410 Dr. W. Marshall on new Stliceous Sponges. 
spot which, if the obstacle was a ball suspended freely in the 
water, would have the form of a regular cone, but otherwise 
may be of various forms according to the configuration and 
position of the interposed object. In our sponge growing as 
acrust, the quieter space of water will have been approxi- 
mately semiconical, at least for a certain distance, until the 
laterally diverted masses of water united again; and here the 
oral cones could become better developed than the anterior 
and older ones, which are more exposed to the force of the 
water, so that, under certain circumstances, the oldest oral 
cones will show the poorest development. 
It may be objected that the freer development of the oral 
cones in P, Pechuélit might indeed indicate a somewhat weaker 
pressure of water, but that the differences in the form and 
arrangement of the spicules of the different species are in this 
way by no means explained; for in P. Pechuélic they are 
more extended and arranged in trains, whereas in the other 
species they are much more considerably curved and abso- 
lutely without any tendency to arrange themselves in bundles, 
both of which characters appear certainly to indicate growth 
in quieter water. I believe that this is only apparent ; more 
strongly curved spicules will be able to interlock more closely 
than less curved or straight ones; they will form more com- 
pact masses, and so be able to oppose a more considerable 
resistance to the force of the currents, which will operate 
with effect in opposition to the efforts of the latter to arrange the 
skeletal elements of the sponge, whether they be proper to 
it or foreign bodies incepted to strengthen it, in trains in its 
own direction. A sponge with straight spicules under the 
very strong influence of a current acting in one direction 
must have a difficult task to maintain itself in position 
unless its spicules are remarkably spinous, or unless it 
differentiates auxiliary spicules in the form of hooks &e. 
In my opinion, to which I have already often given expres- 
sion, the forms and arrangements of the skeletal elements of 
the sponges are for the most part to be referred to influences 
of purely mechanical nature, which, as they will frequently 
come into conflict with the inherited tendencies which dwell 
even in these structures, have led to that enormous abun- 
dance of often very wonderful adaptations to conditions which 
are usually, at least directly, still obscure. Besides the move- 
ments of the water acting from without, the currents in the 
interior of the sponge caused by the position ot the flagellate 
chambers and the action of their ciliary organs will also be 
important factors in the construction of the skeleton ; but ver 
frequently also the position and nature of the flagellate cham- 
