Cuvier included them in his /Radiata/, a class comprising all the animals whose parts diverge or radiate from a central axis. Recent classification has divided the radiate animals into several classes. This arrangement of parts is obviously quite different from that of bilateral symmetry, or the disposal of parts on each side of a longitudinal axis. The type of radiate structure is shown in polyps. The body is a sac, in the center of which is another sac or axis. This is the digestive cavity. Vertical partitions extend from the central to the outer sac, forming distinct [pg114] divisions or chambers. The number of divisions varies with the different species and also with the age of the animal. Other partitions start from the outer sac, and extend toward the central axis, but do not unite with it. These partitions, called /mesenteries/, are always in definite multiples, varying in different species, new divisions growing between the first partitions in regular order. On the inner edge of these partitions the eggs of the animal are formed, which, when mature, drop into the chambers and pass through openings into the inner sac, or digestive cavity, and out of the mouth into the water. [Illustration: Diagram of radiate structure.] The animals are classed according as the eggs are formed on all or on special partitions, those being of the highest order where a limitation and constancy of function is maintained. The upper surface of the body has hollow tentacles, each one of which opens into one of the chambers and extends outward. All parts of the animal communicate, and whatever enters the mouth circulates through the whole structure; and when assimilation is completed the residue returns by the same road and is expelled through the mouth. This structure is common to all polyps; but there are great differences in their texture, some being soft and some horny, while others deposit a calcareous skeleton (corals). Some grow in colonies, like the hydroids and corals, and are stationary, others are free and independent; some have but few, others have many tentacles; and they differ widely in size, form, and color. Hydroids, sea-anemones, corals, sea-fans, starfishes, and sea-urchins are different examples of the radiate structure. They are not, however, all of them polyps. The /Cœlenterata/ are divided into four classes: =Hydrozoa=, which include the colonies of zoöphytes which resemble seaweeds, the small jellyfishes which are born of these colonies, and the /millepores/, which are colonies of zoöphytes which secrete a stony instead of a horny skeleton, yet differ in some respects from other [pg115] stony corals; =Scyphozoa=, large jellyfishes, many of which do not have the double form of development; =Actinozoa=, the sea-anemones and the different forms of stony corals, including sea-fans, sea-pens, etc.; =Ctenophora=, the jellyfishes commonly known as comb-jellies. [pg116]