336 



MOLLUSCA. 



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 Brachiopods, 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 lamina? 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 % nor mesentery. Several have 

 secretions, which 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 MoUusca of the same order, 

 sometimes of the same family. 



The Mollusca, in general, seem to be animals of inferior developement: hebetous 



• Previous to my system, the Testacea were considered a peculiar 

 order ; but the transitions from the naked to the shelled Mollusca are 

 so insensible, and their natural divisions are so interlaced, that this 

 distinction can be no longer retained. Moreover, there are several 

 Testacea which are not Mollusca. 



+ The student will find the formation of shells, and their structure, 

 ad.mrably explained by Mr. Gray, in a paper, on the economy of Mol 

 luscous animals, inserted in the Phil. Trans., 1833.— Ed. 



A Professor Grant maintains that there is a pancreas, or its rep-8- 

 i^ntative, in all classes of Mollusca.— Ed. 



