MATERIALS FOH A MONOGRAPH OF THE ASCONS. 511 



In coriacea it is, in my experience, rare to find a spicule 

 with rays exceedinj^ about 20 fx in length which has apical 

 cells present on all the rays, but here and there a spicule ray 

 occurs which far exceeds this length, and still bears a formative 

 cell at its apex, while the other rays of the spicule may show 

 no trace of any such cell. In PI. 38, fig. 9, is seen a spicule 

 with rays averaging about 20 /n in length, and only one of the 

 rays retains its apical formative cell. The spicule in fig. 10 

 has rays of about 45 fx in length, and still bears a formative 

 cell at the apex of one of its rays. Fig. 14 shows a ray of 

 about 60 ju in length, in which the apical cell appears to be in 

 the act of leaving the ray. This is the longest ray I have ever 

 seen bearing an apical cell in this species, and as a rule the 

 apical cells disappear long before the ray reaches so great a 

 length. 



In other species, so far as my observations go, the apical 

 cells may persist for a much longer time, — that is to say, until 

 the spicule ray has attained a much greater size. This is 

 especially the case in Clathrina, sp. dub. (see PI. 39, fig. 19), 

 in which species the apical cell disappears relatively late in the 

 history of the spicule; in fact, it is usually found adherent to 

 the tip of the ray after the basal cell has begun to migrate 

 from the base outwards, and sometimes the basal cell may be 

 found to have approached fairly close to the apical cell before 

 the latter disappears. 



In Clathrina contorta also the apical cells persist to a 

 late stage (PL 41, fig. 38). It seems as if there was some rela- 

 tion between the size to which the spicule is destined to grow 

 and the persistence of the apical cells. In the species charac- 

 terised by larger spicules the apical cells persist to a relatively 

 later period, as measured by the growth of the rays, than in 

 the species with smaller spicules. Unfortunately it is in 

 species of the former kind that the facts are most difficult to 

 ascertain, on account of the greater thickness of the wall and 

 the number of spicules crossing each other in all directions. 



We see, then, that the period at which the apical formative 

 cell disappears varies very much, and that in all cases it 



