OF THE SPONGIAD^. 155 



the determination of the orders to which many exotic 

 species belong, it would lead in numerous cases to inex- 

 tricable confusion. The term Leuconida is also objection- 

 able, as all calcareous sponges are not white, and colour is 

 at best but a very uncertain character even in the determi- 

 nation of a species ; I have therefore adopted the principles 

 of the arrangement of Professor Grant, with the following 

 modifications of position and descriptions of the charac- 

 teristics of each order. 



1. Calcaeea. Sponges the skeletons of which have as 

 an earthy base carbonate of lime. 



2. SiLicEA. Sponges in which the earthy base consists 

 of siliceous matter. 



3. Keratosa. Sponges in which the essential base of 

 the skeleton consists of keratose fibrous matter. 



While thus assuming the principles of arrangement 

 enunciated by the learned Professor, I have been induced 

 to vary the mode of the disposition of his Orders from the 

 following considerations. 



In the highest vertebrated animal types we invariably 

 find the skeleton principally composed of phosphate of 

 lime with a small portion of carbonate of lime and other 

 substances, the whole consoHdated by cartilage. As we 

 descend the scale of the Vertebrata we fi;nd the salts of 

 lime decrease in proportional quantity until they occur in 

 minute detached patches only, and cartilage becomes the 

 essential base of the skeleton. 



In the great tribe of MoUusca we find carbonate of lime 

 prevailing in their shells to the exclusion of phosphate of 

 lime, and in the compound Tunicata we have a structure 

 analogous to that of the cartilaginous tribe of Fishes. In 

 the massive subcartilaginous body of this tribe there is no 

 continuous or connected earthy deposits. This material of 

 the skeleton exists only in the form of detached masses of 

 radiating spicula. As we descend in the animal scale we 

 find carbonate of lime entirely absent, and silex replacing 

 it in the elaborate and beautifully constructed loricse of the 

 marine and freshwater infusoria. 



If we are to reason from these gradations of structure 



