34 
GUIDE TO THE CORAL GALLERY. 
High Gases 
IV.-YI. 
High Cases 
I. II. and 
Table Case 1. 
in lakes and rivers attached to stems of reeds or the piles of locks, &c. 
These sponges are often of a bright green colour, and are easily 
mistaken for waterweeds. The green colour, which is due to the 
presence of chlorophyll, does not occur in specimens living in shady 
places, the sponges then being pale buff. Alcohol dissolves out the 
colour, forming a clear green solution. 
Spongilla lacustris forms green crusts from which long digitate 
branches arise. 
The black Parmula batesii (Case YI. 3), from the Amazons, is 
often found attached to branches of trees submerged during the rainy 
season, the sponges being left high and dry when the floods subside. 
Many fresh-water sponges produce little seed-like buds or 
gemmules, which possess a hard resistant capsule perforated by a 
pore at one point. When the favourable season arrives, the contents 
of the gemmule burst through the pore and develop into a sponge. 
In the Chalinid Sponges (Cases Y., YI.), the skeleton forms a 
network of horny fibres cored by siliceous spicules ; if the latter 
were absent from the fibres, the sponges would be Horny Sponges, 
and it is generally supposed that the pure horny sponges have been 
derived from siliceous forms which no longer secrete silex. 
Keratosa (Horny Sponges). 
The Horny Sponges possess a skeleton of horny fibres, which 
generally form a close network, as in the Bath Sponges, or the fibres 
may branch in a tree-like manner. Yery commonly, foreign bodies, 
such as sand grains, the spicules of other sponges, &c., are present in 
the body of the sponge or in the axis of the fibres ; even in the 
finest bath sponges there are scattered sand grains in the main 
fibres. 
A series of commercial sponges is set out in Case I. and Table 
Case I. 
The Fine Turkey Sponge, Spongia officinalis (Fig. 18), has a cup- 
shaped body with a black or dark skin. The oscules are situated on 
the floor of the cup. A section of the body shows a comparatively 
uniform pale yellow surface, the canals being slightly darker in tint. 
Croups of pores on the outer surface lead by short fine canals into 
spaces just below the skin ; from the floor of these spaces canals pass 
inwards, branching and gradually diminishing in size, till they reach 
groups of pear-shaped whip chambers, with the cavities of which they 
communicate through minute orifices in the walls of the latter. 
