STUDIES IN SPICULE FORMATION. 271 
and certain Clathrinide, and the finished regular ‘‘ symmetri- 
cal” form of spicule in the encrusting Clathrinide “is doubt- 
less correlated with the reticular form and growth of the 
sponges themselves” (Minchin). 
Spicules, again, which develop in a mass of sponge-jelly, 
and which are therefore not in close proximity to two parallel 
surfaces, are also irregular in form; though, in these larval 
spicules, there necessarily exists an initial tendency to assume 
the triradiate form, yet, owing to the absence of the structures 
and hence forces which, in the adult sponge, ensure the sym- 
metrical form of the spicule, this is not preserved in the 
Tpxt-FIc. 10.) 
ensuing growth. ‘T'o speak in more detail, the thin unilaminar 
wall of such sponges as the Clathrinide must be continually 
subject to pressures incident perpendicularly to its surface, 
and, granted the presence of a young spicule of regular tri- 
radiate form, this wall must tend to be invaginated in each of 
the three areas situated between the rays to an equal extent, 
which means that a groove is formed in line with each ray of 
the spicule along which the apical cell must tend to travel, as 
1 Tam indebted to Mr. Chubb of University College for this figure, which 
well illustrates my meaning. 
