120 



ZOOLOGY 



The elements of the skeleton differ in character in the different 

 classes. In the Calcarea they consist of calcareous spicules, usually 

 tri-radiate in form. Each of these spicules is developed from special 



cells the scleroblasts (Fig. 

 89). In the remaining groups 

 of Sponges the skeleton 

 either consists of spongin 

 fibres alone (Fig 90, A), 

 or of siliceous spicules 

 alone, or of a combination of 

 spongin fibres with siliceous 

 spicules (B) : in some Demo- 

 spongia (the Myxospongia) 

 skeletal parts are altogether 

 absent. Spongin is a sub- 

 stance allied to silk in chemi- 

 cal composition: the fibres 

 are exceedingly fine threads, 

 consisting of a soft granular 

 core and an outer tube of 

 concentric layers of spongin. 



FIG. 89. Development of a tri -radiate spicule ofrpu 4-1* ^"U l> J 



ciatbrina. sd, scleroblasts. (After Minchin.) Ihese threads branch 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 in the mesoglo3a which are called the spongin- 

 blasts, derived from the ectoderm. In certain exceptional cases 

 the spongin assumes the form of spicules. The siliceous spicules 

 (Fig. 91) 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 forms siliceous spicules and spongin fibres 

 combine to form the supporting framework, the relative develop- 

 ment of these two elements varying greatly in different cases. 

 But in certain groups, including the common Washing-sponges 

 (Fig. 90 A), spicules are completely absent, and the entire 

 skeleton consists of spongin. In some forms 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. 90, C). In others, again, such as the Venus's 

 Flower-Basket (Euplectella), the Glass-Rope Sponge (Hyahnema), 

 and Pheronema (Fig. 92), 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 



