Animal Diversity

The diverse appearance of animals is mostly superficial; the bewildering variety of known forms, some truly bizarre, can be assorted among a mere half-dozen basic body plans. These plans are established during the embryonic stages of development and limit the size and complexity of the animals. Symmetry, number and relative development of tissue layers, presence and nature of body cavities, and several aspects of early development define these fundamental modes of organization.  

Parazoa: a cellular level of organization

Although the two phyla in this subkingdom, Porifera (sponges) and Placozoa, lack clearly defined tissues and organs, their cells specialize and integrate their activities. Their simplicity has been adaptive, and sponges have remained important in benthic marine habitats since their origin. The sessile, filter-feeding way of life shown by sponges has favoured a body plan of radial symmetry, although some members have become asymmetrical. The shape of the creeping, flattened placozoans is irregular and changeable.

Radiata: a tissue level of organization

The two coelenterate phyla (Cnidaria and Ctenophora) advanced in complexity beyond the parazoans by developing incipient tissues—groups of cells that are integrally coordinated in the performance of a certain function. For example, coelenterates have well-defined nerve nets, and their contractile fibres, although only specialized parts of more generalized cells, are organized into discrete muscle units. Because discrete cells of different types do not carry out the internal functions of the animals, coelenterates are considered to be organized at only a tissue level.

The integration of cells into tissues, particularly those of nerve and muscle, permits a significantly larger individual body size than is possible with other modes of body movement. Flagella and cilia become ineffective at rather small size, and amoeboid movement is limited to the size a single cell can attain. Muscles contract by a cellular mechanism basically like that used in amoeboid locomotion—interaction of actin and myosin filaments. Through coordinated contraction of many cells, movement of large individuals becomes possible.
Coelenterates, like parazoans, have only two body layers, an inner endoderm primarily for feeding and an outer ectoderm for protection. Between the endoderm and the ectoderm of coelenterates is the mesoglea, a gelatinous mass that contains connective fibres of collagen and usually some cells. Both layers contain muscle fibres and a two-dimensional web of nerve cells at the base; the endoderm surrounds a central cavity, which ranges from simple to complex in shape and serves as a gut, circulatory system, and sometimes even a skeleton. The cavity is also used for gamete dispersal and waste elimination.
Cleavage of a fertilized egg produces a hollow sphere of flagellated cells (the blastula). Invagination of cells at one or both poles creates a mouthless, solid gastrula; the gastrula is called the planula larva in species in which this stage of development is free-living. The inner, endoderm cells subsequently differentiate to form the lining of the central cavity. The mouth forms once the planula larva has settled. Although the details of early development are different for parazoans and coelenterates, most share a stage in which external flagellated cells invaginate to form the inner layer, which lines the cavity, of these diploblastic (two-layered) animals. This is characteristic of invagination during the development of all animals.
All coelenterates are more or less radially symmetrical. A radial form is equally advantageous for filtering, predatory, or photosynthetic modes of feeding. Tentacles around the circumference can intercept food in all directions.