CHAPTER XII. 



MOLLUSCS, OR SHELL-FISH. 



General characters of the Mollusca An outline classification The 

 Chitons, their habits and structure The common limpets Forms 

 with coiled shells Their general characters Tops and periwinkles 

 Species of periwinkles Some allied forms Carnivorous forms 

 Whelks, purples, and their allies Their egg-capsules and develop- 

 ment The cowry. 



HHHE Mollusca form a very large group, including animals 

 _L which are usually well defined and easy to recognise. 

 The fact that most possess a shell which is easy to study 

 and to preserve has rendered them general favourites among 

 those interested in shore animals. Probably, indeed, most 

 people have at some period of their lives made collections 

 of shells ; all know how beautiful in form and colour they 

 often are. Interesting as shells are, however, it cannot be 

 denied that as a whole the Mollusca are a group of con- 

 siderable difficulty. The shells are much more external 

 structures than the coats of the Crustacea, have a less 

 intimate connection with the body, and are therefore not 

 of much use as guides to affinities, except to a very general 

 degree, while the study of the internal structure is not easy. 

 It is a natural result of this, that while much has been 

 written on the shells of Mollusca, their internal structure 

 is still in many cases insufficiently known. 



Perhaps the easiest way to get a general notion of the 

 structure of Molluscs is to begin with the study of some of 

 the limpets. Knock off the rocks a f e w large specimens of 

 the common limpet, and look for the largest specimen you 

 can find of the little tortoise-shell limpet, or its relative 

 the little pink limpet, to be found far out on the rocks 



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