CHAPTER 11. 

 CNIDARIA. 



Systematic : I. Hydrozoa. 



1. Hydroidea 



2. Siplionopliora. 

 II Anthozoa. 



III. SCYPHOMEDUSiE. 



I. HYDROZOA. 



I. Hydroidea. 



The sexual products of the Hydroidea are usually matured 

 in specially organized individuals, which are either free- 

 swimming, and then attain to the high degree of organization 

 of the hydroid medusa, or remain united throughout life with 

 the polyp colony, and then, as sessile medusoid gonopJiores 

 (Sporosacs), exhibit that organization only in a degenerated 

 condition. In Hydra, on the contrary, the sexual products 

 develop in the ectoderm of the body-wall of the polyp. 



The eggs of the hydroid medusEe are generally extruded, 

 by the dehiscence of the wall of the gonad, into the sea- 

 water, where they are fertilized and undergo development. 

 But in those forms which possess sessile gonophores the 

 first stages of development take place within the gonophore, 

 and the embryo does not become free until it attains the 

 stage of a planula or actinula. 



In the following account we separate as metagenetic forms 

 those which produce free-swimming medusae (forms with 

 alternation of generations) from those whose sexual indi- 

 viduals remain sessile, as medusoid buds (forms with masked 

 alternation of generations, Hatschek). A third group em- 

 braces those Hydroids in which there is developed out of the 



