12 THE VOYAGE OF H.M.S. CHALLENGER. 



TEEMS MADE USE OF IN THE DESCRIPTION OF THE STRUCTURE 

 OF THE HYDROCORALLIN^E. 



All the Hydrocorallin^e are compound structures, and consist of numerous polyps of 

 various forms and of a common mass of hard and soft tissue by which these are supported. 

 This common support constitutes what according to Allman's nomenclature must be 

 termed the hydrophyton, or " common basis by which the several zooids of the colony 

 are kept in union with one another." The hydrophyton in the Hydrocorallin.e consists 

 of a hard calcareous mass which is penetrated throughout by a network of soft tissue 

 contained within a corresponding network of canals excavated in its substance. The hard 

 tissue is here termed ccenosteum, to distinguish it from the Anthozoan corallum, whilst 

 the soft tissue, which together with it constitutes the hydrophyton , is termed ccenosarc. 

 In all Hydrocorallin^e two forms of zooids are present ; one form has a mouth and 

 gastric cavity, the other is devoid of these and has a purely tentacular function. 



For the former the term gastrozooid is here adopted, and for the latter that of 

 dactylozooid . 



The pore in the corallum occupied by the gastrozooid is termed gastropore, and 

 that of the dactylozooid dactylopore. In the more highly differentiated Stylasterid^e 

 the pores are arranged in regular circular systems, simulating the calicular systems of 

 Anthozoans in appearance. These systems are termed cyclo-systems. 



