106 
THE VOYAGE OF H.M.S. CHALLEHGEK. 
collencytes. This tissue extends from the ectosome inwards, forming a wall to the 
larger incurrent and excurrent canals. Near the free surface of the canal walls or of the 
ectosome the granule- cells are crowded closely together in cavities only just large enough 
for them ; but further inwards they are separated by wider intervals, and the cavities in 
which they lie are sometimes several times the diameter of the cells themselves both in 
length and width (PL XL fig. 27). The walls of these cavities are usually very thin, mere 
immeasurable films, in which, however, collencytes can be discerned under favourable 
conditions. The granule-cells in some parts of the tissue lose their granules, and are 
reduced to a finely granular protoplasm, in which a nucleus and nucleolus are to be seen 
(Pi. XL fig. 26). In some cases these cells take a far fainter stain than usual, while the 
surrounding matrix becomes stained, so that, instead of appearing as darkly coloured 
cells on a colourless ground, they are seen as pale cells on a darker ground (PI. XL 
fig. 25). 
The mesoderm of the choanosome does not take a very deep stain ; more gelatinous 
matrix appears to be present than is usual in sarcenchyme, and it might perhaps be 
more properly termed a highly granular collenchyme. 
The Skeleton. — The oxeate rhabdi are collected together into bundles which run more 
or less parallel to the sides of the larger canals, and at right angles to the external 
surface of the sponge ; the tissue surrounding them is not modified to form a spicular 
tract. The calthrops appear, for the most part, to be scattered through the sponge 
without any arrangement, they are densely crowded together, their actines crossing each 
other in all directions. Near the exterior of the sponge, however, two or tliree of the 
actines tend to lie tangentially with the surface, and a similar disposition is to be observed 
in those calthrops which lie adjacent to the walls of the canals. Near the bundles of 
oxeas also the calthrops show a tendency to definite arrangement, three of the actines 
extending more or less at right angles to the course of the bundles, and the fourth 
parallel to it ; in this case the calthrops is orientated in the same manner as a trisene 
would be in a corresponding position. 
The microstrongyles form a dense layer immediately beneath the outer epithelium 
(PL XL fig. 25), and occur in a single layer beneath the epithelium of the canal walls, 
they are also irregularly scattered throughout the tissues of the sponge generally. The 
spirasters appear to be restricted to a position immediately beneath the lining epithelium 
of the canals, occurring in conjunction with the microstrongyles, or occasionally by them- 
selves, in the latter case usually only in the vela of the canals, or in the smaller canals 
of the choanosome. 
In the earliest observed stage of the calthrops (PL XL fig. 4) its actines are traversed 
by disproportionately large axial fibres, which are probably continued into the substance 
of the surrounding scleroblast, since in nitric acid preparations the axial canals are 
freely open at the extremities of the actines. 
