34 



GUIDE TO THE CORAL GALLERY. 



High Cases 

 IV.-VI. 



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 Avaterweeds. 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. 



Tlie black Parmula hatesii (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 ])urst through the pore and develop into a sponge. 



In the Clialinid 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. 



High Cases 

 I. II. and 

 Table Case 1. 



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 spouges, &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 '^])o\\gQ,Spongia officinalis (Fig. 18), has a cup- 

 shaped body with a black or dark skin. The oscides 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. 

 Groups 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. 



