68 MANUAL OP ZOOLOGY. 



difficulty which is ordinarily experienced by the student in 

 mastering the details of this class of animals, it has been 

 thought advisable to introduce here a short explanation of 

 some of the technical terms which are in more general use 

 in describing these organisms. 



GENERAL TERMINOLOGY OF THE HYDROZOA. 



Individual. We have already seen (see Introduction) that 

 the term ' individual,' in its zoological sense, must be re- 

 stricted to ' the entire result of the development of a single 

 fertilised ovum,' and that in this sense an individual may 

 either be simple, like an Amoeba, or may be composite, like a 

 Sponge, which is produced by an aggregation of amoebiform 

 particles. If all the parts composing an individual remain 

 mutually connected, its development is said to be ' continu- 

 ous ; ' but if any of these parts become separated as indepen- 

 dent beings, the case becomes one of ' discontinuous ' de- 

 velopment. 



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 de- 

 note 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 may be used either in re- 

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

 the entire hydrosoma, whether simple or compound. 



Ccenosarc. 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. 



Zooids. In continuous development the partially inde- 

 pendent beings which are produced by gemmation or fission 



