Miscellaneous. 431 



It is to this that is due the general coloration of the shell, as also 

 that of the spots and bands. The second layer, which is colourless 

 and is commonly designated nacre, includes several strata of prisms 

 arranged horizontally, and the axes of which, in two successive 

 strata, are nearly perpendicular to each other. The cuticle and the 

 calcareous layers are produced by different regions of the mantle, 

 approaching nearer to the collar in proportion as the layers are 

 more superficial. The cuticle is formed by an apparatus which we 

 believe has not yet been described, and which we shall call the 

 cutogenic apparatus (appareil cutogene). It is composed of two 

 special organs situated immediately behind the collar. One of them 

 consists of a furrow, to which we have given the name of the 

 pallial groove, parallel to the margin of the mantle, and at the 

 bottom of which glandular caeca open ; the other, situated behind 

 the former, appears, upon an antero-posterior section, like an epithe- 

 lial wedge buried in the substance of the mantle. It is formed of 

 long vertical cells, shaped like bottles, the orifices of which open at the 

 base of the organ which we shall name provisionally the epithelial 

 organ. These cells contain granules, which are isolable in potash, 

 and a nucleus situated in their deeper-seated part. We must regard 

 them as differentiated epithelial cells. 



The cutogenic apparatus exists in the embryo when still enclosed 

 in the ovular envelopes. A.t this epoch the shell is already fur- 

 nished with a cuticle. It persists during the whole of the growth 

 of the young snail, the epithelial organ showing itself under the 

 appearance of a white border surrounding the margin of the mantle. 

 In the adult it disappears and is replaced by the tissue of the mantle. 

 The long-necked cells become converted into ordinary epithelial 

 cells, capable of producing nacre. The pallial groove, on the other 

 hand, always persists ; but the glandular caeca which line its bottom 

 are gradually atrophied. The consequence of this disappearance 

 and atrophy is that a scar in the shell can no more be covered with 

 cuticle in this region than in the other parts of the mantle, whilst 

 this does take place when the animal is still in its period of 

 growth. 



According to our observations the function of the cutogenic organ 

 is as follows : — The glands of the pallial groove secrete mucus, 

 perhaps that substance which chemists have found associated with 

 the calcareous matter of shells, and which they have named con- 

 chioline. The long-necked cells of the epithelial organ afterwards 

 deposit the granules they contain in the membrane originating from 

 the pallial groove. The cuticle is the result of this double secretion. 

 What justifies our thinking that this is the case is, that when a young 

 Helix increases its shell it may be seen to apply closely the margin 

 of the mantle to the last-formed part in such a manner that the 

 cutogenic apparatus borders this part. Above the groove a mem- 

 brane destitute of lime soon makes its appearance. The animal is 

 so adherent to the shell that it does not yield to those slight irrita- 

 tions which at other times would cause it immediately to withdraw. 



