or THE SPONGIAD^. 127 



admirably calculated to resist the force and motions it has 

 to sustain in such encounters. But these spicula, although 

 exceedingly numerous, are not the only organs capable of 

 retaining intruders into the body of the sponge with which 

 it is furnished, there is in addition numerous large multi- 

 hamate birotulate spicula dispersed in various positions on 

 the sides of the interstitial cavities of the sponge, each of 

 the rotulse consisting of seven or eight stout recurved 

 flattened radii,; which, if immersed in any struggling 

 animali would be capable of sustaining a vastly greater 

 amount of force than many of the spiculated quadriradiate 

 ones combined, could endure without injury ; and that their 

 especial office is that of auxiliary retentive organs, is well 

 demonstrated by the fact that the trenchant edges of the 

 flattened radii are all at right angles to the line of force 

 required to tear away their hold of any body in which they 

 may have been inserted. Thus they appear destined by 

 nature to secure the prey while its own struggles among 

 the lacerating organs contributes to its destruction (Figs. 

 294, 296, Plate XVIII, and Fig. 60, Plate III). 



In the modification of the structure of the contort biha- 

 mate spicula, and their peculiar adaptation to the retention 

 and destruction of intruders within the sponge, which I 

 have described when treating on the internal defensive spi- 

 cula, and which is represented in Fig. 293, Plate XVIII, and 

 Fig. 112, Plate V, we have precisely the same physiological 

 principle carried out, but by means widely difierent from 

 those I have previously described. 



If we consider the whole of these extraordinary organs to 

 w^hich I have referred in relation to each other, we cannot 

 fail to see that, however varied their forms may be, there 

 is every appearance of perfect harmony of design in the 

 purposes they are destined to effect in the economy of the 

 Spongiadse. 



