CNIDARIA 



153 



gives rise to medusae by transverse fission (strobilation) ; with vertical 

 partitions (mesenteries) in the enteron of some forms and larval 

 enteron of others ; velum and nerve ring absent ; endodermal gonads ; 

 and skeleton absent. 



AcTiNOZOA. Solitary or colonial cnidarian polyps without medu- 

 soid phase; vertical partitions (mesenteries) in the enteron; endo- 

 dermal gonads; with or without a skeleton. 



Fig. 113. The planula of a hydromedusan, Clava squamata. A and B, Swim- 

 ming about in the sea. C, Coming to rest on a rock. D, Developing tentacles, 

 oral cone and stolon, or.c. oral cone ; stn. stolon. Magnified. From Allman. 



Class HYDROZOA 



The most typical life histories of the "hydroids" are those in which, 

 the phenomenon of '* alternation of generations" is presented. That 

 is, there is a regular alternation of phases, hydroid colonies giving 

 rise to free-swimming medusae and the fertilized eggs laid by the 

 medusae each giving rise to a new colony of polyps. In the first two 

 orders of the Hydrozoa, the Calyptoblastea and the Gymnoblastea, 

 alternation of generations is well shown in the typical genera. As will 

 be shown, there is a progressive suppression of the medusoid "genera- 

 tion " in other members of these orders. In the other orders there is, 

 however, complete suppression of the polyp phase in the Tracho- 

 medusae and Narcomedusae, and in the Siphonophora remarkable 

 colonies are found which appear to^^have originated by budding from 

 a medusa. 



The following orders are contained in the class : 



Calyptoblastea {Leptomedusae). Hydrozoa in which the coenosarc 

 is covered by a horny perisarc, produced over the nutritive polyps as 

 hydrothecae and over the reproductive individuals as gonothecae; 



