128 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The interior of these frequently contains vacuolar spaces 

 (Fig. 24, B, C). A valvular "pylorus" separates the gastric 

 from the somatic cavity in the Calycophoridce. Long tenta- 

 cles, frequently provided with unilateral series of branches, 

 are developed, either one from the base of each hydranth, or,' 

 independently of the hydranths, from the ccenosarc. 



In the Calycophoridw and many Physophoridce, complex 



Fig. 25.—AthoryMa rosacea.— A, a hydranth with villi (a). B, one of the villi in its 

 elongated state, enlarged. 6', a small retracted villus, still more magnified, with 

 its vacuolar spaces and ciliated surface. 



organs, containing a sort of battery of thread-cells, terminate 

 each lateral branch of a tentacle (Figs. 24 and 26). Each 

 consists of an elongated saccidits, terminated by two fila- 

 mentous appendages, and capable of being spirally coiled up. 

 In this state it is invested by an involucnan^ which surrounds 

 its base. The somatic cavity is continued through the branch, 

 which constitutes the peduncle of this organ, into the saccu- 

 lus and its terminal filaments. In the latter it is narrow, and 

 their thick walls contain numerous small spherical nemato- 

 cysts. In the sacculus the cavity is wider. One wall is very 

 thick, and multitudes of elongated nematocysts, the lateral 

 series of which are sometimes larger than the rest, are dis- 

 posed parallel with one another, and perpendicular to the 

 surface of the sac. Like the other organs, each of these 

 tentacular appendages commences as a simple diverticulum 

 of the ectoderm and endoderm, and passes through the stages 

 represented in Fig. 26. 



In Physalia the tentacula may be several feet long. They 

 have no lateral branches, but the large nematocysts are situ- 



