THE STRUCTURE OF XENIA HICKSONI. 269 



with similar relations to those above described in the cylin- 

 drical portion of the stem. In this portion of the stem the 

 canals embrace or pass round each coelenteron at a distance of 

 about "2 mm., and communicate by means of branches with 

 the coelenteron and with the neighbouring longitudinal canals. 

 Tlie superficial canals on the convex summit are continuous 

 at the edge of the summit with the corresponding canals of 

 the cylindrical portion of the stem. In the stem the super- 

 ficial canals frequently communicate with the neighbouring 

 coelentera and longitudinal canals. Thus, by means of branch 

 canals, this superficial system of canals is placed in commu- 

 nication with the remaining cavities lined by endoderm, and 

 by means of strands of cells is placed in communication 

 with the ectoderm and with the neighbouring cells in the 

 mesogloea. 



Where a stem divides there are extra canals which establish 

 thorough communication between the superficial canals of the 

 two branches. 



As the base of the colony is approached there is a tendency 

 for the superficial canals to send inwards wide branches, and 

 in the lowest 2 mm. of the stem such branches are given off 

 in large numbers, and unite with the complicated anastomosis 

 of longitudinal canals j)resent in that part of the stem. 



This system of canals is of great importance, as all the young 

 buds produced in the colony are formed by enlargement and 

 growth outwards and inwards of one of these canals, the endo- 

 derm and lumen of the canal forming respectively the endoderm 

 and coelenteron of the young polyp (see also p. 291). 



Histology of the Superficial 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. 27, fig. 29). This is caused by the cells on 

 the outer side 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 coelentera, but their protoplasm is somewhat less vacuo- 

 lated, and therefore does not so markedly present the reticulate 



