The differences between plants and animals led to a division of all life into the two kingdoms Plantae and Animalia. It was found that some were plantlike while others, PROTOZOA, resembled animals in that they had locomotion by means of flagella, or pseudopodia, and digested food. At first, protozoa were placed in a subkingdom of Animalia. This classification had problems: some forms showed mixed characteristics, and some groups had members that were plantlike with close relatives that were animallike. Eventually a five-kingdom system came into use, in which Plantae and Animalia are more restricted in definition and protozoan groups are assigned to the kingdom PROTISTA.
The phylum Placozoa, is the simplest
animal; like a tiny plate, but with tissue layers. 
The phylum Porifera consists of
sponges. Their nutritional needs are met through ingestion by individual
cells that are located along internal water canals that open to the
outside of the body. They are nonmotile organisms. The tissue is the
highest organizational level obtained.
The phylum Cnidaria consists of Hydrozoans, corals, jellyfishes, and sea anemones. Cnidarians are tentacled, radial animals.
Their mouths are encircled by tentacles. Present within these tentacles and elsewhere in the body are nematocysts--harpoonlike structures that are discharged from specialized cells (cnidocytes) in which they are cocked when not in use. The nematocytes are used to stun prey, and the tentacles serve to bring the prey into the mouth. Coelenterates have a "two-layered" body. The ectoderm is the outer layer, and the endoderm is the inner layer. The two layers surround the digestive cavity, the coelenteron, which has only one opening.
The phylum Flatworms, Roundworms, and Rotifers are simple bilateral, cephalized animals at the organ-system level of construction.
The
body plan is organized along a longitudinal axis, and one half is a mirror
image of the other. The organ system is the highest level of complexity
reached.
The flatworms have a digestive cavity, or gut, with only one opening and an excretory system is present. The space between the ectoderm and endoderm is occupied entirely by the mesoderm.
The roundworms is a little different. The mesodermal layer does not completely occupy the region between the ectoderm and endoderm. The free space is a body cavity, in this case a "pseudo," or false, body cavity. The cavity is present only because the germinal tissue (meso), which ordinarily gives rise to a coelom, does not develop. This cavity is not formed within tissues that appear in this region in the course of embryonic development. The tubular gut has openings to a mouth and an anus.
The Rotifers are abundant in plankton. They have a pharynx, esophagus, digestive glands, a stomach, and usually an intestine and anus. the head has nerve cell cluster. All rotifers have a crown of cilia used in swimming and drawing food to its mouth.
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