SPONGES 



39 



appears as a point. In this type, however, the angles at which the 

 rays meet one another are subject to considerable variation, as well 

 as the rays themselves. 



(4) The polyaxon type of spicule (Fig. 48, m, n, 0), laid down 

 along numerous axes which typically radiate from a common 

 centre. 



Subordinate variations of these different types will be described in 

 dealing systematically with the subdivisions of the Porifera. We may 

 mention here, however, one differentiation of the spicules which is often 

 of importance, the distinction, namely, between skeletal spicules or 

 megascleres, which by their union in various ways build up the general 

 supporting framework of the body, and flesh spicules or microsderes, 



FIG. 48. 



Types of spicules (microscleres). a and 6, sigmaspire viewed in different directions ; c, toxa- 

 spire ; d, spiraster ; e, sanidaster ; /, amphiaster ; g, sigma ; h, chela (isochela) ; j, one end of 

 another form of chela ; k, I, other forms of chela ; m, spheraster ; n, oxyaster ; o, the same, with 

 six actines ; p, another, with four actines ; q, another, with rays reduced to two (diactinal 

 monaxon) ; r, tylote microrhabdus ; s, oxeote microrhabdus ; {, rosette. 



which lie scattered more or less freely in the tissues. In many sponges 

 no such distinction can be drawn ; in others the distinction is purely 

 functional, and in so far as it has any effect on the morphological 

 characteristics, affects only the size of the spicules. In some cases, 

 however, the difference of function in the two classes of spicules is corre- 

 lated with divergent morphological characters, so that the distinction 

 between megascleres and microscleres may become a perfectly sound 

 and useful one. 



All spicules, whatever the material of which they are composed, 

 are deposited within cells, termed scleroblasts. The origin and 

 relations of these cells will be discussed below in dealing with the 

 histology ; we may consider here the development of spicules 

 themselves, which shows important variations. In the first place, a 



