OF THE SPONGIAD.fi. 29 



within the oscula and other spaces requiring such pro- 

 tection ; but when especially formed for and appropriated 

 to defensive purposes, they are always free from spines and 

 usually terminate acutely ; and they are frequently pro- 

 vided with widely extended basal radii, so as to fix them 

 rigidly and firmly in their proper positions, as exemplified in 

 the various forms of spiculated triradiate spicula represented 

 by Figs. 85, 86, and 87, Plate IV. 



The best illustrations of the application of the simple 

 defensive spicula are to be found in the cloaca in several 

 species of Grantia, as in G. ciliata, Johnston, and G. 

 tessettata and ensata, Bowerbank, MS. In all these 

 species this great central cavity is abundantly furnished 

 with spiculated triradiate spicula, which are so disposed 

 that while the basal radii are firmly cemented on the 

 surface of the cloaca, the spicular or defensive rays are 

 projected from its surface, not at right angles to its plane, 

 but always at such an inclination towards the mouth of 

 the cloaca as to present a combined series of sharp points 

 in the best possible position of defence, so that an 

 intruding assailant could scarcely escape being seriously 

 wounded by them, while a retiring enemy would pass 

 with impunity over their inclined apices. In some species, 

 as in G. tesselhita, the defensive ray is naturally curved to 

 the desired angle for defence (Fig. 86, Plate IV), and it is 

 also of such a form as to be readily released from the 

 creature it has wounded, either by being attenuato-acuate 

 or ensiform, as in Fig. 85, Plate IV, from G. ensata, and 

 as represented in situ by a small portion of a longitudinal 

 section of the cloaca of a specimen of Grantia tessettata 

 in Fig. 286, Plate I, in which the defensive radii are all 

 curved in the direction of the mouth of the cloaca. 



In the second division the internal defensive spicula 

 are usually short and straight, and more or less covered 

 with strong conical acutely pointed spines, projected either 

 at right angles to the axis of the spiculum, or recurved 

 considerably towards its base ; generally speaking the 

 spines are dispersed on all parts of the spiculum without 

 any approach to order, as represented in Fig. 66, Plate III, 



