202 J. H. ASHWORTH. 



is really a fine network of numerous canals, which have a similar struc- 

 ture and appearance in all parts of the colony. The canals are about 

 •08 mm. in diameter. In the intervals between these canals there are 

 usually cords of ectoderm cells {Ect. Str.) which pass from the ecto- 

 derm to cells in the deeper parts of the mesoglcsa. In many cases the 

 superficial endodermic canals are themselves closely connected with the 

 ectoderm by strands of cells passing across the mesogloea from the 

 ectoderm to the outer walls of the canal lying beneath. From the 

 inner wall of these canals cords of cells frequently pass inward into the 

 mesogloea, and are connected with other cells, with the ccelentera, or 

 with longitudinal canals. 



The superficial canals are also present on the convex summit of tha 

 stem (see Fig. 8), and form there a plexus of canals, with similar 

 relations to those above described in the cylindrical portion of the stem. 

 In this portion of the stem the canals embrace or pass round each 

 ccelenteron at a distance of about "2 mm., and communicate by means 

 of branches with the ccelenteron and with the neighbouring longitudinal 

 canals. The 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 superficial canals 

 frequently communicate with the neighbouring ccelentera and longi- 

 tudinal canals. Thus, by means of branch canals, this superficial 

 system of canals is placed in communication with the remaining cavities 

 lined by endoderm, and by means of strands of cells is placed in com- 

 munication 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 present 

 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 out- 

 wards and inwards of one of these canals, the endoderm and lumen of 

 the canal forming respectively the endoderm and ccelenteron of the 

 young polyp (see also p. 222). 



