12 THE VOYAGE OF H.M.S. CHALLENGER. 
TERMS MADE USE OF IN THE DESCRIPTION OF THE STRUCTURE 
OF THE HYDROCORALLINA. 
All the HypRocoRALLIN® 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& 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 cenostewm, to distinguish it from the Anthozoan corallum, whilst 
the soft tissue, which together with it constitutes the hydrophyton, is termed cenosare. 
In all HyprocoraLLin&® 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® 
the pores are arranged in regular circular systems, simulating the calicular systems of 
Anthozoans in appearance. These systems are termed cyclo-systems. 
