SPONGES. 
33 
tropical seas, where they are very numerous and varied, 
has species peculiar to itself. 
A Sponge, as it is used in domestic economy, is merely 
a skeleton : it is the solid frame-work which in life sup- 
ported the softer flesh. This skeleton is composed of one or 
two of the following substances, — flint, lime, and a peculiar 
horny matter. The first two are crystallised, and take the 
appearance of spicular needles either simple or compound, 
varying greatly as to their longth, thickness, shape, and 
curvature, but constant in form in the same species. The 
horny matter, of which the common domestic Sponge affords 
an example, is arranged in slender, olastic, translucent, 
tough, solid fibres, united to each other irregularly at 
various points, and in every direction, and thus forming 
an open netted mass commensurate with the size of the 
whole sponge. The homy Sponges are almost confined to 
the warmer seas, but the siliceous and calcareous kinds are 
common with us, especially the former. 
The solid parts are, during life, invested with a glairy 
transparent slime, so fluid in most species as to run off 
when the Sponge is taken out of its native element ; yet 
this clear slime is the flesh of the animal. 
■The spicula, whether of flint or lime, or the homy fibres, 
are so arranged as to form numberless pores, with which 
the whole animal is perforated ; it is to these that our 
common Sponge owes its most valuable property of 
imbibing and retaining water, as wc shall presently see 
when we investigate the history of this species in detail. 
In life the surrounding water is made to flow through these 
pores by a continual current (interrupted, however, at the 
will of the animal) from without into the interior of the 
o 
