256 J. H. ASHWORTH. 



are situated in the outer portion of the mesogloea, just beneath 

 the ectoderm of the stem. This system of canals extends 

 all round the cylindrical stems, and also runs on the umbrella- 

 shaped areas from which the free portions of the polyps arise 

 (fig. 8). The cavity of this system of canals is invaded 

 throughout by zooxanthellse, which are especially numerous 

 in the canals in the upper part of the stem. 



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 mesoglcea, 

 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 

 complicated, 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 communi- 

 cation 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 de- 

 scribed 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 eacli 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. 



