SPONGES, 



35 



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 length, 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, elastic, 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 horny 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 oft 

 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 horny 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 we 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 



