CHAPTER IX 

 THE MOLLUSKS OR SHELLFISH Subkingdom MOLLUSKA 



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 Molluska 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 Molluska comprise only such forms as the octopus, cuttlefish, 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 mollusks are met with, 

 and the extreme depth to which molluskan 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 Molluska; for whenever a 

 limited stretch of coast is carefully searched by the conchologist, considerable 

 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. 



Mollusks are soft, cold-blooded animals, without any internal skel- 

 eton; 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 defense. 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. Mollusks are more or less bilaterally symmetrical; but this bilateral sym- 

 metry 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 character- 

 istic of most Mollusks. The shells, in the vast majority, consist either of a single 

 piece, as in the snail, whelk, etc., or of two portions (valves) as in the oyster, 

 (3288) 



