154 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



is evidently fibrous to the eye, but which, when examined under 

 the microscope with reflected light, resembles that of an assemblage 

 of basaltic columns. The shell is thus seen to be composed of a 

 vast number of prisms, having a tolerably uniform size, and usually 

 presenting an approach to the hexagonal shape. These are arranged 

 perpendicularly (or nearly so) to the surface of the laminae of the 

 shell ; so that its thickness is formed by their length, and its two 

 surfaces by their extremities. A more satisfactory view of these 

 prisms is obtained by grinding down a lamina until it possesses a 

 high degree of transparency ; and it is then seen that the prisms 

 themselves appear to be composed of a very homogeneous substance; 

 but that they are separated by definite and very strongly-marked 

 lines of division. When such a lamina is submitted to the action 

 of dilute acid, so as to dissolve away the carbonate of lime, a 

 tolerably firm and consistent membrane is left, which exhibits the 

 prismatic structure just as perfectly as did the original shell ; the 

 hexagonal divisions being apparently those of the walls of cells 

 resembling those of the pith or bark of a plant, in which the cells 

 are frequently hexagonal prisms. In very thin natural laminse 

 the nuclei can often be plainly distinguished." The Microscope 

 and its Revelations by W. Carpenter, M.D., London, 1857, 2nd 

 edit. pp. 546-7. 



I believe the basaltic structure here referred to is a portion of 

 the outer surface ; and the hexagonal cells are, in the species I 

 mention, the nerve-tubes or portions of the capsules for the sense- 

 organs. I do not undertake to say that this is the case in the 

 particular species referred to by I)r. Carpenter, because I have 

 not examined it. But I have seen a similar structure in shells I 

 have examined which I explain as above. 



Yet as a general rule I am hardly inclined to state that there is 

 a uniformity in the way in which the plates of shelly matter 

 succeed each other which is applicable to every species. They are 

 continually being added as the growth of the shell progresses. 



It has already been observed that the shell is not to be regarded 

 as an inorganic investiture ; but one which is developed with the 

 growth of the animal, and has an intimate association with all that 

 is considered living structure. The mere fact of its taking for its 

 constituents elements which are not found in other portions of its 

 economy, and which belong more properly to the mineral kingdom, 

 does not place it outside the pale of living tissue. It lives with 

 the animal and is just as much subject to disease or dissolution as 

 any other part. In a section taken from the columella of Cerithium 

 ebeninum, Brug., I have found no less than 15 or 16 thin layers 

 of new shelly matter added in succession from within. All these 

 are regularly perforated by the tubes or nerve-channels. There 

 are, it would seem, two kinds of growth in the shell ; one from 



