Dr. W. Marshall on new Siliceous Sponges. 409 
water will not admit of it. The sponge must then seek in 
some other way to help itself, and this it can do only by 
forming a thin crust in which a large surface is developed 
with a small volume of body. This consideration leads to a 
series of consequences, to which I will here refer only en 
passant. On looking at a large, round, conical or cylindrical 
sponge with a smooth surface, we can assert, @ prior, that it 
is produced under favourable cir cumstances, and has had not 
only abundance of food, but also the necessary quiet; another, 
composed of mzandrically united plates, interwoven branches, 
&c., and traversed by numerous intercanals, will have had 
quiet but a more slender diet ; but a thin crust, unless it has 
been mechanically confined 1 in extension by growth between 
stones, will have passed its life with very little rest and with 
a badly supplied table; and in the last-named case the sponges 
are also usually polyzoic with small personal regions. In 
connexion with this, the facts of individual development are 
very instructive ; all young sponges are rounded, conical, or 
cylindrical, and, as a matter of course, the only question 1S, 
under what conditions they are further developed. By these 
their form is governed, and many species are in consequence 
exceedingly polytropic, showing an almost infinite power of 
form-variation ; while others are in so high a degree mono- 
tropic that they rather die than make concessions to external 
circumstances in their form. ‘There are extremely variable, 
but also extremely constant sponges, and these latter are 
naturally the rarer ones. 
It is clear that the specimen of P. Pechuélit, under some- 
what different circumstances, with a less pressure of water, 
may have been more freely "developed than the specimens of 
the other two species ; but the influence of the moving water 
is unmistakable in the position of the oral cones and their 
tendency in one direction. Probably also the serial arrange- 
ment of the oscula which we recognize in all the species may 
be referred to the same cause; in P. Pechuélit they lie also 
in the direction of the strike (direction of the greatest dia- 
meter) of the oral cones. In this sponge probably the central 
persons were not first developed, but those in the margin 
indicated by @ in fig. 10. In very strongly moving water a 
sponge will scarcely be able to bud in all directions ; the 
buds will rather always be formed in one direction, one be- 
hind the other, so that the younger will be somewhat pro- 
tected by the older ones from the disturbing influence of the 
flowing water; this may lead to a radiate arrangement, for a 
current of water breaking upon an obstacle opposed to it does 
not reunite immediately behind it, where there is rather a quiet 
Ann. & Mag. N. Hist. Sev. 5. Vol. xii. 30 
