MATERIALS FOR A MONOGRAPH OP THE ASCONS. 571 



the spicule is often very difficult to make out ; but in coriacea, 

 which I studied carefully, I was able to see clearly the state of 

 things in a large number of instances. As the filaments ap- 

 proach the centre they swell out slightly and become less 



Fig. C. — Tracing with the camera lucida of the spicule sheath {sp. sh.) and 

 axial filament (ax./.) of Clathrina coriacea, after treatment with 

 nigrosin and picric acid. X 800. 



definite ; finally they end at a short distance apart, and when 

 seen with insufficient magnification appear independent of 

 each other, but with a high power they are seen to be con- 

 nected by three delicate curved filaments, forming a minute 

 triangle enclosed by curved lines, concave towards the ex- 

 terior. 



This structure was seen by me in all the spicules in which I 

 could make out clearly the central portions of the filaments; 

 but considerable variations were observable in the dimensions 

 of the central triangle, and in some cases it was so small that 

 the filaments appeared to touch each other. I believe that the 

 central triangle is easily explained by the development of the 

 spicule, and the fact that the rays are at first separate from 

 one another. The abrupt termination of the filaments before 

 they meet represents the distance between the rays at their 

 first origin, and the delicate connecting threads represent the 

 secondary union of the three sclerites. I have not had time 

 yet to make extended observations upon the union of the fila- 

 ments in different species, as I hope to do, but I have made 

 and examined a preparation of reticulum, a species which 



