CHAPTER IX 



THE SHELL, ITS FORM, COMPOSITION AND GROWTH — DESIG- 

 NATION OF ITS VARIOUS PARTS 



The popular names of 'shells,' 'shell-fish,' and the like, as 

 commonly applied to the Mollusca, the intrinsic beauty and 

 grace of the shells themselves, resulting in the passion for their 

 collection, their durability and ease of preservation, as compared 

 with the non-testaceous portion, — all these considerations tend 

 to unduly exalt the value of the shell as part of the organism as 

 a whole, and to obscure the truth that the shell is by no means 

 the most important of the organs. 



At the same time it must not be forgotten that the old 

 systems of classification, which were based almost entirely on 

 indications drawn from the shell alone, have been strangely 

 little disturbed by the new principles of arrangement, which 

 depend mainly on structural points in the animal. This fact 

 only tends to emphasise the truth that tlie shell and animal are 

 in the closest possible connexion, and that the shell is a liv- 

 ing part of the organism, and is equally sensitive to external 

 influences. 



A striking instance of the comparative valuelessness of the 

 shell alone as a primary basis of classification is furnished by 

 the large number of cases in which a limpet-shaped shell is 

 assumed by genera widely removed from one another in cardinal 

 points of organisation. This form of shell occurs in the com- 

 mon limpet (Patellidae), in Ancylus (Limnaeidae), Hemitoma 

 (Fissurellidae), Cocculina (close to Trochidae), Umbrella and 

 Siplionaria (Opisthobranchiata), while in many other cases the 

 limpet form is nearly approached. 



Roughly speaking, about three-quarters of the known Mol- 

 lusca, recent and fossil, possess a univalve, and about one-fifth 



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