STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 191 



Sycon (figs. 2— 8). 



Here, again, we meet with distinct gastral and tubar skeletons_, 

 as shown in fig. 2, and the general plan of the skeleton is the 

 same as in Sycetta. Quadriradiates are usually, if not invari- 

 ably, present in the gastral skeleton, the apical ray projecting 

 freely into the gastral cavity (figs. 2, 4, 7), and doubtless 

 serving as a protection against the ingress of parasites. The 

 gastral skeleton may be very strongly developed so as to form 

 a thick cortex, which may be continued inwards along the 

 exhalant canals, as in Sycon boomerang (fig. 7). Where 

 this is the case — but it is very rare — it seems to me to indicate a 

 folding of the wall of the gastral cavity. The tubar skeleton is 

 always "articulate,'^ and the number of ''joints^' which it 

 exhibits depends upon the length of the chambers. Usually 

 the spicules composing it are more or less strongly sagittal and 

 triradiate, but occasionally apical rays may be developed, as in 

 S. boomerang (fig. 7). The position of the spicules is always 

 the same as in Sycetta, with the basal ray pointing away from 

 the gastral cavity; where an apical ray is developed it projects 

 freely into the cavity of the chamber. Usually a special set 

 of spicules, known as " subgastral sagittal triradiates '' are 

 developed, as is well shown in Sycon Carteri (fig. 2). These 

 have the oral rays extended very widely in the outer part of 

 the wall of the gastral cavity, and thus forming a part of the 

 gastral cortex, while the basal ray is usually very long, and 

 points towards the distal end of the chamber in whose wall it 

 lies. These spicules, or at any rate their basal rays, form the 

 first "joint" of the articulate tubar skeleton, which is com- 

 monly a good deal longer than any of the succeeding joints 

 (fig. 2). The subgastral sagittal triradiates may, however, be 

 almost, if not quite, indistinguishable from the ordinary tubar 

 triradiates of the sponge. Usually the three rays of the tubar 

 triradiates do not lie all in one plane, but the oral rays are 

 curved or inclined towards one another, so as to partially 

 embrace the chamber in the thickness of whose wall they lie 

 (fig. 5). 



VOL. 35, PART 2. NEW SER, P 



