i6o 



CHAPTER XI. 



PORIFERA— Continued. 



Classification and Principal Groups of the Sponges. 



There is, perhaps, no single group of the animal kingdom in which 

 it has proved so difficult to establish a natural classification, as has 

 been found to be the case with the Sponges. Even at the present 

 day there is no extant classification which can be regarded as final. 

 It is, however, now generally admitted that the calcareous Sponges 

 are so far separated from all the other groups of Sponges as to pro- 

 perly constitute a distinct class of Sponges. The non-calcareous 

 Sponges may be grouped together in a second class under the name 

 of Plethospongice, proposed for them by Professor Sollas. As regards 

 the ordinal divisions of the Plethospongice, the grouping followed by 

 Professor Zittel may be adopted, with some modification, and the 

 general classification of the Sponges is thus expressed in the following 

 table : — 



SUB-KINGDOM PORIFERA. 



Class I. Plethospongle (Sollas). 



Order i. Myxospongi^e (Halisarca). 



„ 2. Ceratospongi^e (Euspongid). 



„ 3. Monactinellid^e (Halichondria, &c.) 



,, 4. TetractinellidjE (Geodia, Tethya, &c.) 



,, 5. Lithistid^e (Discodermia, &c.) 



„ 6. Hexactinellid^e (Holtenia, &c.) 



„ 7. Octactinellid^e (Astrceospongia). 



„ 8. Heteractinellid^e (Asteractinella, &c.) 



[The last six of the above-mentioned orders are regarded by Hinde as 

 sub-orders, and are included in the single order Silicispongice?^ 



