12 



BRANCH CCELENTERATA. 



FIG. 1C. 



E 



cavity are the cavities of the tentacles, which are simple 

 outpushings of the body wall, being given their form by 

 the nutritive fluid they contain. If this liquid is with- 

 drawn, the tentacle sinks down to a mere elevation. On 

 being irritated, the Hydra may allow most of the liquid 

 to escape from its body, contracting itself into a small 

 knob. It is thus without the supporting skeleton of the 

 sponge. 



The Campanularian Hydroid (kam pan u la'ri an hy'droid), 

 Obelia gelatinosa (o be'li a je lat i no'sa), is another repre- 

 sentative of this class. It looks 

 like a Hydra surrounded by a cup, 

 into which, on contracting, it can 

 withdraw. The cup may be sup- 

 ported at the tip of a long branch- 

 ing stock, the base of which is at- 

 tached by root-like expansions to 

 some support, as the side of a spile 

 or the frond of a sea-weed. 



When these branching animals 

 are examined during the summer 

 months, some of the cups, or bells, 

 are found to be without mouth or 

 tentacles, and to have in place of 

 the body, a central stock from 

 which "buds" are growing. These 

 buds finally break from their sup- 

 port and swim away. This free form, of minute size, is 

 called a Medusa (me du'sa), is bell-shaped, and swims by 

 alternately expanding and contracting its rim. Around 

 the edge, are several small sacs, connected together by a 

 nervous cord and filled with liquid, in which are small, 

 hard particles. When the water is disturbed, these im- 



OWVi a ge lat \ no'sa. 

 S, Supporting stem. E, Cup. C, 

 Contracted individual. H, Ex- 

 panded individual showing its 

 mouth at the center. S ' , Support 

 bearing Buds (5), which event- 

 ually leave the surrounding cup 

 and swim away as A, the Me du'sa 

 of an allied form. (Enlarged.) 



