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- 
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,@) assume the external form and habits of the sea- 
Fig. 21.—A jelly-fish (2hizostoma), about one- 
fourth natural size. 
