128 



COBLENTERATA. 



c 



FIG. 112. Group of polyps of 

 nodosa. A gastrozooid surrounded 

 by five dactylozooids B ; o mouth ; 

 c tentacles ; ii tubes (after Moseley). 



C P 



of the soft parts of this genus and 

 of the Sti/lasteridae, that the Hydro- 

 medusan affinities of these animals 

 were clearly proved, and that the order 

 Hydroeorallinae was definitely estab- 

 lished. 



The colonies have the form of 

 encrusting or arborescent masses con- 

 sisting of a network of coenosarcal 

 canals (Fig. 113) permeating a hard 

 calcareous support called the coeno- 

 steum. The canals consist of ectoderm 

 and endoderm, and the coenosteum is 

 calcareous matter secreted by the 

 ectoderm, and comparable to the peri- 



sark of other Hydroid colonies. From the surface of the colony 



which is covered by a layer of ectoderm the polyps project. They 



are of two kinds gastro- 



zooids with a mouth and 



tentacles, and more nu- 

 merous dactylozooids (Fig. 



112) with or without 



tentacles, but always 



without a mouth. They 



are lodged in pits or caly- 



cles excavated in the sur- 

 face of the coenosteum, 



and lined by a continuation 



of the surface ectoderm. 



The relation of the surface 



ectoderm to the ectoderm 



of the polyps and of the 



canals is not known. The 



polyps of the two kinds 

 are either scattered irregu- 

 larly over the surface of 

 the colony, or gathered 

 into groups more or less 

 regular, in each of which 

 a centrally placed gastro- 

 zooid is surrounded by a 



--D2 





FIG. 113. Vertical section through a group of retracted 

 zooids of Allopora profuwht, (after Moseley). DZ dac- 

 tylozooids; P calycles of dactylozooids separated by 

 pseudosepta ; Z gastrozooid giving off canals which 

 join the coenosarcal network ; GZ calycle of gastro- 

 zooid ; DZ' dactylozooids of an adjacent group; G 

 gonozooid (gonophore). 



