62 



INVERTEBKATA 



CHAP. 



rc 



sp 



B 



ten 



mouth, and the whole medusa becomes free by the absorption of 

 the stalk of the bud. Generative cells are only matured after 

 a considerable period of free-swimming life. 



On the other hand, accord- 

 ing to Gotte, in Clava the bell- 

 rudinient is formed but the 

 umbrella-cavity never opens to 

 the exterior, nor are there any 

 radial canals formed, and the 

 whole bell-rudiment is absorbed 

 before the germ cells are shed. 

 In such forms as Clava the 

 generative cells of at least one 

 sex in the case of Clava of 

 both sexes can be detected 

 in the stalk of the person 

 (blastostyle) which bears the 

 gonophores. Their develop- 

 ment in this genus has been 

 described by Harm (1903). 

 They appear to arise from 

 amongst the interstitial cells 

 of the ectoderm, but, as is also 

 the case with the genital cells 

 of Tubularia, they migrate 

 early into the endoderm, where 

 they grow. They reach the 

 spadix when fully ripe, and 

 burst through the ectoderm 

 there (Fig. 42). 



In Cordylophora the bell- 

 rudiment remains a solid mass 

 of cells, part of which is 

 converted into generative 

 cells. 



In Sertularia the gono- 

 phore is a broad-based lateral 

 swelling on the blastostyle, 

 the bell-rudiment separates 

 from the ectoderm and splits 

 imperfectly, the vestigial um- 

 brella-cavity being crossed by 

 cell trabeculae ; and finally, in 

 Halecium no bell-rudiment at 

 all is formed. 



The last two forms belong to that division of Hydromedusae 

 termed Calyptoblastea, which possess hydrothecae, and in which the 

 mouthless person, or blastostyle, which bears the gonophores, is 



FIG. 40. Two longitudinal sections through the 

 developing medusa-bud of Podocoryne carnea. 

 (After Gotte.) 



A, before umbrella-cavity is open to the exterior. 

 B, after umbrella - cavity is open to the exterior, 

 e.c, circular canal ; gen, developing genital cells ; r,c, 

 radial canal ; sp, (in A) cavity of the spadix,' (in B) wall 

 of the spadix ; ten, tentacle ; u, umbrella-cavity. 



