86 MANUAL OF ZOOLOGY SECT. 



The elements* of the skeleton differ in character in the 

 two sub-classes into which the sponges are divided. In 

 the Calcarea, of which Sycon is an example, they consist 

 of calcareous spicules, usually triradiate in form. In the 

 Non-Calcarea the skeleton either consists of spongin fibres 

 alone (Fig. 39, A), or of siliceous spicules alone, or of a 

 combination of spongin fibres with siliceous spicules (B) : 

 in some (Myxospongias) skeletal parts are altogether 

 absent. Spongin is a substance allied to silk in compo- 

 sition ; 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. 40) 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 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. 39, C). In others, again, 

 such as the Venus's flower-basket (Euplectelld], the glass- 

 rope sponge (Hyalonema), 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 



