io8 



SPONGES 



triradiate to which an adventitious gastral ray has been added. It is 

 remarkable that this fourth ray is derived from a distinct source from the 

 other three, its scleroblast, or gastral actinoblast, as it may be termed, being 

 derived from a porocyte at a comparatively late period in the growth of 

 the basal system. After the three basal rays have reached a certain 

 length, the nucleus of a neighbouring porocyte divides, and a portion of 

 the cell, with one of the nuclei, becomes constricted off, grows out towards 

 the minute triradiate, takes up a position over it i.e. internal to it and 

 secretes a minute spicule ray which becomes fused and tacked on to the 

 basal triradiate system (Fig. 75, 4). The secretion of the gastral ray 

 may commence before its actinoblast is completely separated from the 

 porocyte. In the further development the nucleus of the gastral actino- 

 blast may remain single or divide into two or four nuclei, according to 

 the size of the ray to be formed. In all cases, however, the protoplasm 



FIG. 75. 



Development of equiangular triradiates and quadriradiates in Clathrina. 1, trio of actino- 

 blasts ; 2, sextet, with young spicule ; 3, late stage in the growth of the spicule, after loss of 

 the apical formative cells ; 4, division of a porocyte to form a gastral actinoblast ; 5, late stage 

 in the secretion of the gastral ray. tr.syst, triradiate system ; b.f.c, basal formative cell ; g.act, 

 gastral actinoblast ; g.ray, gastral ray ; p, dermal aperture of pore. 



of the actinoblast remains undivided, and covers at first the whole ray 

 (Fig. 75, 5), but later only its tip, in the form of a granular plas- 

 modium, very different in appearance from the formative cells of the 

 basal system which, at first granular, soon become very clear and free 

 from conspicuous granulations. 



It is evident from their development that the many-rayed spicules of 

 Clathrinidae, and probably of all Calcarea, are compound spicules, repre- 

 senting a spicular system derived from fusion of primitively distinct 

 monaxons. Even the apparently monaxon spicules, always of large size 

 in this family, seem to be derived from a modification of the compound 

 triradiate type. In the Leucosoleniidae, on the other hand, the monaxon 

 spicules are always true primary monaxons, derived each from a single 

 mother-cell, and are the first spicules to arise in the development. The 

 triradiate systems of Leucosolenia are formed just as in Clathrina, from 



