THE STRUCTURE OF XENIA HICKSONI. 203 



Histology of the Stiperficial Endodermic Canals. — The endoderm lining 

 the cavity of the superficial canals is always much thicker on the outer 

 side of the canal than on the inner side (PI. XXIL, Fig. 29). This is 

 caused by the cells on the outer sides being columnar and longer than 

 the cubical or slightly flattened cells of the inner side of the canal. 



The cells lining these canals resemble the endoderm cells of the 

 ccelentera, but their protoplasm is somewhat less vacuolated, and there- 

 fore does not so markedly present the reticulate appearance which is 

 so usual in the endoderm of the ccelentera. None of the cells of these 

 canals bear flagella. Among the bases of the cells there are small cells 

 which 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 processes. 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 direction, and commence in the 

 umbrella-shaped portion at the top of each stem. Each canal runs in 

 a sinuous or zigzag course in the mesogicea, about equidistant from the 

 surrounding ccelentera. x 



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 communicates by small transverse 

 canals with the neighbouring ccelentera and canals, and the 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 (he branches 

 are almost as large as the main canal. At the base of the colony these 

 longitudinal 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 ccelentera. Owing to the 

 presence of so many canals in this region of the stem, the mesogicea is 

 penetrated in all directions by a complicated network of canals, which 

 place all the cavities lined by endoderm in intimate communication 



