OF THE SPONGIADjE. 31 



the bases of Miplectella aspergillum and E. cucumer, Owen. 

 The long attenuated basal portions of the shaft being 

 without spines, are incorporated with the longitudinal 

 fasciculi of the skeleton, while the apical pontions of them 

 are projected from the base of the sponge, and embrace 

 and hook on to any extraneous mass near which it may be 

 situated ; and this free portion is thickly beset with strong 

 acutely conical spines, reflected at about the same angle 

 and in the same direction as the radii of the quatemate 

 apex, and to which they are auxiliary as prehensile organs ; 

 and as we proceed towards the central portion of the 

 spiculum, the spines successively decrease in length, until 

 at about one third of the length of the spiculum from its 

 apex they become obsolete. I am indebted to my friend 

 Dr. Arthur Farre for the specimen figured of this singular 

 and interesting form of spiculum ; and the only sponge in 

 which they have been found in a perfect state, is the deh- 

 cate and beautiful one designated by Professor Owen 

 Euplectella cucumer. They occur in great profusion, 

 embracing the mass of matter at its base in every direction. 

 I propose, therefore, to designate this form as an apically 

 spined recurvo-quaternate spiculum (Fig. 59, Plate III : a, 

 the apical portion of the spiculum ; b, a portion from that 

 part of the shaft at which the spines become obsolete). 



Defensive Spicula. 



There are two classes of defensive spicula : — 



1st. Those of the exterior, 

 2nd. Those of the interior of the sponge. 



They are neither of them necessarily present in every species, 

 nor are they confined to particular genera, but occur occa- 

 sionally, and in certain species of various genera, apparently 

 as the necessities of the animal may render their presence 

 requisite. If the exterior of the animal be amply supplied 

 ■with them, the interior rarely possess them. Their office 

 is evidently to defend the sponge from the attacks of 

 predacious animals that would otherwise very probably 



