34 
THE VOYAGE OF H.M.S. CHALLENGEE. 
sponge. The mesoderm consists of sarcenchyma, except about the spicular tracts, where 
it passes into collenchyma. 
The Canal System. — The dermal membrane is perforated by pores which lead 
directly into the subdermal cavities, from these canals are continued inwards through the 
cortex and descend radiately into the choanosome, where they soon lose all regularity of 
direction, and finally communicate with the flagellated chambers, which vary from 0’0276 
to 0‘0395 mm. in diameter, and open into the excurrent s37-stem by narrow aphodal canals. 
S'pieules. — The cortex is without special oxeate spicules, but the radiating spicular 
fibres are well developed, and already present traces of a spiral arrangement. They are 
composed of all the forms of spicules which are present in the adult. The oxeas do 
not call for special remark, but the anatrisenes present a very interesting series of forms; 
the simplest are terminated distally by a swollen bulb-like thickening (tylote), within which 
the axial fibre ends simply, or in a slight enlargement ; in other cases small processes 
representing the axial fibres of cladi proceed from it, but end within the bulb, the cladi 
themselves being absent (PI. V, fig. 15). Stages in which only one or two cladi are 
present, occur along with others in which all three are developed. The cladi extend at 
right angles to the rhabdome for a considerable part of their course, and are suddenly 
bent back near the ends. The protrisenes are similar to those of the adult, though cases 
in which only two cladi are present appear to be more numerous. Microscleres which 
are absent in the adult do not appear in the embryo. 
Spicule-Cells or Sclerohlasts (PL II. fig. 20). — These somewhat fusiform darkly 
staining granular cells are very numerous and well displayed. They are about O’Ol mm. 
broad by 0'023 mm. long, and the contained granules, which are spherical, are about 
0'002 mm. in diameter. Granules similar in size and appearance can in many cases be 
traced from the cell along the whole length of the spicule, showing that the spicule does 
not simply penetrate the scleroblast, but lies wholly immersed in it. In the case of the 
anatrisenes the film coating the rhabdome thickens over the cladome into a mass of 
protoplasm, completely embedding it up to the points of the cladi. 
Planula Stage (PI. XL. fig. 5). — ^The preceding embryos, fully developed, but 
for the absence of an oscule which I could not find, are large and consistent enough to 
be separated from the mother sponge and sliced separately ; less developed embryos are 
also present in the basal part of the mother sponge, but they so readily fall to pieces that 
they can only be sliced along with the tissue in which they occur, and as this is traversed 
by stout spicules, it is very difficult to obtain slices as perfect as could be wished. I 
have! succeeded, however, in preparing serial sections of the planula of this sponge, the 
only other stage in its development I have met with. It may be added that ova were 
not observed in any part of this preparation. 
The planula is egg-shaped, about 0‘8 mm. long by 0'5 mm. broad where broadest. 
The hypoblast consists of oval masses, often polygonal by appression, about 0’065 by 
