270 J. H. ASHWORTH. 



appearance which is so usual in the endoderm of the coelentera. 

 None of the cells of these canals bear flagella. Among the 

 bases of the cells there are small cells wliich are probably stages 

 in the formation of the larger ones. Some of the cells of the 

 canals appear to be provided with very slender muscle pro- 

 cesses. There are numerous zooxanthellse in the lumen of 

 the canals and embedded in the endoderm lining the cavity. 



The Internal Canal System. — The canals forming the 

 main portion of this system are chiefly longitudinal in direc- 

 tion, and commence in the umbrella-shaped portion at the 

 top of each stem. Each canal runs in a sinuous or zigzag 

 course in the mesogloea, about equidistant from the surround- 

 ing coelentera. 



The longitudinal canal communicates with the superficial 

 canals lying around its origin (fig, 8). During its course 

 down the stem the longitudinal canal very frequently com- 

 municates by small transverse canals with the neighbouring 

 coelentera and canals, and tlie longitudinal canals in the outer 

 portion of the stem communicate also with the superficial 

 canals. Owing to the frequent occurrence of branches, the 

 longitudinal canals are nearly always angular in transverse 

 section, one (or more) of the angles being usually produced 

 into a small branch canal. The branch canals vary greatly in 

 size ; in the upper and middle portions of the colony being 

 small, and their lumen very small or obliterated altogether, 

 while near the base of the colony the branches are almost as 

 large as the main canal. At the base of the colony these longi- 

 tudinal canals give off several rather larger lateral branches on 

 all sides, some of which unite with similar branches from 

 adjacent canals, while others open into neighbouring coelentera. 

 Owing to the presence of so many canals in this region of the 

 stem, the raesogloea is penetrated in all directions by a com- 

 plicated network of canals, which place all the cavities 

 lined by endoderm in intimate communication with each other. 

 Very close to the base of attachment of the colony, a canal 

 may usually be seen passing from each side of the lowest 

 portion of each primary ccelenteron, so that in longitudinal 



