98 THE STORY OF ANIMAL LIFE 



CHAPTER IX 



MOLLUSCA, THE SHELL-FISH 



THE shell-fish are called Mollusca, the soft- 

 bodied animals. It will easily be seen that this 

 name was intended to point out the distinction 

 between them and the Arthropoda, as regards 

 the way in which the skin is protected. In the 

 latter, as we have seen, the skin itself is hard- 

 ened. In the shell-fish, the skin secretes a cov- 

 ering which lies outside it. Just as our skins 

 pass out superfluous moisture to the outside, in 

 the form of perspiration, so the skin of the mol- 

 lusc continually passes to the outside the solid 

 substances which the body has taken in from the 

 sea-water ; and by the continual accumulation of 

 these, the shell is formed. This, at least, is the 

 view taken by modern authorities of the forma- 

 tion of the shell in most instances. 



The juvenile shell-collector usually begins his 

 knowledge of the classification of the Mollusca, 

 by learning that shells are classified as Univalves 

 and Bivalves. This distinction is useful as a be- 

 ginning. Univalves, that is to say shells which 

 consist of one piece, are those of the snail-like 

 animals, Gasteropoda, or Gastropoda, as some 

 prefer to spell it. Bivalves, or shells which con- 

 sist of two flaps, are those of the Lamellibranchiata 

 or animals with plate-like gills, such as the mussel 

 or oyster. 



Let us begin with the former. Everybody 

 knows the snail. The snail proper bears a typical 

 univalve shell : though in its relatives (the slugs), 

 the shell is more or less suppressed. The name, 



