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 Spongilla 
fluviatilis, 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 ‘wncludes those forms which possess a 
skeleton of calcareous spicules, generally triradiate in shape. The 
collar cells are large. 
This class comprises two orders : 
G.) 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. 
(i.) Heterocoela, in which the endoderm is differentiated into 
(a) flat 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-CALCAREA possesses a skeleton of siliceous spicules 
or spongin ; very rarely none at all. The spicules may be isolated, 
articulated, or fused. The collar cells are markedly smaller than 
those of the Calcarea. 
The group includes three classes: (.) 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. 
