40 THE VOYAGE OF H.M.S. CHALLENGER. 



nearer to the external surface ; for here the meshwork is much closer, and the mass of 

 soft living tissue much greater in proportion to the calcareous structures secreted by it, 

 than is the case in the deeper regions. Further, the deeper canals are of greater calibre 

 than those nearer the surface. Towards the deepest regions of the coenosarc the canals 

 are shrunken and atrophied, and pass off into effete and almost dead fragments of tissue, 

 which form the inner boundary of the living lamina. 



The largest trunks of the meshwork are those which proceed directly from the bases 

 of the zooids and gonophores. These are soon reduced in size by branching, and are lost 

 in the general anastomosis. 



Around the sacs containing the zooids the canals of the coenosarc have a special 

 radiate disposition (PI. X. fig. 3). The radial canals occupy an area circular in outline 

 extending all around the outer sides of the sacs of the zooids. They pass directly inwards 

 radially, from the margins of the areas where they take origin from the general meshwork 

 to join themselves on to the walls of the zooid sacs, towards the centres of the areas. 

 They branch but seldom on their course, and then only towards their outer origins 

 where they not uncommonly bifurcate. 



As may be seen from the figure, the radial canals, which lie at successive depths 

 from the surface, do not correspond in any way in position with those above or below 

 them, but are quite irregular as far as radial disposition is concerned. In vertical 

 sections, however, of the living lamina (PI. III.), these radial canals are seen to succeed 

 one another at tolerably regular intervals, in vertical disposition, with a somewhat regular 

 series of interspaces between them. 



This radial disposition of the canals is less marked around the sacs of the larger 

 dactylozooids than around those of the gastrozooids, and is hardly apparent around those 

 of the smaller dactylozooids. Traces of it are to be seen around the sacs of the gono- 

 phores, as at G', Plate XXXVI. Although towards the periphery of the area occupied 

 by them these radial canals contain endoderm cells, and appear similar in structure to 

 the other canals of the coenosarc, towards their inner extremities, where they join the 

 zooid sacs, they become diminished in size, and often appear as mere slips of transparent 

 tissue having a fibrillate appearance. 



Muscular filaments, continued from ovoid muscular cells embedded in the walls of 

 the zooid sacs, pass outwards along the radial offsets, and are attached to them in the 

 region about the mouths of the sacs (PL X. fig. 3, EM). 



Attached to the radial offsets, and often extending over the interspaces between 

 adjacent ones, slips of a fine transparent membrane, containing minute nuclei and 

 striated in appearance as if composed of fine fibrillar, are constantly to be seen ; but 

 they seem to occur at altogether irregular intervals, and only towards the more super- 

 ficially lying parts of the zooid sacs (PL X. fig. 3 ; PL III. AA). 



A continuous layer of ectodermal tissue extends over the outer surface of the coral. 



