74 



HYDROZOA. 



repetition is more or less distinctly marked. An 

 example of the first of these methods is presented 

 by Pelagia ; of the second, by JEginopsis ; of the 

 third, by Cordylophora or Sertularia. 



In discontinuous development the detached 

 portions of the individual are termed 'zooids' 

 that which is first formed being distinguished as 

 the ' producing/ that which separates from it, the 

 ' produced' zooid. If there be more than two 

 successive series of zooids, the terms ' protozooid,' 

 € deuterozooid,' and ' tritozooid,' may then be re- 

 spectively applied to them. Thus, the medusoids 

 budded by Sarsia are, probably, tritozooids. The 

 term zooid is also extended to the several parts of 

 a connected structure which increases by vegeta- 

 tive repetition ; for example, to the polypites, and 

 other appendages of the composite Hydrozoa. 



The producing zooid may either possess or want 

 generative organs. In the latter case the pro- 

 duced zooid may take on the performance of the 

 reproductive function, as in so many orders of 

 Hydrozoa. 



In this class we have seen that the produced 

 zooid may resemble the producing zooid, as in 

 Hydra, or be dissimilar to it, as shown by the 

 free-swimming gonophores of the Corynidce and 

 Sertularidce. The first case affords an illustration 

 of simple ' gemmation with fission ' ; the latter, of 

 the process known as 'metagenesis/ If the pro- 

 ducing zooid possess sexual organs, and the pro- 

 duced zooid present the morphological, but not 

 the physiological, characters of an ovum, then the 

 process is one of ' parthenogenesis.' All these 

 varieties of discontinuous development are col- 

 lectively denominated ' agamogenesis,' as distin- 



