44 ZOOLOGY 



Artificial fission has been successfully adopted in the cultiva- 

 tion of sponges for commercial purposes. 



All sponges are marine, with the exception of the family 

 Spongillidae, the commonest species of which is Sfongilla 

 flimatilis, an incrusting mass with crater -like oscula, commonly 

 found on the woodwork of locks, weirs, etc., in our rivers. It 

 may be coloured green by the presence of chlorophyll. Many 

 other sponges are brightly coloured, and they may assume a 

 very great variety of shapes. Some emit a powerful and un- 

 pleasant odour, which may, like the presence of the spicules, 

 tend to prevent their being devoured. 



The classification of the Porifera cannot be regarded as 

 settled. That at the head of this chapter is suggested by 

 Vosmaer. 



A. The Calcarea. includes those forms which possess a 

 skeleton of calcareous spicules, generally triradiate in shape. The 

 collar cells are large. 



This class comprises two orders : 



(i.) Homocoela, in which the endoderm consists wholly of 

 collared cells, which line the central cavity. Ex. Leuco- 

 solenia. 

 This group includes all those forms which were formerly 

 known as Ascones. 



(ii.) Heterocoela, in vjhich the endoderm is differentiated into 

 (a) fat epithelial cells, lining the central cavity and 

 excurrent canals, and (b) collar cells, confined to the 

 flagellate chambers. Ex. Grantia, Sycon, Leucandra. 



B. The NoN-CALCAEEA possesses a skeleton of siliceous spicules 

 or spongin ; very rarely none at all. The spicules may he isolated, 

 articidated, or fused. Tlie collar cells are ma^-kedly smaller than 

 those of the Calcarea. 



The group includes three classes : (i.) Hyalospongiae 

 (Hexactinellidae), (ii.) Spiculispongiae, and (iii.) Cornacu- 

 spongiae. The first of these three classes is much more clearly 

 defined than the other two ; indeed the latter tend to run into 

 one another. 



