144. W. WOODLAND. 
occur in the case of other siliceous sponge spicules. For 
several reasons I very much doubt if this assumption of the 
desertion of the siliceous spicule, hexactinellid or otherwise, 
by the scleroplasm, is justifiable, at least in the majority of 
cases. It must be remembered that the scleroplasm which 
builds up the triradiate and other spicules of calcareous 
sponges differs from the syncytial scleroplasm of siliceous 
and, indeed, many calcareous spicules, in that it consists of 
separate scleroblasts, cylindrical or semi-cylindrical in shape 
(5), which are not united to form a syncytium entirely 
enveloping the spicule, and which are in consequence free to 
leave the spicule at almost any period of deposition. On the 
other hand, seeing that the syncytium which envelops a 
complex hexactinellid megasclere, e.g. (and the micro- 
scleres certainly retain their syncytia) is correspondingly 
distended and configurated, it is difficult to understand how 
the syncytium could take its leave as a whole, and it is 
exceedingly doubtful if the small portions containing nuclear 
matter separate themselves off from the rest. Personally I 
believe the scleroplasm of all or, at any rate, most siliceous 
sponge spicules clings to the spicule as long as it persists, 
although in many cases it becomes so attenuated as to be 
invisible. Indeed, the presence of a nucleus is frequently 
the only visible indication of the existence of a scleroplasmic 
film enveloping the spicule. 
Ijima again suggests the probability that the six rays of 
the hexactinellid spicule originate as separate sclerites, but 
supplies no evidence in support of this view, merely men- 
tioning the mode of origin of the triradiates and quadri- 
radiates of Calcarea as possibly presenting an analogous 
case. ‘That no analogy can be drawn from the mode of 
development of these calcareous spicules is, I think, suffi- 
ciently manifest on remembering that each of the three 
monaxons of the calcareous triradiate is produced from, and 
throughout subsequent growth is constantly related to, one 
of the three cells which “ fused ” to form the “‘ trefoil ”’ (4, 5), 
a feature entirely absent in the case of the hexactinellid 
