Johnson discusses the following 9 key evolutionary characteristics (each exemplified by a phyla) that show the progressive development of animal body architecture and animal evolution in general. These phyla and their evolutionary characteristics will be considered here:

PhylumPorifera

Key evolutionary advance : Multicellularity. The body of a sponge consists of several different types of cells whose activities are coordinated with each other. These cells, however, are not arranged into tissues or organs.

Other characteristics of sponges: Sponges consists of specialized cells embedded in a gel-like matrix. Tiny holes or pores perforate the body of the sponge and along with the flagellated cells called choanocytes (which comprise the innermost layer of cells), allow the sponge to act as filter feeders. Sponges anchor themselves to the seafloor and the beating of the flagella of the choanocytes draws water in through the pores of the body cavity. Any food particles in the water are trapped and later ingested by the choanocytes or other cells. It should be noted that some of the cells of sponges consists of tough protein called spongin which strengthen the sponge; these spongin cells are what comprise our bath sponges.

Phylum Cnidaria

Key evolutionary advances: Symmetry and tissues. The cells are organized into specialized tissues. Cnidarians are also radially symmetrical. In radial symmetry, the parts are arranged around a central axis so that any plane passing through the axis divides the organisms into halves that are approximate mirror images. (This is distinguished from bilateral symmetry discussed below).

Other characteristics of Cnidarians: As noted above cnidarians consist of two phyla-Cnidaria and Ctenophora-which are the most primitive animals to exhibit radially symmetry and tissues. The cell layers of the embryo differentiate into the tissues of the body. The Cnidarians possess an ectoderm and endoderm but no mesoderm so they have a relatively simple body construction. The ectoderm gives rise to the outer coverings of the body and the primitive nervous system and the endoderm gives rise to the digestive tissues.

Cnidarians are carnivores and feed upon animals such as fish and shellfish. They capture animals with stinging cells called cnidocytes and tentacles that form a ring around their mouth. Cnidocysts contain nematocysts which are harpoons used to spear the prey and then draw the harpoon prey back to the cnidocyte.

Phylum Platyhelminthes

Key evolutionary advances: Internal organs and bilateral symmetry. An organ is a collection of different tissues that function as a unit. Bilateral symmetry means that the animal has a right and left half that are mirror images of each other. When the animal is divided into two planes, that are perpendicular to each other, the top half of the animal is called dorsal and the bottom ventral while the front is called anterior and the back posterior. Bilateral symmetry allows animals to become specialized in different ways. Bilaterally symmetrical animals have sensory organs concentrated in the head or front of the animal. The process of having a "head" is called cephalization and allows the animal to detect food or overt danger more effectively.

References

1. Brusca, R. C. and Brusca, G. J.: Invertebrates, 1990. Sinauer Associates, Inc., Sunderland, MA.
2. Johnson, G. B.: The Living World, 2^nd Edition, 2000. McGraw-Hill Co., Inc., New York
3. Pechenik, J. A.: Biology of the Invertebrates, Ed. 3, 1996. McGraw-Hill Companies, Inc., New York.
4. Ruppert, E.E. and Barnes, R. D.: Invertebrate Zoology, 1994. Saunders College Publishing, New York.

These are the simplest animals in which organs occur.

Other Characteristics of Platyhelminthes: Most members of this phylum of flatworms have an anterior brain which is connected to at least one pair of nerve cords. Eyes are present and respond to light. They also have a series of specialized organs called protonephridia (first kidney), which look like cups and contain cilia. The cilia beat the fluid into the cup which is connected to a tubule that connects to an excretory pore on the outside of the animal. Platyhelminthes are hermaphroditic and contain male and female sex organs. Many species may also reproduce asexually by budding or fission. The digestive tract consists of a mouth, larynx, and simple gastrovascular cavity (some species have lateral diverticula which increase the surface area). Digestion starts outside the animal when digestive juices are excreted onto the prey and food particles, then move into the mouth. Since the mouth is the only opening, wastes also leave through this opening. Platyhelminthes do not have specialized respiratory organs or circulatory systems. Oxygen and carbon dioxide diffuse across the body surface.

Free-living and parasitic forms exist. The free-living species are mostly marine although a number are found in fresh water and a few are found in extremely humid terrestrial areas. The aquatic species usually live on the bottom of the ocean, lake, pond or river. Most free-living species are carnivores and feed on protozoa, rotifers, insect larvae, small crustaceans, snails and annelid worms. Some species feed on algae.

