361 
of the Ventriculidse of the Chalk. 
likely to offer some difficulty in the way of the free circulation 
of the sea-water. Confident however that, if my interpretation 
of the nature of these animals were correct, the means of free 
circulation must exist and might be discovered, I resumed the 
examination with an increased series of specimens. The result 
was the strengthening of all conclusions as to the physiology 
of the whole family in general, and of this species in particular, 
by the discovery of a contrivance by which this end was perfectly 
effected; a contrivance which, for its novelty and simplicity, may 
well claim the inquirer^s attention. 
Equidistant, or nearly so, along each lateral margin, I found 
what at first I took for a larger form of depression, but which, on 
dissection, I found to be actual perforations* through the mem- 
brane. These are found present from near the root to the ex- 
treme angular expansion already named. A comparison of many 
specimens satisfied me that these perforations are never absent ; 
that they are always ranged in the same way, and that their size 
is proportioned to that of the entire animal and to the width be- 
tween the walls of the arms. They are usually circular, some- 
times oval. They vary from half a line to two lines in longest 
diameter, seldom however attaining this last size. 
It became at once evident that this simple provision could 
have but one end ; but that that end it would fully effect. That 
end was the fulfilling of the very purpose whose incomplete- 
ness had before been felt. A constant access and circulation 
of sea-water would be maintained over all the inner surfaces 
of these lobes, deep and narrow as they are ; the water, admitted 
at the opening of the great central cavity, coursing out, by 
the continual action of numberless ciliated tentacles, through 
these marginal perforations. And herein it is that these perfo- 
rations differ from those in the group Apertij and do not bring 
this species within that group. The large central opening was 
amply sufficient, in this case, for the admission of sea-water, but 
the peculiar form of the arms interposed difficulties in the way 
of its free change and circulation, which is equally necessary to 
the well-being of these creatures. Hence this beautifully simple 
contrivance for the water, admitted by the central orifice, to pass 
out through these perforations. In the group Aperti, on the 
other hand, each perforated lobe offered the principal, if not in 
every case the only, means of both access and circulation of the 
* In some of Michelin's figures traces of some of these are found, which, 
when their nature is known, will be recognized, but which, as they appear 
in those figures, suggest nothing but irregular depressions : they attracted, 
indeed, no attention of that author, but, in his descriptions, they are wholly 
unnoticed, the margins being only said to be, — in the same words as the 
“ superficiebus laminum,” — “pc^/ora/w,” which, as applied, is erroneous. 
