1088 
DE. J. S. BOWEEBAi^K ON THE ANATOMY 
through internal canals and ejecting it through appropriate orifices ; having an internal 
flexible or inflexible skeleton composed of either carbonate of lime, silex, or keratode ; 
with or without either of these earthy materials. Calcareous skeletons always spicular. 
Siliceous skeletons either spicular or composed of solid, laminated, and continuous 
siliceous fibre. 
Propagation by ova, gemmulation, or spontaneous division of its component parts. 
Dr. Geant, in his learned and elaborate “ Tabular View of the primary divisions of 
the Animal Kingdom,” published in 1861, has divided the Porifera into three orders, 
based on principles which I have adopted. The first order is Keratosa, in which the 
skeletons are essentially keratose and fibrous ; the second, Leuconida, is composed of the 
calcareous sponges ; and the third, Chalinida, consisting of the siliceous sponges. I 
have not adopted the full and precise definition of each of these Orders as given by the 
learned Professor, as, if the whole of the distinctive characters in the first and thnd of 
them were msisted on in the determination of the orders to which many exotic species 
belong, it would lead in numerous cases to inextricable confusion. The term Leuconida 
is also objectionable, as all calcareous sponges are not white, and colour is at best but a 
very uncertam character even in the determination 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 characteristics of each order. 
1. Calcarea. 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 Pro- 
fessor, 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 consolidated by cartilage. As we descend the scale of the Verte- 
brata we find 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 Mollusca we find carbonate of lime prevailing in their shells to 
the exclusion of phosphate of lime, and in the compound Tunicata we have a structm’e 
analogous to that of the cartilaginous tribe of Fishes. In the massive subcartilaginous 
body of this tribe there is no continuous or connected earthy deposit ; 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 loricae of the marine and freshwater 
infusoria. 
If we are to reason from these gradations of structure and apply our reasoning to the 
