232 
THE VOYAGE OF H.M.S. CHALLENGER. 
of a large oscule appears as a dark circle surrounded by an annulus, — the slightly everted 
margin, which by its snow-whiteness stands in contrast with the slightly brownish tint 
of the general surface ; the smallest oscules are indicated by a white dot, the wall and 
not the lumen of the oscular tube being alone visible. The appearance given by these 
white dots and circles to the oscular surface is very characteristic. The pores. 0‘03 to 
OT mm. in diameter, occur in sieves (PI. XXII. fig. 14) overlying chones, which, like 
the oscular chones, are provided with thick walls of fibrous collenchyma, in which are 
frequently dispersed small oval vesicular cells, 0‘015 mm. in diameter, containing 
numerous deeply stained, minute, spherical granules. Similar cells are scattered through 
the collenchyma and adjacent sarcenchyma of the choanosome, chiefly when it surrounds 
the larger canals. The thickness of the collenchymatous wall and the diameter of the 
lumen of the chone are highly variable, and stand in no constant relation to one another 
(PI. XXII. figs. 15-18). Thus in the case of the poral chones we have the following 
measurements : — In one case, diameter of the lumen 0'019 mm., diameter of the entu’e 
chone from sterraster to sterraster 0’193 mm.; in another, diameter of the lumen 0T29 
mm., of the chone 0T93 mm.; and in a third, diameter of the lumen 0 039 mm., and of 
the chone 0'039 mm., or practically no distinct wall at all. This variation does not 
depend on the state of contraction of the fibrous collenchyma, either it is accidental or 
due to age or some other unknown cause. About the pores, on the one hand, and the 
inner termination of the chone on the other, the fibrous collenchyma passes into true 
myenchyma, the myocytes of which are arranged concentrically, sphincter-like. 
The pore-sieves are more richly scattered over some portions of the porous surface than 
others. The margins of the sieves are surrounded by the sterrasters of the cortex, the 
margins of the pores by the spherasters of its outermost layer. This at least is the appear- 
ance presented by a slice taken tangentially ; as a matter of course the spherastral layer 
extends over the whole exterior of the sponge, but the sterrasters do not extend into the 
poral roof, or do so only occasionally. 
The poral chones lead into incurrent canals, which, like the excurrent canals, are pro- 
vided with collenchymatous walls of very variable thickness ; both sets of canals are 
frequently crossed by velar diaphragms which convert them in many cases into a succes- 
sion of vesicles. Sometimes in the same collenchymatous tract a double row of vesicles 
may be seen running side by side, and communicating laterally at intervals, the result 
apparently of the conversion of an originally single canal into two by an excessive 
development of collenchyma. 
The incurrent canals sometimes descend directly from the chone radially into the 
sponge, sometimes they first extend tangentially to the inner surface of the cortex. 
The oscular chones are similar to the poral but larger, and without the sieve-like roof 
(PI. XXII. fig. 13). They vary in size considerably; a somewhat small example measured 
0'478 mm. in diameter, and the lumen 0‘318 mm. in diameter, the walls in this instance 
