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THE INVERTEBRATA 



They are divided into the following classes : 



Hydrozoa. Cnidaria, nearly always colonial; typically with free or 

 sessile medusoid phase, arising as buds from the polyp-colony: no 

 vertical partitions in the enteron; medusae with a velum and nerve 

 ring; tentacles of polyp usually solid; ectodermal gonads; and an 

 external skeleton. 



ScYPHOMEDUSAE. Cnidaria in which the polyp stage is inconspicu- 

 ous and may be absent altogether : the polyp, where present, gives rise 

 to medusae by transverse fission (strobilization) ; with vertical par- 

 titions (gastric ridges) in the enteron ; velum and nerve ring absent ; 

 endodermal gonads; and skeleton absent. 



AcTiNOZOA. Solitary or colonial cnidarian polyps without medusoid 

 phase; vertical partitions (mesenteries) in the enteron; endodermal 

 gonads; with or without a skeleton. 



Fig. III. 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 "generation" in 

 other members of these orders. In the other orders there is, however, 

 complete suppression of the polyp phase in the Trachomedusae and 



