BRITISH ZOOPHYTES. 29 



is seen to form a source of communication between all 

 tlie members of the colony. The receptacle, or part 

 of the polypary in which the polypites of the sub-order 

 Thecaphoea are lodged, is called the hydrotheca or 

 cahjcle. The whole company of polypites formiug a 

 hydroid colony is called the trophosome. 



A constant circulation of nutrient particles is kept 

 up throughout the coenosarc, by which means every 

 portion of the colony is nourished, and, to aid in the 

 circulation of these particles, the endoderm of the 

 coenosarc is richly ciliated. 



The calycles of the various species of Thecaphora 

 are often very beautiful structures, and serve very 

 largely for identification. In some species they are 

 open, but in others they are furnished with au operculum 

 or hinged valve, which serves as a further protection 

 for the polypites. This valve varies in shape, in some 

 species it is simple and in one piece, while in others it 

 is complex, the various parts converging to a point. 



The enlargement of a colony proceeds by gemmation 

 from the coenosarc. The plan of gemmation is very 

 uniform, each species having its own characteristic 

 mode of branching, and the buds being produced at 

 such places on the coenosarc as best serve to preserve 

 the typical form of the species. 



In addition to the ordinary polypites of the colony 

 which may be called alimentary zooids, there are pro- 

 duced at certain seasons of the year sexual zooids for 

 the purpose of forming new colonies, and the phases of 

 reproduction in the hydroida present some of the 

 most curious life-histories known to science. 



The sexual zooids or gonozooids are produced in 

 gonophores or reproductive buds which spring from 



