156 ANATOMY AND PHYSIOLOGY 



and apply our reasoning to the SpongiadaB, we should then 

 give precedence to the calcareous sponges as representing 

 in the class the highest order of secretive power ; and if 

 we add to these considerations the regularity of structure 

 and function and the full development of ciliary action that 

 exists in Grantia ciliata and compressa and the allied 

 species, I think it scarcely allows of a doubt that this order 

 should take precedence of the others in an arrangement of 

 the Spongiadae. 



The siliceous sponges naturally follow in succession, and 

 the Keratosse, from their imperfect secretive powers and 

 their low order of organization in other respects, would 



. 



indicate their position to be thd last in the series. 



ON THE GENERIC CHARACTERS OF THE SPONGIADAE. 



The foundation of the genera of the Spongiadse has 

 hitherto been based principally upon form and other 

 external characters of an equally unstable description, and 

 in many instances genera have been named without the 

 slightest attempt to characterise them. As a generic 

 character form is inadmissible, inasmuch as each variety of 

 it is found to prevail indiscriminately in genera differing 

 structurally to the greatest possible extent. 



I will not enter on the history of the genera that have 

 been proposed by previous writers on the Spongiadse, as 

 the greater portion of those which have been published will 

 hereafter be found to have been adopted, with certain 

 revisions of their characters, in the series of genera I pro- 

 pose to establish, but I shall beg to refer such of my readers 

 as may be desirous of further information on that subject 

 to page 70 of Dr. Johnston's admirable introduction to his 

 ' History of British Sponges and Lithophytes.' 



Having thus rejected form and other external characters 

 as the foundation of generic descriptions, we naturally 

 resort to the anatomical peculiarities of the animal for these 

 purposes ; and here fortunately we rind a variety in struc- 

 ture and form, and a constant adherence to their respective 

 types that admirably adapt them to our purpose. 



