223 



CHAPTER XV. 



H YD ROZOA— Continued. 



HYDROCORALLINES AND STROMATOPOROIDS. 



Sub-class Hydrocorallin^e. 



The name of HydrocorallineB has been proposed by Professor 

 Moseley for two groups of marine Hydrozoa which produce a reg- 

 ular skeleton of carbonate of lime, and which, on the strength of 

 the skeleton alone, were formerly referred to the true Corals (Ac- 

 tinozoa). The two groups in question are the Milleporidce and 

 Stylasteridce, the former being represented by the well-known Mille- 

 pora, which contributes largely to the formation of coral-reefs in 

 various regions. The " coenosteum " or calcareous skeleton of 

 Millepora (fig. 108) is usually in the form of a foliaceous or 

 laminar expansion, or is simply branched, and though minutely 

 spongy, is of considerable density. The main mass of the skeleton 

 is essentially composed of a complex network of anastomosing cal- 

 careous fibres, so disposed as to give rise to a correspondingly com- 

 plex network of anastomosing and tortuous canals (fig. 109, c c). In 

 the living condition, this canal-system is filled with anastomosing 

 stolons of the ccenosarc, by which the different zooids of the colony 

 are placed in organic connection. The general spongy skeleton, 

 constituted as above described, is traversed at intervals by the 

 vertical tubes in which the zooids were contained. These tubes 

 are in two series, differing slightly in size, and they open on the sur- 

 face by apertures correspondingly different in dimensions, of which 

 the larger ones are called the " gastropores " and the smaller the 

 " dactylopores " (fig. 108, g and d). The gastropores and dactylo- 

 pores may be irregularly distributed, or the dactylopores may be 

 arranged in more or less definite systems round the gastropores. 

 These two sets of tubes, as shown by Moseley, lodge respectively 

 larger and smaller zooids, which differ in structure and function. 



