MOLLUSCA. 757 
while the circumstance of its surface being smooth, or variously grooved, 
tuberculated, or marked with spines, supplies others. The immense variety 
in the coloring, also, affords obvious means of specific distinction. All the 
mollusca are oviparous. The reproduction is therefore necessarily effected 
by sexual impregnation. In some of the orders of these animals, the sexes 
are separate, as in the Sepiaria. These animals, however, do not copulate, 
but the males shed a fecundating fluid upon the ova deposited by the females. 
It appears that the other mollusca, such in particular as the Gasteropoda and 
Trachelipoda, have the two sexes united in the same individual. Of these 
hermaphrodites, some require a reciprocal copulation, while others appear to 
fecundate themselves. The ova of the mollusca are not in general hatched 
until after they have been deposited. Some have acrustaceous covering like 
the ova of birds and reptiles, as is the case with the helices; others are 
sometimes surrounded with a sort of jelly, by which they are attached to- 
gether, as in the planorbes, lymnee, &c., and others are contained in mem- 
branous sacs, of very different forms, sometimes solitary, but more commonly 
in groups; each of the sacs containing several young individuals, which 
issue from them alive, with their shell already formed, as is the case with 
the Buccina, &c. The mollusca are in general aquatic animals. Many 
species, however, are terrestrial; and some appear to live almost constantly 
under ground, such as the testacelle, but this is rare. A great number are 
found on the surface, such as the limaces, helices, &c.; while some are to a 
certain degree amphibious, as the lymnee. By far the greater part, however, 
iive in water, fresh and salt. Of the fresh water kinds, some remain free at 
the surface of the mud; others adhere to other bodies. Of the latter, the 
circumstances, in this respect, are considerably varied. Some are found 
only on the coasts, and are termed littoral species, as the genera patelia, turbo, 
&e. ; others appear to exist only at a distance from the shores, and in deep 
water, whence they are called pelagic species; and the Sepzaria wander in 
the depths of the ocean. With respect to their geographical distribution, 
little is known, this subject not having been submitted to sufficient investi- 
gation. They are found, however, in all parts of the world, whether in the 
seas, rivers, and lakes, or on land. Certain tribes are confined to particular 
zones, While others appear to inhabit all. Thus the Sepzaria occur in all 
seas, While the nautilus and spirula are found only in the torrid zone. The 
food of the mollusca consists of almost all sorts of substances, animal and 
vegetable, in all states, living or dead, fresh or putrid; but each species is in 
general confined to acertain kind. ‘The uses of the mollusca, in the econo- 
my of nature, are varied and extensive. They afford food to numerous 
animals, especially fishes and birds, and to man himself. The savage tribes 
which live along the coast, in many parts of the world, employ them much 
as an article of food. Even in civilized countries, the mollusca frequently 
form a considerable pertion of sustenance, although in general they are nei- 
ther very pleasant nor wholesome. The cuttle-fish furnishes a fluid from 
64 
