86 ANATOMY AND PHYSIOLOGY 



in the skeleton beside pure keratode ; and the secretion of 

 it by its own inherent power appears to be the next 

 natural step in the development of the animals. 



In the third division, those having the skeleton formed 



* O 



of speculated keratose fibre, the gradual development is 

 also well marked, as in one group we find spicula only in 

 the primary or radiating fibres of the skeleton, while in 

 another group they are found in both the primary and 

 secondary fibres, and are developed simultaneously with the 

 keratode of the young fibres of the skeleton. 



1. Calcarea . a. Spicula dispersed on membranes. 



2. Silicea . . a. Spiculo-radiate skeletons. 



b. Spicula dispersed on membranes. 



c. Spicula cemented together by keratode. 



d. Solid siliceous fibre. 



3. Keratosa . a. Keratose fibre only. 



b. Arenated keratose fibre. 



c. Spiculated keratose fibre. 



These divisions afford a general view only of the principal 

 types of the skeleton structure. Other well-defined varia- 

 tions of these divisions, on which the sub-orders will be 

 based, will be pointed out and described at length when 

 we arrive at that portion of our subject in which we shall 

 treat on the classification of the Spongiada?. 



The essential parts of the skeleton of the Spongiada? are 

 keratode, carbonate of lime, silex, and membrane ; and on 

 the different modes of the combination and arrangement of 

 these materials their division into orders, sub-orders, and 

 genera will depend. It will not be necessary to enter here 

 into a detailed account of the structure of these respective 

 parts, as each of them are treated on at length under their 

 respective heads in the portion of this work devoted to 

 organography and terminology. I shall therefore confine 

 my observations to a brief general view of the combinations 

 of the parts essentially necessary to the construction of the 

 skeleton. 



Keratode, in its application to skeleton structure, has an 

 exceedingly wide range. In the order Keratosa it is the 



