8 CORALS 



and calcareous structures of the Hydrozoa, Zoantharia, and 

 Alcyonaria. In the group of the Polyzoa the derivative 

 word " polypide " is usually employed for those parts of 

 the body of the animals that are capable of extension and 

 retraction, and the word polyp is rarely used with reference 

 to the solitary sea-anemones. 



In the case of several H}'drozoa and some of tlie 

 Alcyonaria the animals that build up the structure of a 

 single colony are found to be of two or three different 

 kinds, performing different functions in the economy of the 

 colony as a whole. In these cases great confusion has arisen 

 as to the use of the word polyp. 



Thus Kolliker suggested that in the dimorphic Alcyonaria 

 the word polyp should be restricted to those individuals 

 of a colony that exhibit the full number of tentacles and 

 mesenteries, and the word " zooid " employed for those 

 individuals in which the tentacles and mesenteries are re- 

 duced in number or absent. This proposal is obviously 

 inconvenient, although it has been constantly maintained 

 by the German writers, because it suggests a homological 

 difference between the two kinds of animals of such a 

 colony which does not exist. It would be better to use 

 some prefix to the word polyp, such as auto- and siphono-, 

 to indicate the difference in structure. But this has not 

 been done. Another word has been introduced which lends 

 itself more readily and euphoniously to the use of prefixes, 

 and instead of auto-polyp and siphono-polyp, for example, 

 the words auto-zooid and siphono-zooid are employed. The 

 word " zooid " then may be regarded as a synonym of the 

 word " polyp," but, whereas the latter is used only in a 

 general sense, the former may be used with a prefix to 

 signifv a particular kind of polyp or zooid. 



Thus, in speaking of the Precious coral, it may be said 

 that the colony is formed by a number of polyps, and that 

 these polyps are of two kinds, which are called the auto- 

 zooids and the siphonozooids respectively. 



The word polyp has thus come to be used in very much 

 the same sense as Milne-Edwards used it in the sub-title of 

 his book, " polypes proprement dits," but the word coral 



