160 ANATOMY AND PHYSIOLOGY 



Order I. CALCAREA. 



The number of species of calcareous sponges that are 

 known are comparatively so few, and the four genera into 

 which I have divided them are naturally so well charac- 

 terised as to render the establishment of suborders unne- 

 cessary. Hereafter, when we are acquainted with a greater 

 number of species and other varieties of organization 

 become known, the genera now established may become 

 the types of suborders, for which office their distinctly 

 different modes of construction render them eminently 

 efficient. 



Although the calcareous structure of the species of this 

 order appear to entitle it to precedence in the arrangement 

 of the Spongiadse, it does not maintain in the structure of 

 its skeleton throughout the whole of the genera the same 

 high type of formation that is exhibited in Grantia com- 

 pressa, Johnston, and the allied species, and we observe a 

 progressive decline in regularity of structure in its genera 

 very analogous to what we find existing among the Hali- 

 chondroid tribe of sponges ; but in this respect they only 

 follow the same laws of gradual degradation that obtain in 

 every other class of created beings, and this gradual decHne 

 in regularity of structure should not therefore militate 

 against the claim of even the lowest in organization of the 

 tribe from taking precedence of the siliceous sponges. 



Dr. Grant was the first naturalist who decided that the 

 spicula of a certain group of small sponges were composed 

 of carbonate of lime, and he separated them accordingly 

 from those the spicula of which were siliceous, and assigned 

 to them the generic name of Leucalia (' Edinburgh Ency- 

 clopaedia,' vol. xviii, p. 844) ; and subsequently, in his 

 ' Outlines of Comparative Anatomy,' he changed that 

 name to Leuconia. In 1828 Dr. Fleming gave to the 

 group the name of Grantia, in compliment to the learned 

 naturalist who had first pointed out their pecuhar 

 structure. 



