DE. J. S. BOWEEBANK ON THE ANATOIVIY 
spicula of Spongilla lacustris (fig. 21, Plate XXIV. Phil. Trans. 1858) they are made to 
serve as defensive organs as well as tension spicula ; and, again, in the spicula of the 
gemmules of the Spongiadee their skeleton spicula also perform the office of defensive 
organs as well, as represented by figs. 13 to 43, Plate XXVI. Phil. Trans. 1858. 
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 exist in so 
many of their bodies must render them anything rather than desii’able 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, that appears to prey upon them. The only animal 
in the stomach of which I have ever seen the spicula of any sponge was a Dons. But 
although appearing to enjoy almost an immunity from the common lot of animals, that 
of being eaten by others, they may yet serve, at their death by natural causes, to supply 
an immense quantity of animal molecules for the sustenance of the myriads of minute 
creatures that exist around them. 
Tension Spicula. 
The primary purpose of the tension spicula is that of strengthening and supporting the 
membranes, both external and internal. They are usually of the same form as those of 
the skeleton, but more slender and shorter in their proportions. On the internal mem- 
branes they are dispersed without any approach to order, and cross each other at every 
imaginable angle. They vary exceedingly in length and diameter, and are attached for their 
whole length to the tissues on which they repose. In some cases they are not readily to 
be distinguished 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 Hymeniacidon ; but in 
other cases, as in some species of Chalina and Isodictya, they may always be distin- 
guished by their position, and by the total absence of keratode around them, while those 
of the skeleton are always more or less coated by that substance. 
In other cases they difier materially in form and proportion from those of the skeleton. 
Thus in Halichondria incrustans, while the skeleton spicula are stout, short, entirely 
spined and acuate, as represented by fig. 30, Plate XXIII. Phil. Trans. 1858, the 
tension spicula are smooth, slender mucronato-cylindrical, as represented by fig. 23, 
Plate XXIV. Phil. Trans. 1858. They are frequently dispersed on the dermal mem- 
branes, 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 
incrustans, they are congregated in flat broad fasciculi, which are disposed on the mem- 
brane with little or no approximation to order. 
The tricurvo-acerate form in all its varieties is better calculated to effect their peculiar 
office in small and irregular spaces, and with greater economy in numbers, than the 
