38 ANATOMY AND PHYSIOLOGY 



It would be almost an endless task to describe every 

 variety of those singularly beautiful contrivances for com- 

 bined defence and offence in the interior of the Spon- 

 giadae. Those which I have particularised are some of 

 the most elaborate and beautiful that I have seen during 

 the course of my researches. In many other cases, where 

 all that is required is defence, the means employed are of a 

 much more simple nature. We find in the Spongiadse, as 

 in other animals, that nature frequently economises her 

 means by the conversion of one organ to the purposes 

 of another by slight adaptations or additions ; thus in 

 Halichondria incrustans, Johnston, and in other sponges, 

 the skeleton spicula are made to perform the duties of 

 internal defensive spicula, by being more or less furnished 

 with spines, as represented in Fig. 28, Plate I, and in other 

 cases where we find them medially or apically spined, as in 

 Figs. 30 and 32 of the same Plate. 



In like manner we find the spicula of the sarcode, by 

 the extreme profusion in which they occur in that sub- 

 stance near the surface of some sponges, are turned to good 

 account for the general purposes of external and internal 

 defence, as well as for their special purpose of protection 

 and support of the sarcode. So likewise in the tension 

 spicula of Sponyilla lacustris (Fig. 90, Plate IV,) they are 

 made to serve as defensive organs as well as tension 

 spicula; and, again, in the spicula of the ovaries of the 

 Spongiadae their skeleton spicula also perform the office of 

 defensive as well, as represented by Figs. 203 and 204, 

 Plate IX. 



As regards, then, their protection from their enemies, 

 there appears to be almost a natural prohibition to the 

 sponges becoming, to any great extent while alive, the food 

 of other creatures. The keratode of their skeletons 

 appears to be almost indestructible by maceration or 

 digestion, and the abundance of the acutely pointed 

 spicula that exists in so many of their bodies must render 

 them anything rather than desirable or digestible food 

 to the generality of other marine animals ; and in truth I 

 do not know of a single large fish, or other marine creature, 



