CHAPTER IX. 



The Molluscs or Shell-Fish, — Subkingdom 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. . . . . „ 



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 



