once larvae penetrate human skin they begin to mature in the liver, and implant in blood vessels of the small intestine. A weakened person is then more likely to die from secondary diseases.

The Chinese liver fluke, , requires two intermediate hosts. Humans become infected when they eat uncooked fish. Adults migrate to the liver and deposit eggs in the bile duct, which carries the eggs to the intestine. The larval flukes must then pass through two intermediate hosts, a snail and a fish.
Class Cestoda

The class Cestoda consists of the tapeworms, an example of which is. The tapeworm scolex (head/neck region) has hooks and suckers that allow the organism to attach to the host's intestinal wall. Behind the head is a short neck and then a long string of proglottids. Each proglottid segment contains a full set of both male and female sex organs and very little other structure. Since the animal does produce wastes, it retains its excretory canals, but no digestive system is needed. Being sessile organisms in a fairly stable environment, tapeworms have only rudiments of nerves. Following fertilization, proglottids become a bag of eggs that when mature, breaks off and passes out with feces. If the eggs of tapeworms are ingested by pigs or cattle, the larvae become encysted in the muscle of the hosts. The covering of ingested eggs is digested away and the larvae burrow through the intestinal wall and travel by bloodstream to lodge and encyst in muscle; a cyst is a hard-walled structure sheltering a larval worm. If humans eat the meat of infected pigs or cattle and fail to cook it properly, they too become infected.
The Phylum Nemertea: Ribbon Worms |

The phylum Nemertea include approximately 650 species of marine ribbon worms. Ribbon worms have a distinctive eversible proboscis stored in a rhynchocoel. When the walls of the rhynchocoel contract, the proboscis extends out of the body. The proboscis is a long, hollow tube that can be everted and shot outward through a pore located just above the mouth. It is used primarily for prey capture, and for defense, locomotion, and burrowing. This phylum is included as the organisms are also triploblastic. Several of the fossils from the Cambrian-aged Burgess Shale are interpreted as ribbon worms, and some extremely long worms have been found in the Mediterranean (up to 100 feet long) and in the ocean under the Antarctic ice shelves.
The Phylum Rotifera

Rotifers (shown in .e 23) belong to the phylum Rotifera, which contains about 2,000 species. Rotifers are often observed in biology lab preparations. Their digestive tract is the inner tube and the rest of the animal is the outer tube (of a tube-within-a-tube body plan). Rotifers are microscopic and abundant in freshwater. A crown of cilia (corona) forms a rotating wheel that serves as both an organ of locomotion and acts to deliver food to the mouth.


The Tube-within-a-tube Body Plan: Phylum Nematoda

The phylum Nematoda consists of several hundred thousand species of roundworms,. Most are free-living, although some are parasitic (pinworms are thought to infect 30% of all US children). Adult nematodes have a pseudocoelom (tube-within-a-tube), a closed fluid-filled space that acts as a hydrostatic skeleton, aids in circulation and dispersal of nutrients. Nematodes lack a circulatory system, but do have a well developed digestive system.

One nematode, Caenorhabditis elegans, has only one thousand genes in its genome and its developmental pathways are well known. C. elegans serves as a model for eukaryote gene systems and has been extensively studied as part of the human genome project.

Ascaris is a parasitic roundworm, and is. These worms are unsegmented and have a smooth outside wall. They move by whiplike motions. Mating produces eggs that mature in the soil, limiting most roundworms to to warmer climates. When eggs are swallowed, larvae burrow through the intestinal wall, moving to the liver, heart and/or lungs. Once within the lungs, larvae molt and, after ten days, migrate up the windpipe to throat where they are swallowed. In the intestine, the mature worms mate and the female deposits eggs that are passed out of the body with feces. Feces must reach the mouth of the next host to complete the life cycle, thus, proper sanitation is an important aspect to prevent infection.

Humans contract Trichinella (the roundworm that causes the disease trichinosis, illustrated) by eating raw pork containing encysted larvae. Mature female adults burrow into the wall of the small intestine. Live offspring are carried by the bloodstream to the skeletal muscles where they encyst. Religious dietary injunctions against eating pork may in part be a reflection of the prevalence of this disease in the Middle East.

Filarial worms cause various diseases. Dirofilaria, showcauses heartworm in dogs, and is a common filarial worm in temperate zones.

Elephantiasis, a disease associated with tropical Africa, and is also caused by a filarial worm that utilizes mosquitos as secondary hosts. Adult worms reside in and block lymphatic vessels. This results in limbs of an infected individual swelling to monstrous size. Elephantiasis is treatable in its early stages but not after scar tissue has blocked lymphatic vessels.

Referenced Articles:

Reitner, J. 1990. Polyphyletic origin of the "Sphinctozoans". In Rutzler, K. (ed.), New Perspectives in Sponge Biology, Proceedings of the Third International Conference on the Biology of Sponges (Woods Hole). pp. 33-42. Smithsonian Institution Press, Washington, DC.

All text contents ©1995, 1999, 2000, 2001, 2003, 2004, 2007 by M.J. Farabee. Use of the text for educational purposes is encouraged.

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