58 SKELETONS OF SPONGES: PART in. 



extracted the nutritious part, the offal is carried into the 

 sea through the oscula, by the current of water whose flux 

 is maintained by the vibrations of the cilia. In the 

 compressed and many of the tubular sponges the water 

 passes through them in a straight line ; in branched 

 and encrusting sponges, the afferent and efferent open- 

 ings are on the same surface. The water is inhaled 

 continuously and gently like an animal breathing, but it 

 is rapidly and forcibly ejected ; and in its passage it no 

 doubt furnishes oxygen to aerate the juices of the com- 

 pound animal, whose flesh or sarcode is irritable while 

 alive, and which has the power to open and shut the 

 pores and oscula of the canals, for the whole sponge 

 forms one compound creature whose mass is nourished 

 by the myriads of Amoebae of which it is constituted. 



Within the animated sarcode mass of the sponges 

 there is in most cases a complicated skeleton of fibrous 

 network, either horny, calcareous, or siliceous, which 

 supports the soft mass, and determines its form. 



Besides the skeleton, the mass of sponges is for the 

 most part strengthened and defended by siliceous, and 

 more rarely by calcareous, spines or spicules, either 

 imbedded among the fibres of the skeleton, or fixed to 

 them by their bases. The fibres of the skeleton network 

 always unite, whether they be horny, calcareous, or 

 siliceous ; the spicules never, though they often lie in 

 confused heaps over one another. They are of in- 

 numerable forms and arrangements. Some are like 

 long needles lying close together in bundles, pointed or 

 with a head like a pin at one or both ends; a great 

 number are stellate with long or short rays ; there may 

 even be several different forms in the same sponge. 

 Many calcareous sponges have cavities full of organic 

 matter ; and when the calcareous matter is dissolved by 

 dilute acid, the organic base is left. 



The common commercial sponges have a skeleton 



