158 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



arrangement for the exit or entry of nerve-fibres on the inside as 

 well as the outside of the shell. Now, if I am right in my assertion 

 that there is a large nerve-ganglion in the columella of the shell, 

 then we must expect large means of communication between this 

 nervous centre and the soft tissues which lie inside the whorls of 

 the shell. There are equally extensive communications from the 

 sense-organs outside. I have just now stated that they are with 

 difficulty visible in the last named species, but in sufficiently thin 

 sections they are equally conspicuous. What has been observed 

 with reference to the eyes and sense-organs on the outside of this 

 species will be mentioned further on. It should be mentioned also 

 that the different strata of colour, forming fine lines of division, 

 are not always due to different plates of shelly matter, but 

 apparently to differences in the strata or lines of nerve tissue and 

 in the amount of pigment connected with these lines, which varies 

 in a manner I cannot explain. Externally the shell is marked by 

 no bands of colour, except that the nodules and spiral lines are 

 very much darker than the rest of the shell, giving it the appearance 

 in fact of a scorched and blackened piece of wood. 



In this species (Gerithium ebeninum) there is an absence of 

 any appearance of what is called fibrous structure, that is to say, 

 plates of shelly matter with oblique fibres arranged in successive 

 strata at right angles to one another. This shell is of unusual 

 hardness, requiring considerable force to break it. This is the 

 more singular when we reflect how completely permeated it is with 

 animal matter. When once the animal dies, unless the shell be 

 carefully kept from exposure to the weathering influences of air 

 and moisture it soon falls into decay. 



Now this shell is to all appearance non-nacreous, and yet an 

 attentive examination of the shell will show a certain amount of 

 iridescence in the shell structure. In fact nacreous iridescence 

 seems to depend upon the transparency of the plates and the colour 

 of the intercalated membrane which is thus to be seen through 

 the shelly matter. There is a hard, white, compact substance in 

 the shells of Chama gigas, which is so hard as to be worked like 

 stone into bracelets and ornaments ; yet even this, hard as it is, 

 will be found to contain, under the microscope, alternate lines of 

 transparent shelly matter, which with the aid of the membrane 

 underneath, give rise to a nacreous appearance. It is generally 

 thought that only a few shells are nacreous ; but examination 

 with microscopic aid shows that the fact is the other way, only 

 that it is but faintly perceptible in most kinds. This is what we 

 might reasonably expect ; for if nacre be due to the intercalation 

 of finely divided membranous tissue, all shells partake of this 

 structure. If a chip is taken off the enamel, the transparent lines 

 are easily visible with transmitted light. 



