CCELENTERATA. 57 



remain permanently connected with one another, and partly of dis- 

 similar zooids which are detached to lead an independent life, all 

 alike being the result of the development of a single ovum. 



Zooid. — The term " zooid " is indifferently applied to all the more 

 or less completely independent beings which are produced by bud- 

 ding, or by cleavage from a primitive organism. It does not matter, 

 therefore, for the purposes of this definition, whether these beings 

 remain permanently attached to the original organism, or whether 

 they are finally separated to enjoy an independent existence. 



Hydrosoma. — The term "hydrosoma" is one which is very con- 

 veniently applied to the entire organism in any Hydrozoon, whether 

 this be simple, or whether it be composite and made up of a number 

 of connected zooids. 



Poh/pite. — That portion of any Hydrozoon which is concerned with 

 the process of digestion, or, in other words, the " alimentary region," 

 is termed the "polypite" — the more generally current term of "polype" 

 being now restricted in meaning to the same region in the higher 

 Coelenterata (Actinozoa.). In such of the Hydrozoa as the Fresh- water 

 Polype or Hydra, in which the hydrosoma is simple, the whole 

 organism is termed a polypite ; but the term is more generally em- 

 ployed to indicate the nutritive zooids of any compound Hydruzoiin. 



Ccenosarc. — The term " coenosarc " (Gr. koines, common ; sarx, 

 flesh) is employed to designate the common trunk or flesh by which 

 the separate polypites of any compound Hydrozoon are united into a 

 single organic whole. 



Distal and proximal. — These are terms applied to difi'erent ex- 

 tremities 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 the term " proximal " is 

 applied. When the Hydrozoiin is fixed to any solid object, it is by 

 the proximal end that attachment is effected ; but in such forms as 

 are permanently free this mode of distinction is inapplicable. The 

 terms may be used either in reference to a single polypite in the 

 compound Hydrozoa, or as regards the entire hydrosoma, whether 

 this be simple or compound. 



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 been not un- 

 commonly employed to designate the very similar structures pro- 

 duced by the much more highly organised Sea-mats and their allies 

 (Polyzoa), but it is better to restrict their use entirely to the 

 Hydrozoa. 



