3 1 4 
THE TANGANYIKA PROBLEM. 
The spicular systems of the third class are seldom found in any other position than 
in the fibres. 
As a rule, the spicules are arranged in the fibres with their axes parallel to one 
another, and in the deeper parts of the sponge the connecting spicules are rather 
numerous, and more strongly developed than in the more superficial parts. The 
connecting spicules are usually the most strongly developed spicules in the whole 
sponge as regards size, differing, however, only in thickness from the smooth, curved 
amphitornota which constitute the fibres (Fig. 2 — a . ). Speaking generally, the largest 
spicules of the first class, together with a few of the second and all the third, form 
the fibres and the connecting links between them, while the smaller spicules belong- 
ing to the first, and nearly all those belonging to the second class, are scattered 
about irregularly in the meshes between the fibres. The smallest spicules of all 
seem to be absolutely independent of the skeletal meshwork, and this is the strongest 
argument that can be adduced in favour of the view that they are microscleres and 
not young megascleres. 
The arrangement of the skeleton at large is reticulate. The most general feature 
of the general conformation of the fibres is the way they pass from the surface of 
fixation of the sponge to the dermal membrane which they support Along their 
course from one surface to the other they present a wavy appearance, often dividing 
and again reuniting, approaching the dermal membrane nearly always at right 
angles, and in many cases expanding into a brush-like structure which supports that 
membrane (Fig. 2 — c. ). In some of the largest lobes of the sponge the fibres nearest 
the centre pursue a straight course, while those furthest from that position curve 
outward, so as to form supports to the dermal membrane which covers the flanks 
of these mound-like elevations. Owing to this arrangement a longitudinal radial 
section of one of these lobes presents an almost fan-like appearance, as regards the 
skeletal fibres. 
(C) The Spongin. — All the skeletal fibres of this sponge are enclosed in a distinct 
sheath of spongin, which is greatly thickened at the points where the connecting 
spicules occur, these being either partially or completely surrounded by it (Fig. 2 — b. ). 
Not only are the fibres and the connecting spicules enclosed in a sheath of spongin, 
but the surface of the sponge is covered by a thin layer or cuticle of the same sub- 
stance, which dips down between the cells of the dermal membrane and communi- 
cates with that which envelopes the fibres (Fig. 2 — a.). 
(3) The Canal System. — Owing to the fact that the material which had been 
preserved for histological study of the sponge had been shaken considerably in 
moving from place to place, a great number of cells had, apparently, become loose, 
and were found lying in the spaces of the canal system. In consequence it was 
impossible to make a complete and thorough study of that system, though individual 
cells were in many places nicely preserved ; nor is Spongilla , for other reasons, a 
favourable object for the study of the canals in the Monaxonida. 
The canal system in Spongilla moorei belongs to the type usually inscribed as the 
third. The dermal pores, which are situated on the flanks of the mound-like 
elevations of the surface and in the intermediate depressions, are small, and open 
into the subdermal cavity, which is lined by flattened epithelium, and considerably 
reduced by the passing through of the skeletal fibres, which are enclosed in a sheath 
of spongin, which is covered by cells of the epithelial layer. 
