CLASS SPONGIDA. 273 



CLASS IV, SPONGIDA. 



The Sponge of commerce is the frame-work which, in the 

 living animal, is covered with a slimy material resembling 

 the white of an egg, but so delicate that „. ,„„ 



^°' Fig. ISG. 



it is usually washed off by merely draw- 

 ing the mass from the water. This 

 investing sponge-flesh consists of numer- 

 ous minute nuclei acting independently 

 of one another, and yet individually con- 

 tributing to the sustenance of the whole 

 colony. Canals permeate this gelati- 

 nous substance in every direction. The 

 smaller ones permit the ingress of water ^"^^^ '*°"9'* i^amfiea. 

 to take the place of that constantly rushing out by the larger 

 orifices. These currents are due to the action of cilia in 

 enlarged portions of the canals, and thus food is diffused 

 throughout the mass. The cells along the route select and 

 appropriate the nutritive particles. The Common Sponge 

 {Uuspongia officinalis) is not a single sponge but a commu- 

 nity, " representing," as Huxley remarks, " a kind of sub- 

 aqueous city, where the people are arranged about the streets 

 and roads in such a way that each can choose his food from 

 the water as it passes along." The skeleton is composed of 

 horny fibres interlacing and strengthened with spicules of 

 lime or flint. The frame-work in some species is calcareous 

 or silicious. In the Eupledella the silicious fibres form a 

 beautiful mesh of lace, known as Venus's Flower Basket, 

 formerly considered an unique specimen of Chinese industry. 



