CHAPTEK IX 



THE SHELL, ITS FOEM, COMPOSITION AND GEOWTH DESIGNATION 



OF ITS VAEIOUS PAETS 



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 w^hole, 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 the shell and animal are in the 

 closest possible connection, and that the shell is a living 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 assmned 

 by genera widely removed from one another in cardinal points of 

 organisation. This form of shell occurs in the common limpet 

 (Patellidae), in Ancylus (Limnaeidae), Hemitoma (Fissurellidae), 

 Cocculina (close to Trochidae), Urribrella and Siphonaria (Opis- 

 thobranchiata), while in many other cases the limpet form is 

 nearly approached. 



Eoughly speaking, about three-quarters of the known Mollusca, 

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



