OF THE SPONGIAD.E. 33 



I will not attempt to describe the whole of the numerous 

 variations in the modes of application to defensive pur- 

 poses, but select a few of the most interesting cases as 

 illustrations of the general principles of combined internal 

 defence and aggression. 



The spinulo-recurvo-quaternate spiculum (Fig. 76, Plate 

 III), presents an admirable illustration of the combined 

 defensive and aggressive character of some of those internal 

 defensive spicula. The sponge in which they occur 

 belongs to the Halichondroid tribe, the skeleton being 

 composed of a network of spicula cemented together by 

 their apices, which cross each other at the angles of the 

 areas of the reticulations. The recurvo-quaternate spicula 

 are not dispersed on all parts of the skeleton, but are 

 congregated in groups, frequently consisting of as many as 

 fifteen spicula, the whole of their bases being concentrated 

 on one of the angles of the reticulations of the skeleton, 

 while their shafts and apices radiate thence in every 

 direction into the interstitial spaces of the sponge ; they 

 are thus placed on the strongest and most elastic portion 

 of the skeleton, with their hemispherical bases firmly 

 imbedded in the cementing keratode of the skeleton, 

 which abounds at the angles of the network, and which 

 by its inherent elasticity and strength renders the insertion 

 of the base of the spiculum, in strength and extent of 

 action, quite equivalent to the powers of the ball-and- 

 socket joints in the higher tribes of animals. A small 

 annelid or other minute intruder entangled amidst these 

 numerous sharp hooks would struggle hopelessly in such a 

 situation, as the spicula, from the nature of their attach- 

 ment, would yield readily to its struggles in every possible 

 direction, and at every new contortion arising from its 

 efforts to escape it would inevitably receive a fresh series of 

 punctures and lacerations. 



Fig. 292, Plate XVIII, represents a small portion of 

 the skeleton of the sponge bearing the spinulo-recurvo- 

 quarternate spicula in situ. 



The gradual development of this form of spiculum is 

 interesting and very instructive. In an early stage of 



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