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THE VOYAGE OF H.M.S. CHALLENGER. 
spherical part, from which a narrow, cylindrical neck projects into the interior of the 
cavity, the length of the whole structure being 0‘0118 mm. 
The vesicles are frequently crowded together so as to reduce the matrix to very thin 
partitions (PI. XXXIV. fig. 12). Numerous granules are dispersed through the matrix, 
and it takes a distinct stain with hsematoxylin ; branching and fusiform collencytes, 
with nuclei like those seen within the vesicles, are also present. The tissue is therefore 
a collenchyma containing vesicular cells, similar to those which occur in Pleroma and 
other Lithistids. The vesicular cells are, 1 am inclined to think, residual cells, developed 
from smaller but similar cells, which are scattered through the matrix, and which can be 
traced downwards to a size not exceeding 0'008 mm. in diameter. It appears possible 
that they may have been filled when fresh with some kind of oil, since in Pachymatisma, 
which possesses similar cells, multitudes of oily globules immiscible with water, are set 
free on cutting through the cortex. If this oil were soluble in alcohol, the oil-bearing 
cells in spirit specimens would resemble the vesicular cells above described. The 
choanosome is distinguished from the ectosome, partly by the presence of flagellated 
chambers, partly by the character of the mesoderm, which consists of sarcenchyma. 
There is no sharp line of demarcation between the two regions, however, and vesicles 
precisely similar to those of the ectosomal tissue extend from it for some distance into the 
sarcenchyma. The sarcenchyma also contains numerous problematical little bodies, as 
much as 0‘004 to 0'005 mm. in diameter, subangular in outline, apparently homogeneous, 
and very darkly stained by hsematoxyliu. The flagellated chambers (PL XXXIV. fig. 10) 
are small, about 0‘024 to 0‘031 mm. broad, by 0‘018 to 0*024 mm. long; they open into 
narrow aphodi, and are supplied by short, wide prosodi. The choanocytes present 
comparatively large, deeply stained, basal portions, about 0*0035 mm. wdde by 0*004 
mm. long, the collum appears to present double-contoured walls, and ends in the usual 
fenestrated diaphragm ; the total length of the choanocyte is 0*012 mm. 
Formation of the Desma (PI. XXXIV. figs. 3-6). — In slices cut from specimens frozen 
in gelatine jelly, several young desmas are seen lying close to the outer epithelium 
bounding the ectosome. They are granular throughout, and are separated from the sur- 
rounding collenchyma by an interval which is partially filled by dark, granular, deeply- 
stained protoplasm, forming an incrusting layer 0*009 mm. in thickness, and containing 
oval nuclei 0*008 mm. in length, with spherical nucleoli. This darkly -stained material 
is in striking contrast to the surrounding collenchyma, and evidently associated with the 
young desmas, so that I am inclined to regard it as representing a layer of scleroblastic 
cells. 
