280 PLATE LXXXIII. 



To tlie unassisted eye the surface of the sponge 

 appears quite smootli, nor even when portions of it 

 are mounted in Canada balsam, and examined with a 

 power of 100 linear, are there any projecting spicula 

 visible. When thus examined the numerous long, 

 slender acuate tension spicula are seen crossing each 

 other in every possible direction in the same plane, 

 and in their irregular interstices there are mimerous 

 external defensive and retentive attenuato-cylindrical 

 spinous spicula in a recumbent position. No two of them 

 are precisely alike in form and size, and the variation 

 in the size and disposition of the spines upon them is 

 equally remarkable ; they are slender and acutely ter- 

 minated, and are frequently twice the length of the 

 diameter of that part of the spiculum whence they 

 spring. Occasionally the distal extremity of the spicu- 

 lum terminates with one of these large spines, and the 

 spiculum then assumes the attenuato-acuate form. 

 These remarkable spicula constitute the prominent 

 character of the species and when separated by boiling 

 nitric acid and mounted in Canada balsam they require 

 a power of about 200 linear to render their pecuharities 

 of structure distinctly to the eye. When a thin section 

 of the sponge at right angles to the dermal surface is 

 mounted in Canada balsam, their position amid the 

 tissues is different from what it is when attached to 

 the dermal membrane ; there they appear always to be 

 recumbent, performing the offices of both retentive and 

 defensive spicula, but within the interstices of the skele- 

 ton they are almost always projected at nearly right 

 angles from the sides of the skeleton fasciculi, and in 

 this position they are purely internal defenses ; occa- 

 sionally, but very rarely, a recumbent one may be 

 observed on the interstitial membranes ; these spicula 

 and the long slender tension spicula so abundant on 

 tlie dermal membrane are both of comparatively rare 

 occurrence in the interstitial ones. 



I could not detect any other form of spiculum in the 

 sponge than those which I have described. 



