CHAPTER IX. 
The Molluscs or Shell-Fish,—S ubkingdom MOLLUSCA. 
Science is never stationary, and consequently the scope of many groups of the 
animal kingdom has considerably altered since they were defined by their original 
founders. Such has been the case with the Mollusca of Cuvier. Besides the 
animals which constitute this subkingdom, as now understood, he included in it 
the Tunicata, Brachiopoda, and Cirripedia—branches of the zoological system, 
which more recent anatomists have long since removed elsewhere. At the present 
time the Mollusca comprise only such forms as the octopus, cuttle-fish, etc., all the 
marine shell-bearing animals of the whelk tribe, and other kinds, land and fresh¬ 
water snails, slugs, the tooth-shells, and bivalves of every description. The number 
of known species is very large, and fresh forms are constantly being discovered. 
Probably some fifty thousand recent species have already been described, the 
number of aquatic being more than double that of the terrestrial species. The 
aquatic kinds, however, will eventually be found to preponderate still more, for the 
sea appears to be inexhaustible in the production of new forms. It matters not 
in what ocean the dredge is let down, be it to a great depth, or in shallower 
water, something new is certain, sooner or later, to be gathered in. Drop the 
dredge to three thousand fathoms (more than three miles), and still molluscs are 
met with, and the extreme depth to which molluscan life extends has yet to be 
ascertained. The great coast lines of South America, Africa, Asia, and parts of 
Australia have been but imperfectly explored for the smaller kinds of Mollusca; for 
whenever a limited stretch of coast is carefully searched by the conchologist, con¬ 
siderable numbers of new species are forthwith discovered. On the contrary, 
with the terrestrial forms the case is different. They are more easily acquired, as 
they come under actual vision, and all the inhabitants of a given district can in 
course of time be known. 
Molluscs are soft, cold-blooded animals, without any internal 
Definition. . . . J 
skeleton; but this is compensated for in the majority of cases by an 
external hardened shell, which serves at once the purpose of bones and as a means 
of defence. Their bodies are not divided into segments like those of insects and 
worms, but are enveloped in a muscular covering or skin, termed the mantle, the 
special function of which, in the majority of species, is the formation or secretion 
of the shell. Molluscs are more or less bilaterally symmetrical; but this bilateral 
symmetry in some cases, particularly among the Gastropods, is to some extent 
obscured by the contortion of the body. The foot, which serves the purpose of 
locomotion, or is used in burrowing in sand, wood, and rock, etc., is an organ 
highly characteristic of most Molluscs. The shells, in the vast majority, consist 
