154 ANATOMY AND PHYSIOLOGY 



in Grantia, Geodia, Tethea, and other genera, regular and 

 systematical structures which are very far removed from 

 shapelessness. I have therefore thought it advisable to 

 adopt Dr. Grant's designation of Porifera, a term which 

 embraces the whole of the Spongiadee, and which is truly 

 descriptive of the most essential general action of the 

 animal's power and mode of imbibing nutriment, which in 

 every species with which I am acquainted is, by a series of 

 minute pores, distributed over the external membrane of 

 the sponge. 



Besides this universally existent character there are others 

 which are strikingly characteristic of the class, although not 

 so universally prevalent as the porous one. Thus the 

 skeletons of the SpongiadcG are always internal, but in the 

 material and mode of construction they vary to a very con- 

 siderable extent. Sponges may therefore be defined as 

 fixed, aquatic, polymorphous animals ; inhaling and imbib- 

 ing the surrounding element through numerous contrac- 

 tile pores situated on the external surface ; conveying it 

 through internal canals or cavities, 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 con- 

 tinuous siliceous fibre. 



Propagation by ova, gemmulation, or spontaneous division 

 of its component parts. 



Dr. Grant, in his learned and elaborate ' Tabular View 

 of the primary divisions of the Animal Kingdom/ published 

 in ]8G1, 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, C//ali.>iida, 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 cha- 

 racters in the first and third of them were insisted on in 



