There is another group of jellyfish-like marine animals which have been given the name of Ctenophora. By some they are regarded as a divergent sub-class of the Cœlenterates, by others as a distinct main group; in any case they appear to be important from our point of view. The structure of a typical member is shown in Fig. 38, and a few other forms are illustrated in Fig. 39. Our typical example is pear-shaped, with the mouth at the lower pole. The internal cavity is complex, but is on a different plan from that of the Medusa. There is a central cavity communicating with the outside not only by the mouth but also by two canals opening near the upper pole. There are two radial canals, each of which divides into four, the branches of which lead at right angles into other canals, running from pole to pole and blind at both ends. There are two tentacles, as shown, which can be withdrawn into special sacs. At the opposite end from the mouth are sense organs, seemingly of smell and balance respectively. On the outer surface, above each of the longitudinal canals, is a row of small plates bearing cilia. It is by the movement of these cilia, like a multitude of minute oars, that the animal swims--a method of locomotion which does not occur in the true Cœlenterates. An additional feature is the formation, at an early stage of development, of a definite third layer of cells between the outer and the inner. This layer ultimately forms the greater part of the jelly-like mass of the body. [Illustration: FIG. 39.--Group of Ctenophora.]