STUDIES IN SPICULE FORMATION. 237 
have assumed the equiangular triradiate form had it remained 
undisturbed,—and it is only the more distal portions of the 
three rays that become depressed towards the horizontal— 
this equally showing that some external cause must have 
exerted a disturbing influence during the later stages of 
growth of the spicule. It is true that in other sponges this 
depression of the paired rays in triradiates—the “ alate” 
form—cannot always be attributed to a tendency to invagina- 
tion of the sponge-wall, since mere contact with a lining 
membrane appears to be capable of producing the same effect. 
Nor is it a fact that in all sponges the triradiates situated at 
the edge of the osculum have depressed paired arms, since in 
many Ascons they protrude through the thin body-wall - 
instead, so resembling the monaxons. But despite these ex- 
ceptions I think it will be admitted (judging from the figure 
above provided) that the forms of the Sycon spicules (and 
these same modified forms occur in numerous other vertical 
and cylindrical sponges, both Ascons and Sycons) bear a 
relation to the incident forces I have briefly indicated, and, 
taken in conjunction with the general hypothesis to be 
elaborated below as to the cause of the symmetrical disposi- 
tion of the spicules, little room for doubt as to this relationship 
can remain. 
Another example of the modification of the ideal triradiate 
form due to environmental influence is supplied by the varying 
length of the vertical “ posterior”’ rays of triradiates situated 
in different regions of the sponge. If text-fig. 1 above, e. g. 
be again examined, it will be observed that the posterior 
rays of triradiates situated at the base of the sponge are 
longer than is consistent with the equiradiate type of spicule ; 
and that, on the other hand, the posterior rays of triradiates 
situated at the extremity of the oscular rim are shorter. One 
possible explanation of this difference of length is provided, 
as before, by the fact that the sponge is constantly under- 
going flexion (see p. 260). In the upper regions of the Sycon 
flexion attains its maximum, and hence there is, on this account, 
less room for elongation of the vertical posterior ray (as indi- 
