88 ELEMENTARY ZOOLOGY 



Skeleton. A very few sponges have no skeleton at 

 all. The others have a skeleton or hard parts composed 

 of interwoven fibres of the tough, horny substance called 

 spongin, or of hosts of fine needles or spicules of silica or 

 of carbonate of lime. The siliceous skeletons of some 

 of the so-called glass-sponges (fig. 12) are very beautiful. 

 The lime and siliceous sponge spicules exhibit a great 

 variety of outline, some being anchor-shaped, some cross- 

 shaped, and some resembling tiny spears or javelins. 



Structure of body. The skeleton of a sponge whether 

 composed of interlacing fibres or of short spicules is 

 always invisible from the outside when the sponge is alive. 

 It is embedded in, or clothed by, the soft, fleshy part of 

 the body. The soft part of the sponge is composed 

 simply of two layers of cells, one constituting the external 

 surface of the body, and the other lining the interior 

 cavities and canals of the body. Between these two cell- 

 layers there is a mass of soft gelatinous substance all 

 through which protoplasm ramifies, and in which are em- 

 bedded numerous scattered cells. There are, as seen in 

 the case of Spongilla and Grantia, no systems of organs 

 such as characterize the higher animals. No heart, lungs, 

 alimentary canal, nervous system, organs of locomotion, 

 eyes, ears, or other organs of special sense; the sponge 

 has none of these. It is simply an aggregate of cells, 

 arranged in two layers, and supported usually by a skele- 

 ton of horny fibres or calcareous or siliceous spicules. Its 

 body is usually shapeless, unsymmetrical and without 

 front or back, right or left. It is not to be wondered at 

 that sponges were for a long time believed to be plants. 



Feeding habits. The sponges feed on minute bits of 

 animal or plant substance and on the microscopic unicel- 

 lular plants or animals which float in the water which 

 bathes their bodies. The water entering the sponge- 

 body through the various openings of the surface is moved 



