250 On the Form, Growth, and Construction of Shells. 



Formerly, shells were supposed to have a merely crystal- 

 line structure ; in fact, that they were a mere inorganic exuda- 

 tion. Look, for example, at the exterior of the operculum of 

 Turbo sarmaticus (see Plate, p. 245, Fig. 3, b) how exactly it 

 resembles tufa formed by spray on a limestone rock (the 

 mantle of the animal does not continuously cover it, on the 

 contrary, it is the door which, when closed, shuts in and pro- 

 tects the mantle ; but at times it certainly does cover it, for 

 it deposits on each little eminence an additional layer of shell- 

 matter) ; or at the broken tube of the great Teredo (Furcella) 

 of Java, which is more than two feet in length, and half an 

 inch in thickness ; or at the shell of the fossil Belemnite and 

 recent Pinna, they have a fibrous structure, which might 

 easily be mistaken for mineral. Drs. Bowerbank and Carpen- 

 ter have advocated the views taken by botanists and anato- 

 mists, namely, that the hard tissues of shells are formed by de- 

 posits of lime in cells, or upon layers of membi'ane. Mr. G. 

 Rainey, having observed that when salts crystallize in syrup, 

 they assume a globular or other concretionary form, not geo- 

 metrical, was led to imagine that a close imitation of shell- 

 structure might be produced by causing carbonate of lime to 

 deposit in minute concretions, which, coming in contact, be- 

 come polygonal in the course of their growth. We Lave every 

 reason to believe that the cells are formed first, and then that 

 lime is deposited in them afterwards, just as the hard tissues 

 of seed-vessels, the albumen of the vegetable ivory, and the 

 silicious cuticle of grain is formed. 



Dr. Bowerbank, however, has been misled in the attempt 

 to compare the shell and the formation of shell too closely 

 with that of bone, a highly vascular structure capable of absorp- 

 tion and internal repair, being permeated by canals in which 

 the blood circulates. 



If we examine any immature shell, we shall find the lip, or 

 growing margin of the shell, much thinner than the rest of the 

 shell, and, indeed, quite soft. The mucus of this soft, growing 

 edge is found to contain granules, which are, in fact, incipient 

 cells destined to unite and form the calcareous matter of the 

 future shell- wall. The gradual formation of shell may readily 

 be traced in all its stages. 



There is an intimate connection between the mantle of the 

 mollusk and its shell. The oyster, as we see it on the supper- 

 table is much smaller than its shell, and adheres only by its 

 glistening shell-muscle ; but when alive, its mantle, extended 

 to the very edge of the valves, lining the whole interior, and 

 having a slight adhesion, especially at the edge of the valves, 

 which is speedily ruptured, however, when the poor animal is 

 forcibly invaded. 



