Pvn H,i I'UitlS

28 CONCHIFERA.
back, for the admission of food and water,
as well as for
the ejection of faecal matter ; while in others it is more
or less closed in front, or open only in that part for the
passage
of the foot. In the latter case the mantle on
one or either side is folded, so as to form a single or
double tube. This usually takes place on the posterior
but in a few instances
side, where the shell is broadest ;
(as in the Kelliidce) the incurrent or alimentary tube is
placed on the anterior side, and the excurrent or anal
tube on the posterior side. The tubes are of various
lengths, and when they do not project beyond the edges
of the shell they are termed " sessile." The excretory
opening is always situate on the upper part of the pos-
terior side. That by which the animal takes in its
nourishment, and which supplies the gills with aerated
water, is usually on the same side,
but below the other
opening. The excretory opening or tube is the smaller
of the two. With respect to the reproductive system of
the Lamellibranchiata, it is by
no means settled whether
any, or which of them, have separate sexes. Loven is
positive that such is the case in Modiolaria,
bryogeny of which he has investigated
the em-
with his usual
care; and Sars assures us that Aximis is unquestionably
also dioecious. I have not myself examined the question ;
but I would refer my readers to what I have already said
in page xxv of the Introduction to the first volume.