22 
THE VOYAGE OF H.M.S. CHALLENGER. 
Around tlie pores in the cloaca! walls myocytes are concentrically arranged, and 
stellate cells occur close to their margins, which send a process to the epithelium on the 
one hand, and one or two processes inwards in the opposite direction. 
Sexual Elements. — Spermatozoa. — Around the lower end of the cloaca the choano- 
some is densely crowded with sperm-clusters, each lying in a distinct cavity, which it 
does not fill, owing no doubt to shrinking produced by immersion in spirits. They are 
of round, oval, or sometimes irregular shape, and vary from 0'006 by 0'012 mm. to 
0‘047 by 0‘075 mm. in diameter. Most of them consist of small spherical cells, about 
0'004 mm. in diameter, with a very obvious nucleus and a dark-stained nucleolus. 
They are therefore not mature, though some few instances occur in which the con- 
stituent cells are of smaller size, and appear to be produced into a tail. A cover-cell 
is certainly not present, but the wall of the containing cavity is lined by epithelium, and 
the adjacent mesoderm stains more deeply in its immediate neighbourhood than elsewhere. 
The sperm-clusters are so numerous that they reduce the surrounding mesoderm to 
little more than a trabecular network, in which flagellated chambers are only rarely 
seen. It would therefore seem that we have here a specialised sperm-bearing region or 
rudimentary testicular tissue, recalling the specialised ova-bearing region or rudimentary 
ovary which Schulze discovered in Euspongia. 
Ova . — Certain remarkable cells of great size and complexity occur here and there 
in cavities of the choanosome; their true nature is doubtful, and if not ova, they must 
be regarded as parasites. They present numerous variations both in form and structure. 
Some suggest a resemblance to gigantic Ehizopoda, extending into large branched 
pseudopodial extensions (PI. IV. fig. 22), which enter the surrounding tissue, and lose 
themselves in it. Others (PI. IV. fig. 21) present a round or oval outline, varying 
from about O'l mm. to 0'2 mm. in diameter. The nucleus is an oval body, 0‘0395 mm. 
long by 0’0276 mm. broad, with evenly and finely granular, faintly-staining contents, 
enclosed in a well-defined limiting membrane. It always presents at least one well- 
marked, highly refringent, non-granular, deeply-stained, spherical nucleolus, varying 
from 0’0039 to O'OllS mm. in diameter. Often, however, two or three nucleoli 
are present, and in one instance no less than six were counted. Outside this nucleus, 
immediately next the nuclear membrane, succeeds a layer of dense, darkly-stained 
protoplasm ; then a clearer zone follows, in which a vesicular structure can sometimes be 
made out ; outside this again is a dense, darkly-stained margin, which is sometimes so 
distinct as to look like a second nuclear envelope ; finally, outside this succeeds the 
general mass of external protoplasm. While the structure just described is of most 
common occurrence, cases also occur in which it becomes simplified, the nucleus for 
instance simply lying in protoplasm, which is rather more deeply stained immediately 
next to it than elsewhere. 
In general character the mass of external protoplasm of these cells is finely granular 