Phylum Nematoda (Roundworms)

Key evolutionary advances: Body cavity between endoderm and mesoderm separating the endoderm-lined gut from the rest of the body. Roundworms and rotifers both possess this type of body cavity known as a pseudocoelom. This is distinguished from subsequent animals which will be discussed that contain a coelom, which is a body cavity completely enclosed within the mesoderm. (The advantages of a coelom will be discussed with the mollusks.) Johnson cites three reasons why the pseudocoelom was an important improvement in animal body design:

1) circulation - fluids that move within the body cavity are able to permit the rapid passage of materials from one part of the body to the other;
2) Movement - fluid in the cavity makes the animal body rigid and provides a structure that muscles can push against, thus permitting muscle-driven body movement;
3) Organ function - in a fluid-filled enclosure, the organs are protected and can function without being deformed by surrounding muscle.

Other Characteristics of Nematoda: Nematodes are probably the most numerous multicellular animals alive today. A million nematodes are found in a spadeful of soil or in a square meter of shallow water sediments of both fresh and marine waters. They occur from the polar regions to the tropics in all types of environments. N.A. Cobb, a famous nematologist, stated "If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans represented by a thin film of nematodes.".

Most free-living nematodes are less than 2.5mm in length (most < 1mm). However, some may be microscopic, some soil species as long as 7mm, and some marine species as long as 5mm. They have a perfectly cylindrical body, and are thus known as roundworms. They move using undulations of the body created when they press against particles. Their food source varies ranging from animals such as sponges and corals to algae, fungi, and bacteria.

The digestive system is more advanced than those previously discussed and consists of a mouth, buccal cavity, "esophagus," long tubular intestine and anus. Digestive enzymes are produced by the pharyngeal glands and in the intestine. The nervous system consists of a nerve ring which is attached to dorsal, ventral, and lateral nerves that are associated with various parts of the body. There are no specialized organs for gas exchange, and no specialized circulatory system or excretory system. Metabolic waste products are discharged with other materials leaving the gut or they diffuse across the body wall. Most nematodes have separate sexes and accordingly, distinct male and female sex organs.

Phylum Rotifera

Key evolutionary advances: Same as Nematoda.

Other characteristics of rotifers: Rotifers have an anterior ciliated surface called the corona. The cilia of the corona beat in waves giving the impression of rotation.

Rotifers predominate in freshwater environments including lakes and ponds. Usually 40-500 rotifers are found per liter of water although densities as high as 5,000/liter have been observed. Free-living rotifers feed on food ranging from algae to zooplankton to detritivores.

The digestive system of rotifers includes a mouth, stomach (which secretes digestive enzymes), short intestine and cloaca. Undigested wastes discharge into the cloaca and then the anus. The rotifer brain consists of a mass of ganglia to which are attached nerves that extend to organ systems, muscles, and sensory receptors including photoreceptors. Excretion is partially accomplished by diffusion across the general body surface. All rotifers also contain a pair of protonephridia like those of flatworms. Connecting tubules lead from the protonephridia to a bladder which also expels excretory products into the cloaca.

Phylum Mollusca (mollusks)

Key evolutionary advances: coelom. As mentioned before, the coelom is completely enclosed within the mesoderm. This allows physical contact between the mesoderm and endoderm which permits the development of specialized tissues. The only problem with the coelom (and not experienced by pseudocoelomates) is that the gut is surrounded by tissue that presents a barrier to diffusion so coelomates have resolved this problem through the development of a circulatory system. Blood carries nutrients and oxygen to the tissues and removes wastes and carbon dioxide via a network of blood vessels. Hearts are usually present to pump the blood. Mollusks have a 3-chambered heart and an open circulatory system.

Other characteristics of Mollusks: Mollusks include three classes with outwardly different body parts, although the body of all three classes consists of three segments: head, a central section with the body’s organs, and a foot. Wrapped around the body organs is a heavy fold of tissue called the mantle which includes the gills. The gills are rich in blood vessels which capture oxygen from the water. Mollusks also have a tongue-like organ used to capture food.

The three classes of mollusks include the following:

1. Gastropods - snails and slugs - use the muscular foot to crawl and their mantle secretes a hard protective shield.
2. Bivalves - clams, oysters, scallops - secrete 2-part shell with a hinge.
3. Cephalopods - octopuses and squids - have modified mantle to create a jet propulsion system that propels them through the water; shell is absent.

References

1. Brusca, R. C. and Brusca, G. J.: Invertebrates, 1990. Sinauer Associates, Inc., Sunderland, MA.
2. Johnson, G. B.: The Living World, 2^nd Edition, 2000. McGraw-Hill Co., Inc., New York
3. Pechenik, J. A.: Biology of the Invertebrates, Ed. 3, 1996. McGraw-Hill Companies, Inc., New York.
4. Ruppert, E.E. and Barnes, R. D.: Invertebrate Zoology, 1994. Saunders College Publishing, New York.

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