168 THE FLOATING ANIMALS 



Of the many-celled animals, the jelly-fish are among 

 the commonest and the most attractive. 



Anyone not familiar with the classification of marine 

 animals would call any animal a " jelly-fish " which has 

 the consistency and appearance of a jelly. But it 

 is necessary to limit the name, even in its popular 

 sense, so as to exclude the swimming Urochordates 

 (pp. 184-188), which are most likely to be mistaken by 

 the uninitiated for true jelly-fish. The former are quite 

 different in shape and structure, and are nearly related 

 to the Vertebrates (cf. Figs. 88-100 with Figs. 149-155). 

 The jelly-fishes may be separated for the sake of 

 convenience into four groups — (1) Hydromedusae, 

 (2) Scyphomedusae, (3) Siphonophores, (4) Ctenophores 

 — each of which has well-marked characters. The 

 jelly-fishes belonging to the first two groups are com- 

 monly called simply Medusas. 



The Hydromedusae are small in size, seldom exceed- 

 ing an inch in diameter, and are caught by means of a 

 tow net. They may be subdivided into two groups : 

 (1) Neritic or shore forms ; (2) oceanic or blue-water 

 forms. The neritic forms occasionally drift out into 

 the ocean, but, as a rule, they become scarce beyond 

 the 100-fathom line. 



The neritic Medusae (Figs. 88, 89) are budded off 

 from small animals called " zoophytes," or " hydroids " 

 (Figs. 90-92), which form colonies, and are perma- 

 nently fixed at the bottom of the sea, or in some cases 

 to floating objects. The hydroids, which thus bud off 

 Medusae, do not develop eggs, and are, therefore, 

 asexual. The little Medusae, after leaving their 

 hydroids, begin an independent free-swimming career. 

 In the course of growth and development they usually 

 pass through a series of progressive stages, and finally 

 become sexually adult. The eggs from the female 



