40 ANATOMY AND PHYSIOLOGY 



not otherwise possess. These spicula are sometimes of the 

 same form .as those of the skeleton, as in Halichondria 

 panicea, Johnston, where we find them thickly, but irregu- 

 larly dispersed on the inner surface of the dermal mem- 

 brane. In some cases they are not readily to be distin- 

 guished from those of the skeleton, as they are frequently 

 so nearly of the same size, and are intimately intermingled 

 with them, as in the genus Hymeniaddon ; but in others, 

 as in some species of CJialina and Isodidya, they may be 

 distinguished by their position, and by the total absence of 

 keratode around them, while those of the skeleton are al- 

 ways more or less coated by that substance. 



In other species they differ materially in form and pro- 

 portion from those of the skeleton. Thus in Halichondria 

 incrustans, while the skeleton spicula are stout, short, en- 

 tirely spined and acuate, as represented by Pig. 28, Plate I. 

 The tension spicula are smooth, slender mucronato-cylin- 

 drical, as represented by Fig. 92, Plate IV. They are fre- 

 quently dispersed on the dermal membranes, much in the 

 same manner as they are on the interstitial ones, abounding 

 most where the areas are largest, and where the areas are 

 small they are few in number or entirely absent ; but in 

 other cases, as in the dermal membrane of Halichondria 

 incruslans, they are congregated in flat broad fasciculi, 

 which are disposed on the membrane with little or no ap- 

 proximation to order. 



In the interstitial membranes the same object is fre- 

 quently attained by the incipient skeleton spicula, and we 

 often find either very young and minute skeleton spicula in 

 the membranous areas of the network of the skeleton, or 

 there will be one or more spicula very little less in size 

 than those of the skeleton, imbedded in the surface of the 

 membrane, but quite unconnected with the surrounding 

 skeleton ; or occasionally connected by one termination 

 only, but ultimately by the development of other spicula, 

 becoming incorporated with, and forming part of the skele- 

 ton. And it is not in the Halichondraceous sponges only 

 that the tension spicula occur, for we find them abundantly 

 dispersed in the dermal membrane of one of the Turkey 



