OF THE SPONGIADiE. 157 



If any portion of the animal remains, whereby we may 

 recognise it as one of the Spongiadse, it is always the skele- 

 ton, 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 por- 

 tion 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 cha- 

 racteristic tissues of their skeletons are always found to 

 harmonise 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 

 are of use in generic descriptions to a certain extent, but 

 none of them are absolutely necessary to the determination 

 of a genus, and occasionally we find one or more of these 

 modes of organization entirely absent; we may therefore 

 consider them not as primary, but rather as secondary or 

 auxiliary generic characters. 



I therefore propose to consider the varieties in the con- 

 struction of the skeleton as the foundation or primary 

 source of divisions into genera, and to dedicate that portion 

 of the animal especially to that purpose ; the auxiliary or 

 secondary characters being resorted to only when required 

 to aid and assist the primary ones ; and it is only to a very 

 limited extent that they are in reality available. Thus the 

 cloaca in the Order Calcarea becomes a very important 

 means of generic distinction, and in some cases in the 

 Order Keratosa it is also a prominent character, while in 

 Silicea it is generally absent. In some species of this 

 order, as in Jjcyoncellum, Polymastia, and Halyphysema, it 

 assumes a normal character, while in several species of 



