OF THE SPONGIADjE. 



the spiculum is simply required to give strength and firm- 

 ness to the skeleton, "as in the greater number of the 

 Halichondraceous sponges, the whole interior of the spiculum 

 becomes rapidly filled with silex ; but where strength is 

 required to be combined with great elasticity and tough- 

 ness, the mode of deposit is especially adapted to the 

 requirements of the occasion ; the amount of the silex 

 deposited is small, and confined wholly to the surface, while 

 the interior appears to be filled with keratode. These laws 

 of deposit will perhaps be best illustrated by my detailing 

 a series of experiments I made by the incineration of the 

 spicula of various sponges in the flame of a small spirit- 

 lamp. I was led to this series of experiments by frequently 

 observing during the course of my investigations the great 

 amount of flexure that many of the large and long spicula 

 would sustain without fracture, and the perfect elasticity 

 with which they regained their original form and position > 

 Thus, in mounting the spicula of Tethea cranium in Canada 

 balsam, the long and slender porrecto-ternate defensive 

 spicula projected from its surface would frequently have the 

 shaft bent in a series of sigmoid curves or even loops ; and 

 the thickest portion of the same spicula, while in their 

 natural condition, may be bent down to the surface of the 

 sponge, from which they spring at right angles, so as to 

 form an arc of the third of a circle with perfect impunity. 

 This great flexibility appeared to me to be so incompatible 

 with a purely siliceous structure, that I determined to select 

 the spicula of Tethea cranium, more especially to work out 

 this problem, and from the large size of those of the 

 skeleton fasciculi they are more than usually favorable for 

 the purpose. If we view these spicula in their natural 

 condition, mounted in either water or Canada balsam, by 

 transmitted light and a linear power of 150, they present 

 all the usual appearances of solid siliceous spicula ; there is 

 a small central tubular cavity, and the substance of the 

 spicula intervening between it and the external surface 

 presents to the eye the linear appearance that characterises 

 a deposit in concentric circles ; and the fractured ends have 

 precisely the same aspect that filaments of the same size of 



