112 



ZOOLOGY 



SCT. 



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

 to form a firm, elastic supporting structure. They are secreted by 

 the activity of certain cells of the mesoderm, which are called the 

 spongin-blasts. In certain exceptional cases the spongin assumes 

 the form of spicules. The siliceous spicules (Fig. 81) 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 the various parts of the Sponge-body. In most Non-Calcarea 

 siliceous spicules and spongin fibres combine to form the support- 

 ing 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 (Fig. 80, A), 

 spicules are completely absent, and the entire skeleton consists of 

 spongin. In some Non-Calcarea which are devoid of spicules, the 



FIG. 81. Various forms of sponge spicules. (From Lang's Text-Book.) 



place of these is taken by foreign bodies shells of Radiolaria, 

 grains of sand, or spicules from other sponges (Fig. 80, C). In 

 others, again, such as the Venus's Flower-Basket (Eiiphctclla), the 

 Glass-Rope Sponge (Hyaloncma), and others, the skeleton consists 

 throughout of siliceous spicules bound together by a siliceous 

 cement. 



Reproduction in the Sponges is effected either sexually or 

 asexually. The process by which, in all but the simplest forms 

 of Sponges, a colony of zooids is formed from the originally 

 simple cylinder or vase, may be looked upon as an asexual 

 mode of reproduction by budding. Asexual multiplication 

 also assumes the form in some cases of a process of production 

 of internal buds in the shape of groups of cells called gemmules, 

 which eventually become detached and develop into new indi- 

 viduals. In the Fresh-water Sponges (Spongilliche) multiplication 

 takes place very actively by means of such gemmules, each of which 

 is a spherical group of cells enclosed in an envelope composed of 

 peculiarly shaped siliceous spicules, termed ampliidiscs (Fig. 81, 



