12 MOLLUSCA. 



cipal parts which it presents are the aperture or mouth, con- 

 sisting of an inner or columellar lip, and an outer lip ; the body 

 of the shell consisting of the last and generally tumid turn or 

 whorl ; the spire, formed of the convolutions, which are only in 

 part seen, because enveloped by the last turn ; and the columella, 

 or axis round which the shell is contorted. When the columel- 

 la is hollow its opening is termed the umbilicus. 



The shell consists of a mixture of calcareous matter (carbo- 

 nate of lime) and gelatinous matter. This is frequently cover- 

 ed externally by a thin layer of the latter substance, forming what 

 is called the epidermis. It exhibits a great variety of colouring. 

 The form of the shell indicates that of the animal which in- 

 habits it ; and is used as furnishing the generic characters, the 

 structure of the animals of this class not being generally known. 

 It also furnishes many of the specific characters ; while the cir- 

 cumstance of its surface being smooth, or variously grooved, 

 tuberculated, or marked with spines, supplies others. The im- 

 mense variety in the colouring also affords obvious means of 

 specific distinction. 



All the Mollusca are oviparous. The reproduction is there- 

 fore 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 hem aphrodites, some require 

 a reciprocal copulation, while others appear to fecundate them- 

 selves. 



The ova of the Mollusca are not in general hatched until af- 

 ter they have been deposited. Some have a crustaceous covering 

 like the ova of birds and reptiles, as is the case with those of the 

 Helices ; others are sometimes surrounded with a sort of jelly, 

 by which they are attached together, as in the Planorbes, Lym- 

 ncece, &c. ; and others are contained in membranous 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^hc. 



The Mollusca are in general aquatic animals. Many species, 



