OF THE SPONGIAD^E. 23 



and it is so difficult, in some cases, to determine which of 

 the two, or whether both, are designed in the structure of 

 the spiculum, that I have not made a distinction between 

 presumed offices indicated by their structure, but have 

 classed the whole under the designation of the defensive 

 spicula. 



When the defensive spicula are internal they usually 

 assume a different character from the external ones. The 

 most common form under these circumstances is that of 

 a short, stout attenuato-acuate spiculum, profusely and 

 entirely spined (Fig. 289, Plate XVII) ; they are firmly 

 based in the substance of the skeleton ; and the greater 

 portion of their length is projected at various angles from 

 the sides of the interstitial canals and cavities of the sponge. 

 They would thus render the passage of minute annelids 

 and other small enemies extremely difficult; and in one 

 instance, the mode in which the protection of the interior of 

 the sponge is provided for is very remarkable and curious. 

 Large spinulo-recurvo-quaternate spicula with attenuating 

 radii are grouped together on the angles of the network of 

 the skeleton, and are projected in a radiating manner into 

 the cavities of the interior of the sponge, forming a most 

 effectual prevention to the passage of any small animal 

 (Fig. 292, Plate XVIII). The occurrence of this complicated 

 and beautiful form of spiculum is a singular deviation from 

 the normal mode of defence, and almost induces the belief 

 that it was intended that such intruders as effected an 

 entrance were meant to be retained, and their decomposed 

 particles appropriated to the nutrition of the sponge. In 

 other cases, where no definite form of defensive spiculum 

 forms a part of the sponge, the office of those organs is 

 frequently performed by the projection of spicula similar to 

 those of the skeleton into the canals and cavities of the 

 interior. 



If I were to attempt to enter upon a description of 

 every variation in the mode of the application of spicula 

 to defensive purposes, it would extend this portion of the 

 subject to a greater length than we can afford under the 

 present circumstances. I shall therefore confine my 

 observations to a description of the general principles of 



