244 The Study of Animal Life PART m 



(Chatoderma, Neomenia) where an incipient shell is represented 

 only by a few spicules or plates of lime. 



The shell is made by the folded skin or "mantle" ; it consists 

 for the most part of carbonate of lime along with a complex organic 

 substance called conchiolin ; it shows three layers, of which the 

 outermost is somewhat soft and without lime, while the innermost 

 shines with mother-of-pearl iridescence. The whole product is a 

 cuticle something formed from the skin ; its varied colours and 

 forms are beautiful ; it is a protective shield ; but there are many 



FIG. 48. The common octopus. (From Chambers' s Encyclop. ; after Urchin.) 



questions about shells which we cannot answer. Where does the 

 carbonate of lime come from, since that salt is often far from 

 abundant in the water in which most molluscs live ? Have they 

 the power of changing the abundant sulphate of lime in sea- water 

 into carbonate of lime, perhaps by an interaction with waste products 

 excreted from the skin ? Is the shell an expression of the constitu- 

 tional sluggishness of the animal since it seems on the whole to be 

 most massive in the most sluggish, least so in the most active forms ? 

 Most molluscs are marine, on the shore, in the open sea, in the 

 great depths ; there are also many freshwater forms, e.g. the 

 mussels Anodon and Unio, and the snails, Lymnaus t Planorbis, 



