CCELENTERATA : HYDROZOA. 89 



multiplication may go on, there ultimately arrives in the history 

 of every individual a period at which sexual reproduction must 

 be called in to ensure the perpetuation of the species through- 

 out time. This truth is expressed by Steenstrup's celebrated 

 law of the " alternation of generations." 



Amongst the Hydrozoa, the individual may be either simple 

 or compound, and the development may be either continuous 

 or discontinuous, the following terms being employed to denote 

 the phenomena which occur. 



Hydrosoma. This is the term which is employed to desig- 

 nate the entire body of a Hydrozoon, whether it be simple, as 

 in the Hydra, or composite, as in a Sertularian. 



Polypite. The alimentary region of a Hydrozoon is called 

 a "polypite ;" the term "polype" being now restricted to the 

 same region in the Actinozoa. In the simple Hydrozoa the 

 entire organism may be called a " polypite ; " but the term is 

 more appropriately applied to the separate nutritive factors 

 which together make up a compound Hydrozoon. 



Distal and Proximal. These are terms applied to different 

 extremities of the hydrosoma. It is found that one extremity 

 grows more quickly than the other, and to this free-growing 

 end at which the mouth is usually situated the term "distal" 

 is applied. To the more slowly growing end of the hydrosoma 

 which is at the same time usually the fixed end the term 

 "proximal" is applied. These terms maybe used either in 

 relation to a single polypite in the compound Hydrozoa, or to 

 the entire hydrosoma, whether simple or compound. 



Caznosarc. This is the term which is employed to designate 

 the common trunk, which unites the separate polypites of any 

 compound Hydrozoon into a single organic whole. 



Polypary. The term " polypary ; ' or " polypidom" is applied 

 to the horny or chitinous outer covering or envelope with 

 which many of the Hydrozoa are furnished. These terms 

 have also not uncommonly been applied to the very similar 

 structures produced by the much more highly organised Sea- 

 mats and their allies (Polyzoa), but it is better to restrict their 

 use entirely to the Hydrozoa. 



Zob'ids. In continuous development the partially inde- 

 pendent beings which are produced by gemmation or fission 

 by the primitive organism, to which they remain permanently 

 attached, are termed " zooids." In other words, " zooids " are 

 the more or less individualised members of which the Hydroid 

 colony is made up. 



In discontinuous development, where certain portions of 

 the "individual" are separated as completely independent 



