OF THE SPONGIADiE. 85 



spicula of the sarcode. But the most singular point 

 in their structure is, that while the curved portion of 

 the hami and the middle of the shaft are perfectly cylin- 

 drical, the inner portion of the hooks and those parts 

 of the shaft immediately opposed to them present sharp 

 trenchant edges, so that each hook assumes to some extent 

 the form of spring hand-shears. The acute termination of 

 the hook and the opposed trenchant edges exhibit every 

 facility for effecting an entrance through the tough skin of 

 the victim, while the perfectly blunt and cyhndrical state of 

 the arch of the hook bespeaks the design of retention as 

 well as of destruction. As soon as the hook has penetrated 

 to the inner blunt surface of the curve it no longer cuts, 

 and the prey wounded in every direction is securely 

 retained for the nutrition of the sponge. This result is 

 not indicated only by the form of the spiculum ; their 

 position in the structure of the sponge bespeaks their 

 ofl&ce equally unmistakably. They are not immersed in 

 the sarcode like their congeners in form, but are firmly 

 cemented by one hook to the reticulating lines of the 

 skeleton, while the other ends are projected at various 

 angles into the interstitial cavities of the sponge in such 

 numbers and in such a manner, that it would be next 

 to impossible for an intruder within the sponge to escape 

 being entangled and destroyed amongst them. Fig. 293,. 

 Plate XVIII, represents a portion of the reticulated 

 skeleton of the sponge with the trenchant contort bihamate 

 spicula in situ, magnified 50 linear ; and Pig. 112, Plate V, 

 one of the spicula, magnified 400 linear, to exhibit the 

 trenchant edges and the cylindrical portions of the hami 

 and shaft. 



This sponge is allied to Hymedesmia by the structure of 

 the skeleton, and it is described by my friend, Mr. J. Yate 

 Johnson, as being a thin coating species spreading, over 

 the surface of rocks and stones to the extent of two or three 

 inches in diameter. 



In Hyalonema mirabilis. Gray, a sponge nearly related to 

 the genus Mcyoncellum Quoy et Gaimard, we find another 

 extraordinary series of internal defences; one portion of 



