STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 523 



composed of styli the term " stylodragmata " might be applied 

 to them. The presence of the stylodragmata in Quasillina, 

 while they are absent in Ridleia, is probably to be attributed 

 to the much less compact character and consequently greater 

 need of support of the choanosome in the former Sponge. 



The Spicules.— The spicules are nearly all styli (i.e. 

 monactinal spicules simply rounded off at the base and pointed 

 at the apex), but occasionally the base is swollen into a head, 

 when the spicule becomes tylostylote. 



They are, as already indicated, of two chief kinds. 



(1) Large, straight, or slightly curved, fusiform styli, usually 

 gradually sharp-pointed at the apex and narrowing consider- 

 ably towards the base. These measure up to about 0*9 mm. 

 in length, with a maximum diameter of 0-0144 mm. 



(2) Small, slightly curved styli, very gradually sharp-pointed 

 at the apex, but not so markedly fusiform as the large ones. 

 These spicules are all of pretty much the same length, viz. 

 about 0*24 mm., but the variation in thickness is very remark- 

 able. Both stout and slender ones occur mixed up promis- 

 cuously in the surface tufts, the stout ones measuring about 

 0*0096 mm. in diameter, and the slender ones only about 

 0-002 mm. The two sizes appear to keep fairly distinct 

 from one another, and one cannot help being struck by the 

 general absence of intermediate stages. The stouter ones 

 appear much more distinctly fusiform than the slender ones. 



The Ec to so me. — The ectosome consists of a somewhat 

 granular but jelly-like matrix, with small, nucleated, multi- 

 polar cells embedded in it. It stains more deeply towards the 

 outside, and on the extreme outside I have detected what 

 appear to be traces of a single layer of flattened epithelial 

 cells. This epithelium appears also to line the subdermal 

 cavities in the ectosome. Fibrous tissue appears to be entirely 

 absent, at any rate in most parts. 



Here, however, we are met with a considerable difficulty in 

 deciding the exact limits of the ectosome. In the wall of the 

 oscular tube there is a very well-developed system of fibrous 

 tissue, which seems hitherto to have entirely escaped observa- 



