MOLLUSCA. 
336 
a long time in parts after they have been amputated. Their skin is naked, very sensi- 
tive, and, in general, bedewed with a humour, which oozes from its pores. No peculiar 
organ of smell has yet been discovered, although they enjoy that sense ; and it may be 
that the entire skin is its seat, for this has much resemblance to a pituitary membrane. 
All the Acephales, the Braehiopods, the Cirrhopods, and some of the Gasteropods and 
Pteropods, are destitute of eyes ; but the Cephalopods possess these organs, with a 
structure equal, at least, in complexity, to those of animals with warm blood. They also 
are the only Mollusca in which organs of hearing have been detected, and in which 
the brain is inclosed in a particular cartilaginous skull. 
Nearly all the Mollusca have a developement of the skin which covers the body, and 
resembles more or less a cloak, but which is often reduced into a simple disk, or is folded 
into a tube, or hollowed into a sac, or, lastly, extended and divided in the form of fins 
or swimmers. 
We call those Mollusca naked in which the cloak is simply membranous or fleshy ; 
but there is commonly formed within it one or several laminae of a more or less solid 
substance, which is deposited in layers, and increases at the same time in extent, as 
well as in thickness, because the recent layers always extend beyond the older ones. 
When this substance lies concealed in the cloak, common usage allows us to extend 
to the species so circumstanced, the title of naked Mollusca. But oftener that substance 
assumes such a size and developement that the animal can contract or withdraw under 
its shelter ; we then give it the name of shell, and the animal is said to be testaceous. 
The skin which covers the shell is thin, and sometimes dried, or wanting: it is commonly 
called [by French naturalists], the drap-marin, [and by the English, and those who 
write in the Latin tongue, the epidermis'].'^ 
The variety in the forms and colour, in the exterior sculpture, composition, and lustre 
of shells, is infinite. The greater number by far are calcareous ; there are some simply 
corneous ; but all are formed of material deposited in layers, or exuded by the skin 
under the epidermis, as are the rete mucosum, the nails, the hair, the horns, the scales, 
and even the teeth. The texture of shells differs according as that exudation is made 
in parallel layers, or in vertical filaments arranged closely against each other, f 
The Mollusca present every kind of mastication and deglutition : their stomachs are 
sometimes simple, sometimes multiplicate, often furnished with peculiar armatures, and 
their intestines are variously elongated. They have, in general, salivary glands, and 
always a liver of considerable size, but no pancreas f nor mesentery. Several have 
secretions, w'hich are peculiar to them. 
They exhibit, also, every variety of generation. Several fecundate themselves, 
while in others, although hermaphrodite, the union of two individuals is necessary to fe- 
cundation : in many the sexes are distinct and separate. Some are viviparous ; others 
are oviparous, and the eggs of these are sometimes enveloped in a more or less con- 
sistent shell, or sometimes only in a simple viscosity. 
These variations in digestion and generation are found in Mollusca of the same order, 
sometimes of the same family. 
llie Mollusca, in general, seem to be animals of inferior developement : hebetous 
* Previous to my system, the Testacea were considered a peculiar I t The student will find the formatiou of shells, and their structure, 
order ; but the transitions from the naked to the shelled Mollusca are admirably explained by Mr. Gray, in a paper, on the economy of Mol- 
so insensible, and their natriral divisions are so interlaced, that this I luscous animals, inserted in the PhU. Trans., 1833. — Ed. 
distinction can be no longer retained. Moreover, there are several t Professor Grant maintains that there is a pancreas, or its repre- 
Testacea which are not Mollusca. | sentative, in all classes of Mollusca. — Ed. 
