134 Mr. W. J. Sollas on the 



cortical crypts. The crypts are separated from each other 

 by a number of fleshy pillars traversed by the shafts of long 

 acerate and trifid spicules ; they communicate laterally, to 

 form a subcortical layer of winding passages. 



Under the microscope the transverse section shows an 

 outermost structureless membrane succeeded by a layer of 

 minute stellates*, the two together having a thickness of 

 0-0004 inch (PL VII. fig. 18, a) . A layer of connective tissue 

 with scattered stellates and of variable thickness succeeds. 



The next layer, 0'03 inch thick, consists chiefly of trichite 

 sheaves arranged in packets — the spaces around and between 

 the packets, but not about the separate sheaves, being filled 

 up with gelatinous connective tissue, the corpuscles of which 

 are fusiform (PL VII. fig. 18, b). 



The cartilaginous-looking layer (PL VII. fig. 18, c) before 

 mentioned next succeeds ; it is about 0*03 inch thick, and 

 consists of long fusiform transparent hyaline fibres with a 

 more refringent, faintly bluish, axial thread : these appear to 

 be muscle-fibres, and form variously oriented fascia? lying 

 chiefly in a plane parallel to the general surface of the sponge 

 (PL VI. fig. 3). 



Just within the proximal edge of the preceding or muscular 

 layer is a discontinuous row of large cells, variable but chiefly 

 elliptical in form, and provided with a large oval nucleus 

 containing some fluid and a spherical nucleolus (PL VII. 

 figs. 18,/; & 26). 



The inner or proximal face of the muscular layer is 

 covered by an epithelial membrane bearing round nuclei. 



The " mark has a very different appearance from that of 

 the gelatinous connective tissue which forms a large part of 

 some sponges; it consists of finely granular protoplasm, which 

 readily stains with reagents : about the borders of the canals 

 it appears fibrous, owing to the presence of a number of 

 granular fusiform corpuscles arranged in parallel order ; fur- 

 ther away from the canals nuclei present themselves similar 

 in appearance to those which occur in the cells on the inner- 

 most face of the muscular layer ; and in some cases the outlines 

 of large elliptical cells can be traced about these nuclei ; but 

 more often the borders of the cells are obscure (PL VII. 



The pillars of the crypts are chiefly continuations upwards of 

 the mark ; but they also contain muscular fibres, lying longi- 



* Whether the external membrane represents a layer of plate-like 

 epidermis, or whether it and the stellates together constitute the epider- 

 mis, is bv no means clear. The stellates have much the appearance of 

 being the contents of epidermal cells. 



