4-2 V. F. .TENKTN. 



due to the bent projecting oxea (sec Fig. 2). The gastral cavity, which is nearly 

 constant in diameter throughout its whole length, terminates at the oscule, which is at 

 the top of an oscular collar about l| mm. long. 



The structure of the l)ody-wall is .shown in Fig. 131. 



Canal System. — Tlicre is an iiKuincnt chamber under the dermal layer which is 

 probably continuous round the whole l)ody of the sponge. The dermal layer is 

 supported over this chamljer by columns of spicules, consisting partly of the basal rays 

 of the chiactines, and partly of oxea which project in spreading tufts outside the 

 dermis. The incurrcnt canals, which are small and irregularly branched, lead inwards 

 from the incurrejit chambc'r. The excurrent canals are large and extend radially from 

 the gastral cavity nearly through the l)ody-wall, often branching once. The flagellated 

 chambers are grouped round these large excurrent canals, the larger number opening 

 directly into them, the remainder being connected by short branches. 



The Skeleton. — The skeleton of the body-wall consists of chiactines. There are 

 also a few triradiates scattered irregularly, and numerous minute spined oxea. There 

 is no gastral skeleton proper; the gastral cortex (see Fig. 132) is only supported by 

 the paired rays of the chiactines. The dermal skeleton is made up of triradiates, whic 

 are approximately equiangular, and lie tangentially to the surface. There are also tufts 

 of oxea which project for most of their length and give the sponge its charactcristi 

 " curly " appearance. 



Oscule. — The thin oscular collar is about l| mm. long and the same diameter 

 as the gastral cavity (see Figs. 133 and 134). On the inside are quadriradiates lying 

 tangentially with the basal ray downwards, and outside are triradiates and oxea. 

 There are numerous minute spined oxea scattered amongst the radiates. At the 

 rim (see Fig. 131) the collar is slightly thickened. Tliere is a ring of small 

 (jUadriradiates forming the edge on the inside ; they do not ajjpear to be very closely 

 or regularly packed. The diaphragm extends from the apical rays of these spicules. 

 There is a scanty fringe of prickly hastate oxea of small size. On the outside the 

 oscular collar is thickly covered by large bent oxea of the same type as those forming 

 the tufts on the body, but shorter. They stick in the collar, often projecting through 

 it into the interior, and are directed upwards usually at about 45° with the axis of the 

 sponge ; some lie tangentially along the surface. 



At the junction of the oscular collar and the body the collar thickens, a few small 

 flagellated chambers appear, the quadriradiates begin to turn their basal rays outwards, 

 and the oxea project more radially, a few even pointing downwards. 



Spicules (Figs. 135 and 13G). 



Oxea. — There are three sorts of Oxea : — 

 (rt) Large projecting oxea, irregular but nearly straight for about three-quarters 

 of their length, then bending (at the outer end) considerably to one side 

 by two or three angular deflections. Tlie iinier end is sharply pointed and 



