THE STRUCTURE OF XENIA HICKSONI. 191 



The internal canal system consists of a series of longitudinal canals 

 (Figs. 8, 9, Long. Can.) which run generally in a sinuous or zigzag- 

 course in the mesogloea, between the coelentera of the polyps. These 

 canals lie in the mesogloea, almost equidistant from the surrounding 

 coelentera, and they run in this position from the top of each stem to 

 the base of the colony. These canals are also endodermic, and have 

 usually only a small lumen. They communicate with the coelentera of 

 the polyps, with the superficial canal system, and with each other. 



At the base of the colony the canal system is exceedingly compli- 

 cated, due to the numerous branchings and anastomoses of the canals, 

 The coelentera are continued down to the base of attachment of the 

 colony, where they are in communication with the numerous branches of 

 the canal systems. The coelentera are readily distinguishable from the 

 canals by their greater size, and the presence in them of eight small 

 ridges, to which the mesenteries in this region are reduced. Besides 

 the superficial and longitudinal canals briefly described above, there 

 are in connection with the longitudinal canals numerous small lateral 

 or transverse canals, with small lumen, which pass from the canals 

 either to other neighbouring canals or to the coelentera (Figs. 8, 9). 

 The coelentera of the polyps do not open into each other directly, but 

 are indirectly connected by these canals. 



The base of the cylindrical main stem is somewhat flattened out, 

 and this basal portion is hard and horny ; and, being closely applied 

 to the rock, provided a firm basis upon which the other parts of the 

 colony were supported. 



Ectoderm. 



The ectoderm is a moderately thick layer, in which large columnar 

 and smaller interstitial cells may with difficulty be distinguished. If a 

 section of a tentacle or pinnule, which was well expanded at the 

 moment of killing, be examined, the ectoderm is then seen to consist 

 of a row of cells elongated at right angles to the free surface, below 

 which are smaller rounded cells which probably correspond to the 

 interstitial cells of the ectoderm of Alcyonium and other coelentera. 

 (PI. XX., Fig. 11). 



The protoplasm of the ectoderm cells is very finely granular ; 

 occasionally cells are met with containing a few small vacuoles. 



