THE STRUCTURE OP XENIA HICKSONI. 205 



dinal canals of Xerda, but this also is probably due to the different 

 modes of growth of the two corals. In Ileliopora the living part is, as 

 it were, spread out in a thin film, and the polyps are a considerable 

 distance (1 mm. to 2 mm.) apart, several coenenchymal tubes being 

 therefore required to place the coelentera in communication with the 

 intervening ectoderm, calicoblasts (or skeleton-forming cells), and meso- 

 gloea. In Xenia each stem is an elongated, cylindrical, compact mass 

 of living tissues, and the polyps are much closer together, the ccelentera 

 being only about *25 mm. to - 4 mm. apart in the stem, and therefore 

 one longitudinal canal, situated in the mesogloea between adjacent 

 ccelentera (together with the auxiliary strands and cords of cells which 

 are so well developed in this Xenia), is sufficient to provide efficient 

 communication between the coelentera and all parts of the narrow 

 mesoglceal column between them. 



Histology of the Longitudinal Endodermic Canals. — These canals are 

 lined by a single layer of cells of equal thickness on all sides (cf. the 

 superficial canals). The cells are more or less cubical in shape, and 

 closely resemble the endoderm cells lining the ccelentera. None of the 

 cells in the canals bear flagella, but some bear slender muscle processes 

 which, like those of the endoderm of the ccelentera, are chiefly circular 

 in direction. In the canals near the base of the colony many nemato- 

 cysts, each in its cnidoblast cell, may be seen lying among the 

 endoderm cells. In the upper portions of the colony nematocysts are 

 seldom seen in the canals. Zooxanthellse are present only in those 

 longitudinal canals which are situated in the circumferential portions 

 of the stem and in the upper portion of the canals, where they approach 

 the summit of the stem. 



Cells in the Mesogloea. — On examining a transverse section of the 

 upper end of a stem of the colony, an almost complete chain of cells is 

 seen surrounding the denser ring of mesogloea round each ccelenteron 

 (Fig. 9, Ect. Ch.). Each ring of cells is situated at a distance of about 

 •06 mm. to - 07 mm. from the endoderm of the ccelenteron within. These 

 cells are the ectoderm of the portion of the polyp enclosed in the stem, 

 as can be seen on examining a longitudinal section through the upper 

 part of the stem (see Fig. 8), when this cylinder of cells is seen to be 

 continuous and in line with the ectoderm of the free portion of the 



