110 - MANUAL OF ZOOLOGY. 



GENERAL TERMINOLOGY OF THE HYDROZOA. 



Individual. We have already seen (see Introduction) that the term "in- 

 dividual," in its zoological sense, must be restricted to "the entire result 

 of the development of a single fei'tilised 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 amcebiform particles. 

 If all the parts composing an individual remain mutually connected, its 

 development is said to be " continuous ; " but if any of these parts become 

 separated as independent beings, the case becomes one of "discontinuous" 

 development. We have seen, also, that however long zooidal multiplica- 

 tion may go on, there ultimately arrives in the history of every individual 

 a period at which sexual reproduction must be called in to insure the per- 

 petuation of the species throughout time. This truth is expressed by Steen- 

 strup'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 fol- 

 lowing terms being employed to denote the phenomena which occur. 



Hydrosoma. This is the term which is employed to designate the entire 

 body of a Hydrozobn, whether it be simple, as in the Hydra, or composite, 

 as in a Sertularian. 



Polypite. The alimentary region of a Hydrozobn is called a " polypite ; " 

 the term " polype " being now restricted to the same region in the Actin- 

 ozoa. In the simple Hydrozoa the entire organism may be called a " poly- 

 pite ; " but the term is more appropriately applied to the separate nutritive 

 factors which together make up a compound Hydrozobn. By Professor All- 

 man the term " hydranth " is used in preference to "polypite." 



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 relation 

 to a single polypite in the compound Hydrozoa, or to the entire hydrosoma, 

 whether simple or compound. 



Hydrorhiza. r Y\i\s term is applied to that portion of the proximal end 

 of a Hydroid colony by which it is attached to some foreign body. 



Ccenosarc. This is the term which is employed to designate the common 

 trunk, which unites the separate polypites of any compound Hydrozobn 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 (Polyzod), but it is better to restrict 

 their use entirely to the Hydrozoa. By Professor Allman the term " peri- 

 sarc " is given to the chitinous investment by which the soft parts of the 

 Plydrozoa are often protected. 



Zobids. In continuous development, the partially independent beings 

 which are produced by gemmation or fission from 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 "indivi- 

 dual " are separated as completely independent beings, these detached 

 portions are likewise termed "zooids ;" that which is first formed being 



