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History of Animals   

Testaceans also differ from one another in regard to the thickness
or thinness of their shell, both as regards the shell in its
entirety and as regards specific parts of the shell, for instance, the
lips; for some have thin-lipped shells, like the mussel, and others
have thick-lipped shells, like the oyster. A property common to the
above mentioned, and, in fact, to all testaceans, is the smoothness of
their shells inside. Some also are capable of motion, like the
scallop, and indeed some aver that scallops can actually fly, owing to
the circumstance that they often jump right out of the apparatus by
means of which they are caught; others are incapable of motion and are
attached fast to some external object, as is the case with the
pinna. All the spiral-shaped testaceans can move and creep, and even
the limpet relaxes its hold to go in quest of food. In the case of the
univalves and the bivalves, the fleshy substance adheres to the
shell so tenaciously that it can only be removed by an effort; in
the case of the stromboids, it is more loosely attached. And a
peculiarity of all the stromboids is the spiral twist of the shell
in the part farthest away from the head; they are also furnished
from birth with an operculum. And, further, all stromboid testaceans
have their shells on the right hand side, and move not in the
direction of the spire, but the opposite way. Such are the diversities
observed in the external parts of these animals.
The internal structure is almost the same in all these
creatures, and in the stromboids especially; for it is in size that
these latter differ from one another, and in accidents of the nature
of excess or defect. And there is not much difference between most
of the univalves and bivalves; but, while those that open and shut
differ from one another but slightly, they differ considerably from
such as are incapable of motion. And this will be illustrated more
satisfactorily hereafter.
The spiral-shaped testaceans are all similarly constructed, but
differ from one another, as has been said, in the way of excess or
defect (for the larger species have larger and more conspicuous
organs, and the smaller have smaller and less conspicuous), and,
furthermore, in relative hardness or softness, and in other such
accidents or properties. All the stromboids, for instance, have the
flesh that extrudes from the mouth of the shell, hard and stiff;
some more, and some less. From the middle of this protrudes the head
and two horns, and these horns are large in the large species, but
exceedingly minute in the smaller ones. The head protrudes from them
all in the same way; and, if the animal be alarmed, the head draws
in again. Some of these creatures have a mouth and teeth, as the
snail; teeth sharp, and small, and delicate. They have also a
proboscis just like that of the fly; and the proboscis is
tongue-shaped. The ceryx and the purple murex have this organ firm and
solid; and just as the myops, or horse-fly, and the oestrus, or
gadfly, can pierce the skin of a quadruped, so is that proboscis
proportionately stronger in these testaceans; for they bore right
through the shells of other shell-fish on which they prey. The stomach
follows close upon the mouth, and, by the way, this organ in the snail
resembles a bird's crop. Underneath come two white firm formations,
mastoid or papillary in form; and similar formations are found in
the cuttle-fish also, only that they are of a firmer consistency in
the cuttle-fish. After the stomach comes an oesophagus, simple and
long, extending to the poppy or quasi-liver, which is in the innermost
recess of the shell. All these statements may be verified in the
case of the purple murex and the ceryx by observation within the whorl
of the shell. What comes next to the oesophagus is the gut; in fact,
the gut is continuous with the oesophagus, and runs its whole length
uncomplicated to the outlet of the residuum. The gut has its point
of origin in the region of the coil of the mecon, or so-called
'poppy', and is wider hereabouts (for remember, the mecon is for the
most part a sort of excretion in all testaceans); it then takes a bend
and runs up again towards the fleshy part, and terminates by the

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