REPORT ON THE TETRACTINELLTDA. 
27 
strands, whicli branch and unite again in the thickness of the wall. In the depressed 
areas mapped out by these ridges, the pore groups are situated. 
On approaching the lower margin of the oscular tube, the ridges affect a longitudinal 
direction, and expanding on each side as they enter it, give to the outermost row of 
depressed areas the form of arched recesses. 
No pores or intercortical cavities are to be detected over any part of the cortex, although 
they have been searched for in several sections of entire sponges; it would appear 
therefore that the oscules and cloacal chambers, so-called, are some of them incurrent in 
function, and in accordance with this we find, proceeding from some of the cloacal chambers, 
subcortical canals, occupying the same relative position in this sponge that the undoubted 
incurrent canals do in Disyringa dissimilis (see PL XLI. fig. 3). 
Notwithstanding this supposed difierence in function we shall continue to speak of the 
flask-shaped recesses and their openings to the exterior, whether excurrent or in current, 
as cloacal chambers and oscules. 
The minute structures of the “ flasks” has been very carefully studied in transverse, 
longitudinal, and tangential sections, and by teasing. 
Oscidar Tube . — The wall of the oscular tube (about 0‘64 mm. thick in average-sized 
examples) is mainly composed (PI. XXXIX. fig. 2) of concentrically arranged myocytes, 
traversed by a few longitudinal strands of similar cells, from which branches arise at 
intervals and proceed towards the free face in a radiate direction ; as they do so their 
constituent cells, diverge from each other upwards and downwards, giving them a fan- 
shaped outline in section. The concentric myocytes represent the fibrous cortex some- 
what modified ; in the immediate neighbourhood of the oscular wall the cortex is a 
collenchyma with tangentially disposed fusiform fibre-cells running through it in various 
directions ; by the reduction of the collenchyma to an almost imperceptible residuum, 
and the consequent approximation of the fusiform cells, which at the same time fall 
into a concentric arrangement, the chief mass of the oscular tube results. The longi- 
tudinal fibres are derived from the walls of the cloacal chamber, but these are simply 
a part of the cortex invaginated, with fibres of fusiform cells running tangentially 
through them just as in the rest of the cortex* 
The epithelium of the cortex is continued inwards, lining the interior of the flask- 
shaped recess; below it in the case of the oscular tube is a layer of tissue, 
about 0'08 mm. thick, sometimes less, which is of considerable interest, since it 
presents in places structures which are similar to sense-cells (PI. XXXIX. fig. 8). 
These are fusiform, finely granular cells, more deeply stained than the adjacent tissue ; 
they are usually somewhat swollen in the middle, where an oval nucleus with a spherical 
darkly-stained nucleolus is situated ; the outer end is elongate, conical, with a rounded 
point, but no apparent sense-hair, it lies either immediately below the epithelium or 
projects between the epithelial cells, at the inner end it terminates in ono or two fine 
I 
