272 J. H. ASHWORTFT. 



and the longitudinal canals of Xenia are perfectly justifiable. 

 It is true the coenenchymal tubes of Heliopora are more 

 numerous than the longitudinal canals of Xenia, but this 

 also is probably due to the different modes of growth of the 

 two corals. In Heliopora the living part is, as it were, 

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

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

 being therefore required to place the coelentera in communica- 

 tion with the intervening ectoderm, calicoblasts (or skeleton- 

 forming cells), and mesogloea. In Xenia each stem is an 

 elongated, cylindrical, compact mass of living tissues, and the 

 polyps are much closer together, the coelentera being only 

 about '25 mm. to "4 mm. apart in the stem, and therefore one 

 longitudinal canal, situated in the mesogloea between adjacent 

 coelentera (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 mesogloeal column between them. 



Histology of the Longitudinal Endoderraic 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 coelentera. None of the cells in 

 the canals bear flagella, but some bear slender muscle pro- 

 cesses which, like those of the endoderm of the coelentera, are 

 chiefly circular in direction. In the canals near the base of 

 the colony many nematocysts, each in its cnidoblast cell, may 

 be seen lying among the endoderm cells. In the upper por- 

 tions 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 coelenteron (fig. 9, Ect. Ch.). Each 



