'Am) PHTSIOLOaT OP THE SPONOIAD^. 
1089 
Spongiadse, 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 dliata 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 Spongiadse. 
The siliceous sponges naturally follow in succession, and the Keratosse, as indicated 
hy their imperfect secretive powers and then- low order of organization in other respects, 
would stand the last in the series. 
On the Generic Characters of the Spongiadce. 
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 characterize 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 propose 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 Litho- 
phytes.’ 
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 find a variety in structure and form, and a constant 
adherence to their respective types that admirably adapt them to our purpose. 
If any portion of the animal remains whereby we may recognize it as one of the 
Spongiadee it is always the skeleton ; and it is therefore advantageous to adopt this most 
persistent portion of the animal as the foundation of our generic descriptions. But this 
is not the sole reason for such a conclusion, as it is not only the most enduring portion 
of the animal, but it is also the most undeviatingly regular in the form and arrangement 
of its component structures. However great may be the variations that exist in size and 
form between different species of the same genus, or between individuals of the same 
species, the characteristic tissues of their skeletons are always found to harmonize in their 
structural peculiarities. It appears, therefore, advisable in these animals, as well as in 
the higher classes, to select the skeleton as the primary source of generic distinctions. 
Other portions of the permanent organs may be occasionally resorted to when necessary 
as auxiliary characters, such as the incurrent and excurrent canals, the intermarginal 
cavities, the cloaca, and the various modes of reproduction. Each of these characters 
is of use in generic descriptions to a certain extent ; but none of them is absolutely 
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