10 ANATOMY AND PHYSIOLOGY 



brown tint, indicating the existence of but a very small 

 amount of animal matter within. This result might be 

 expected ; the spicula, being imbedded in the keratose fibre 

 to give it additional firmness and strength, are not required 

 to be elastic ; they are therefore short, comparatively stout, 

 and sohd in their structure. 



A specimen of Halichondria panicea, Johnston, burned 

 in the flame of a spirit-lamp to a white heat, exhibited no 

 alteration in the mature spicula, in many of which I could 

 not detect a central tubular cavity ; and I presume in these 

 cases the spicula were entirely filled with silex, as in 

 younger spicula it was more or less apparent. When the 

 cavity was very small, the colour had a very faint tinge of 

 brown, and, as in other cases, when the cavity increased in 

 diameter, the amount of colouring matter produced by the 

 incineration of the animal matter within became greater 

 and deeper in its tint, until in the young and immature 

 spicula the internal cavity occupied the greater part of its 

 diameter, and it became perfectly black and opaque; and in 

 one spiculum the gaseous matter generated within expanded 

 one part of the spiculum to such an extent as to cause it 

 to resemble exactly a hydrometer in form. 



The result of the incineration of Halichondria incrustans, 

 Johnston, was very similar to that of Hal. panicea. The 

 adult spicula remained unaltered, and the central canal was 

 rendered more apparent than it was before. 



On burning portions of Spongilla fiwoiatilis and lacustris, 

 Johnston, and of Spongilla cerebellata, Bowerbank, I found 

 the results were similar to those obtained from Halichondria 

 panicea and incrustans, as regards the spicula of the 

 skeleton ; but in the small spinous spicula investing the 

 ovaria of the last-named species there was no apparent 

 alteration, nor could any indication of a central cavity be 

 seen. 



The calcareous spicula of Grantia compressa withstood 

 incineration better than I expected. The surface was studded 

 with numerous little vesicles, generated by the heat, and 

 which interfered with their transparency ; but they re- 

 tained nearly their original colour and proportions, and it 



