38 



ANIMAL FORMS 



preceding group (Fig. 21), but, while the latter are generally 

 very small, these forms are commonly from four to twelve 

 inches in diameter, and some measure one to two meters 

 (three to six feet) across the bell. They are also distin- 

 guished by means of tentacles which extend from the cor- 

 ners of the mouth sometimes to a distance of several feet, 



and together with the 

 marginal tentacles are 

 formidable weapons for 

 capturing small crabs, 

 fishes, and other ani- 

 mals which serve as 

 food. In turn these 

 forms serve as the food 

 of many whales, por- 

 poises, and numerous 

 fishes which hunt them 

 down, though the 

 amount of nourishment 

 they contain is prob-j 

 ably relatively small 

 owing to the fact that 

 in their composition 

 there is a large percent- 

 age of water (99 per 

 cent in some species). The lobed margin of the bell, the 

 absence of a definite swimming organ or velum, and the 

 character of several of the internal organs, distinguish the 

 larger from the smaller jelly-fish ; but the greatest differ- 

 ence, however, is in the method of development. 



39. Development. The eggs arise from the inner layer 

 of the jelly-fish and drop into the gastric cavity, where each 

 develops into a ciliated two-layered sac in some respects 

 like that of a young sponge. Swimming away from the 

 parent, they finally settle down, and attaching themselves 

 (Fig. 22, a) assume the external form and habits of the sea- 



FIG. 21. A jelly-fish (Rhizostoma), about one- 

 fourth natural size. 



