88 



MANUAL OF ZOOLOGY 



SECT. 



in some (Myxospongiae) skeletal parts are altogether 

 absent. Spongin is a substance allied to silk in composition ; 

 the fibres are exceedingly fine threads, which branch and 

 anastomose, or are woven and felted together in such a way 

 as to form a firm, elastic supporting structure. The 

 siliceous spicules (Fig. 39) are much more varied in shape 

 than the spicules of the Calcarea, and in a single kind of 

 Sponge there may be a number of widely differing forms of 

 spicules, each form having its special place in the skeleton of 



FIG. 39. Various forms of sponge spicules. (From Lang's Text-Book.') 



the various parts of the sponge-body. In most Non-Calcarea 

 siliceous spicules and spongin fibres combine to form the 

 supporting framework, the relative development of these two 

 elements varying greatly in different cases. But in certain 

 groups of the Non-Calcarea, including the common washing- 

 sponges, spicules are completely absent, and the entire 

 skeleton consists of spongin. In some Non-Calcarea 

 which are devoid of spicules, the place of these is taken by 

 foreign bodies shells of Radiolaria, grains of sand, or 

 spicules from other Sponges (Fig. 38, C). In others, again, 

 such as the Venus's Flower-basket (Euplectella\ the Glass- 

 Rope Sponge (Hyalonema\ and others, the skeleton 



