130 
THE YOYAGE OF H.M.S. CHALLEHGEE. 
passes about the inner ends of the ectochones into a fibrous tissue in which the fusiform cells 
run tangentially, and are variously orientated. The remaining part of the cortex is that 
which surrounds the tangential canals, the representatives of the endochones and crypts 
of other sponges ; it is partly collenchymatous and partly fibrous in constitution. 
In addition to the elements just described, clusters of granule-ceUs are present, 
chiefly distributed immediately below the outermost layer of fibrous tissue ; they vary 
from about OT by 0'06 mm. to about 0'2 by 0'18 mm. in diameter, though some are 
smaller. Each cell, about 0'016 mm. in diameter, is wholly composed of unstained, 
colourless granules of remarkably uniform size. Between the cells is a thin layer of 
darkly stained protoplasm, which separates them from each other, forming an inter- 
cellular network throughout the cluster. A variable number of the cells, sometimes 
more, sometimes less, are of very difierent characters to the others ; remaining of the 
same size, they present a well-stained oval nucleus about 0‘005 mm. in diameter, enclosing 
a minute spherical nucleolus, and surrounded by a film of darkly stained protoplasm, 
which extends in radiating processes to the periphery of the cell ; or, instead of this 
protoplasm, a number of isolated, deeply stained spherical granules may be present. 
The granules of the unstained cells, on the other hand, are so numerous that they lie 
contiguous in a solid mass. 
The ectosome is continued as a membrane 0T6 to 0T9 mm. thick over the cloaca, 
and forms a rounded margin to the oscule (PI. XIV. fig. 14). This oscular membrane 
is very similar in composition (PI. XIV. fig. 15) to the rest of the ectosome, consisting 
of a stained matrix traversed by fusiform cells, which take chiefly a concentric direction 
around the oscule, but are also partly radiately arranged. Isolated granular cells and 
clusters of these cells are also present, and, at a little distance below the outer epithelium, 
the minute vesicles previously mentioned. On the upper surface of the membrane, near 
the oscular opening, these are very clearly seen, with one fibril running to the outer 
epithelium, and the other losing itself in the general tissue. The tissue of this oscular 
membrane extends down the sides of the cloacal tube as a lining layer, 0‘26 mm. thick. 
This presents below the outer epithelium first a layer of chiasters, then of fusiform 
cells longitudinally arranged, altogether about 0‘03 mm. thick ; this is underlaid by 
clusters of granule-cells, forming a layer, traversed by fibrous strands, of about 0'05 mm. 
thick. The rest of the cloacal wall, about 0T9 mm. in thickness, consists of fibrous 
tissue, the fibres running both longitudinally and concentrically. The walls of the 
larger canals are also formed chiefly of fibrous tissue and associated granule clusters ; 
these fibrous walls are frequently 0‘1 mm. in thickness. 
The Choanosome . — The mesoderm is a typical sarcenchyma, with the sarcencytes very 
clearly defined as small polygonal cells about 0‘012 mm. in diameter. In some of them 
young forms of asters, about 0‘004 mm. in diameter, are well displayed ; the central part 
of the cell, as far as the ends of the actines of the aster, is of much paler colour than the 
