■ OF THE SPONGIAD^. 85 



rial composed of calcareous matter, and this division con- 

 tains but. one group :— 



Spicula dispersed without order on membranous surfaces, 

 as in the genus Grantia as defined by Johnston. 



The second Order, Silicea, comprises those sponges 

 in which' the primary essential material . of the skeleton 

 consists of siliceous matter. It may be divided into four 

 sections or groups. 



1. Those sponges which have the skeleton composed of 

 radiating fasciculi of siliceous spicula, as in Tethea Bictyo- 

 cylindrus, &c. 



2. Those in which the skeleton consists of spicula dis- 

 persed without order on membranous surfaces, as in 

 Hymeniacidon caruncula, Bowerbank. 



3. Sponges having the skeleton consisting of spicula 

 cemented together into a network by keratode, as in 

 Halichondria panicea, Johnston. 



4. Those sponges which have the skeleton composed of 

 solid siliceous fibres, as in Dactylocalyx pumicea, Stutchbury. 



The third order, Keratosa, consists of those sponges in 

 which the primary essential material of the skeleton is 

 keratose fibre, and this may be divided into three sections : 



1. Those which have the skeletons constructed of 

 keratose fibre only, as in the best cup-shaped Turkey 

 sponges of commerce. 



2. Those having skeletons of arenated keratose fibre, as 

 in the genus Dysidea. 



3. Those which have the skeleton formed of spiculated 

 keratose fibre, as in Chalina oculata, Bowerbank, 

 and some of the common West Indian sponges of com- 



I116rCG« ' 



In the first group no earthy material of any kind enters 

 into the structure of the skeleton. ■ 



The sponges of the second group, by a natural transition, 

 pass into the nearly-allied great . division of the HaUchon- 

 droid skeletons ; the inability of the former to secrete silex 

 in an organized form connecting them closely with the 

 purely keratose, while the instinctive habit of appropriating 

 extraneous matters recognises the necessity of other material 



