COELENTERA TA 



55 



tli& hydranth, nor are there capsules surrounding the gono- 



phores. 



The medusae may he vjell developed and free, or they may he 



permanently attached to the hydranth (gonophore), or they may 



not develope hey ond rudimentary 



huds (sporosacs), and finally, 



in Hydra they are not formed 



at all. When free, they may 



he provided with ocelli at the 



hose of their marginal tentacles; 



usually on their outer surface. 



The sexual cells are arranged 



round the manuhrium, either 



uniformly or in hands. Most 



Anthomedusae are small. 



The tentacles of the hy- 

 droid are nearly always solid. 

 Hydra, however, is an exception 

 to this rule; they are usually 

 arranged in one circle as in Hydra, 

 or in two as in Tuhularia; in 

 the family Clavidae, however, 

 they are irregularly scattered over 

 the surface of the hydranth. 



There is no special aggregation of nerve cells in the 

 hydroid stage, but in the medusa a nerve ring is present round 

 the edge of the umbrella ; it is split into an upper and lower 

 nerve ring by the insertion of the velum. The ocelli consist 

 of a collection of pigment spots, and a cuticular lens is present 

 in lAzzia. The sexes as a rule are separate, the genus Hydra 

 again forming an exception. Eeproduction by fission is rare, 

 but sexual reproduction by budding is common in both the 

 hydroid and the medusoid stage. When it takes place in the 



IV. 



Fig. 40. 

 -Sub-umtrella cavity opened, 

 manubrium arising, and the de- 

 velopment of gelatinous tissue. 



1. Ectodei-m. 



2. Endoderm. 



3. Structureless lamella. 



4. Manubrium. 



5. Radial canal. 



6. Velum. 



7. Gelatinous tissue. 



8. Sub-umbrella cavity. 



latter, a medusa is invariably the result. 



Order 2. Calyptoblastea-Leptomedusae. 



Characteristics. — The hydroid forms of this genus never have 

 more than one row of tentacles round the mouth. The 



