The non-spiral

univalves and bivalves are in some respect
similar in construction, and in some respects dissimilar, to the
spiral testaceans. They all have a head and horns, and a mouth, and
the organ resembling a tongue; but these organs, in the smaller
species, are indiscernible owing to the minuteness of these animals,
and some are indiscernible even in the larger species when dead, or
when at rest and motionless. They all have the mecon, or poppy, but
not all in the same place, nor of equal size, nor similarly open to
observation; thus, the limpets have this organ deep down in the bottom
of the shell, and the bivalves at the hinge connecting the two valves.
They also have in all cases the hairy growths or beards, in a circular
form, as in the scallops. And, with regard to the so-called 'egg',
in those that have it, when they have it, it is situated in one of the
semi-circles of the periphery, as is the case with the white formation
in the snail; for this white formation in the snail corresponds to the
so-called egg of which we are speaking. But all these organs, as has
been stated, are distinctly traceable in the larger species, while
in the small ones they are in some cases almost, and in others
altogether, indiscernible. Hence they are most plainly visible in
the large scallops; and these are the bivalves that have one valve
flat-shaped, like the lid of a pot. The outlet of the excretion is
in all these animals (save for the exception to be afterwards related)
on one side; for there is a passage whereby the excretion passes
out. (And, remember, the mecon or poppy, as has been stated, is an
excretion in all these animals-an excretion enveloped in a
membrane.) The so-called egg has no outlet in any of these
creatures, but is merely an excrescence in the fleshy mass; and it
is not situated in the same region with the gut, but the 'egg' is
situated on the right-hand side and the gut on the left. Such are
the relations of the anal vent in most of these animals; but in the
case of the wild limpet (called by some the 'sea-ear'), the residuum
issues beneath the shell, for the shell is perforated to give an
outlet. In this particular limpet the stomach is seen coming after the
mouth, and the egg-shaped formations are discernible. But for the
relative positions of these parts you are referred to my Treatise on
Anatomy.