STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 339 
gelatinous tissue, composed of anastomosing strands of trans- 
parent jelly-like substance, containing small nuclei here and 
there. It is doubtless owing to the presence of this layer of 
very delicate tissue that the wall of the oscular tube can be so 
readily peeled off from the underlying structures. 
( b ) A much thinner layer of deeply-staining fibrous tissue, 
in which the fibres are closely packed and arranged circularly 
around the oscular tube. Judging from its position and the 
arrangement of its component fibres, it seems probable that 
this layer may be muscular, and its fibres myocytes (Sollas, 17), 
which by their power of contraction serve to regulate the 
diameter of the oscular tube. The wall of the oscular tube is 
smooth on the interior, and devoid of diaphragms or special 
circular sphincter muscles, such as occur in many Sponges. 
(Diaphragms are well developed in the genus Spirastrella, 
and circular sphincter muscles in Quasillina; both doubtless 
serve the same function. The condition of Stelospongus in 
this respect is comparable to that of Ridleia (cf. 9) ). 
(c) A continuous layer, only about one cell thick, of cysten- 
chyme. This layer may best be studied by peeling off portions 
of the wall of the oscular tube and preparing and mounting 
them without embedding, for cystenchyme appears to be a very 
delicate tissue, which suffers greatly in the latter process. 
In preparations stained with borax carmine, and mounted in 
the usual way, the cystenchyme is seen to form a continuous 
layer, about one cell thick, of closely packed cells which have 
become somewhat polygonal from mutual pressure. Between 
the cells a deeply-staining, granular, intercellular substance is 
present, and the structure of the individual cells is the same as 
that which I have already described in the case of the ectosomal 
cystenchyme. 
This layer is at first sight deceptively like a layer of large, 
flattened, epithelial cells, and I at first mistook it for such ; 
but the characters of the component cells made me doubt if this 
could be so, and on cutting sections its real nature became 
readily apparent. The individual cells measure about 0 - 03 mm. 
in diameter. 
