268 W. WOODLAND. 
attained a certain length, an osculum is formed, and, as a 
result of this, the young triradiates in the vicinity 
of the osculum immediately assume the oscular 
arrangement (see M in text-fig. 8 below). This fact 
seems to ine clearly to prove that the disposition of the 
spicules is due to the direct action of the environment and 
not to inheritance. 
From what has been said hitherto, it would logically follow 
that in sponges not vertically disposed and not flexible on a 
slender basis, no definite arrangement of the spicules would 
occur. It remains to be pointed out as strong confirmation 
of the above general theory that such is actually found to be 
the case—that in those sponges which are either not subject 
to the pressures resulting from motion of the surrounding 
water or whose conformation is not such as to cause the spicules 
to be influenced by these pressures (as e. g. the numerous 
non-erect encrusting forms of the Clathrinide), there is no 
regular disposition of the spicules, and that the same is the case 
in the very young forms of those sponges which are erect in 
the adult condition, in which, before either the osculum or 
the sponge-wall is formed, the spicules are not only irregular 
in disposition, but also irregular in form, all of which facts 
(except the last, of course) might be anticipated on the above 
hypothesis. 
The more or less vertical position of the monaxon spicules 
in Sycons and other erect Calcarea can be explained in a 
manner similar to that adopted in the case of the triradiates 
of terminally-closed Leucosolenia diverticula. In brief, with 
the exception of those few initially disposed in an exactly 
transverse direction (and such are found), all young monaxons 
more or less inclined to the vertical will tend to be righted by 
the lateral pressures to which they are subjected, as the follow- 
ing diagram suggests (text-fig.9). In addition to this cause, 
invagination of the wall will also tend to cause the monaxons 
to assume a vertical disposition, since (with the exception 
again of those few initially disposed in an exactly transverse 
direction) it is only when they are so disposed that they will 
