44 A MONOGRAPH OP THE 



reticulation are very small in comparison with those of the 

 skeleton, their spread being not more than about one fourth 

 part that of the latter, and the radii are slender and deli- 

 cate in proportion to their diminished size. The interstices 

 of the network are small in proportion, about equal in then- 

 area, but irregular in their form. The excurrent canals 

 are variable in size, but usually rather large, and they are 

 irregularly congregated about the middle of the sponge. 

 They originate near its base, and gradually increase in 

 diameter as they progress in nearly straight lines to its 

 distal extremity ; occasionally, but rarely, a canal may be 

 observed to emerge in a lateral direction. The parieties of 

 these canals are amply provided with defensive spicula; 

 they are of three descriptions. The normal form is that 

 of a stout equiangular spiculated triradiate spiculum ; the 

 spicular ray is not so long as either of the other rays, but 

 is stout and usually slightly curved towards the oscular 

 orifice. The other two forms may be considered as sub- 

 sidiary defences ; they are spiculated rectangular triradiate, 

 aud simple rectangular triradiate ; the rectangulating ray 

 in both is very short, usually not exceeding about one 

 fourth the length of either of the other rays. The defen- 

 sive organ in the first of these two forms is the spicular 

 ray ; in the second, it is the short angulating ray of the 

 spiculum, the two long rays being usually disposed in a 

 line with the long axis of the canal, while the short angu- 

 lating ray is projected at about right angles to the surface 

 of the canal; both these forms are much more slender 

 than the equiangular defensive spicula, which are of about 

 the same " size as those of the skeleton. Immediately 

 beneath the surface of the sponge there are very large 

 fusiformi-acerate spicula disposed singly at nearly equal 

 distances from each other, and in lines at right angles to 

 the surface. Their diameter is three or four times that of 

 the largest sized skeleton spiculum, and their length at 

 least twice as great as the space embraced between any 

 two of their angulating radii. Their distal apices do not 

 appear to pass through the dermal membrane, but are 

 usually slightly below it. A few of these largfe spicula 



