OF THE SPONGIAD^E. 39 



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 Doris. 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. 



Spicula of the Membranes. 



There are two distinct classes of spicula appropriated to 

 the membranous tissues of sponges. The office of the first 

 of these is simply to strengthen and support those delicate 

 tissues when necessary, and to communicate to them a cer- 

 tain amount of tension when it is required. The forms are 

 few in number, and their structure comparatively simple. 



The office of the second class is that of assisting in the 

 retention and protection of the sarcode on the interstitial 

 and other membranous structures. They are usually mi- 

 nute in size, and often very complicated in form. 



I propose to designate these organs as 



1st. Tension Spicula. 

 2nd. Retentive Spicula. 



Tension Spicula. 



In some species of sponges the dermal membrane is 

 without spicula especially appropriated to it, and it then 

 appears, as in Spouf/illa fluviatilis, to be a simple trans- 

 lucent membrane filling up all parts of the network of the 

 external surface of the skeleton, and closely adhering to it ; 

 but the membranous areas thus formed are devoid of pecu- 

 liar forms of spicula. In other cases, as in Spongilla lacns- 

 tris, we find spicula dispersed more or less abundantly over 

 the whole of the surface of the membrane, which are entirely 

 unconnected with the skeleton, and give to the dermal 

 membrane a degree of firmness and tension that it would 



