

ACEPHALOUS MOLLUSCA. 319 



delicate leaves, furnished at the margin with sensitive tentacles and 

 other organs of sense, and with glands sometimes highly coloured. 

 The use of these organs is thus described by Mr. Eymer Jones : 



" When the animal is engaged in increasing the dimensions of its 

 abode, the margin of the mantle is protruded and firmly adherent all 

 round to the circumference of the valve with which it corresponds. 

 Thus circumstanced, it secretes calcareous matter and deposits it upon 

 the extreme edge of the shell, when the secretion hardens and becomes 

 converted into a layer of solid testaceous substance. At intervals this 

 process is repeated, and every newly-formed layer enlarges the dia- 

 meter of the valve. The concentric strata thus deposited remain dis- 

 tinguishable externally, and thus the lines of growth marking the 

 progressive increase of size may easily be traced." 



" While the margin of the mantle is thus the sole agent in enlarging 

 the circumference of the shell," the professor continues farther on, 

 " its growth in thickness is accomplished by a secretion of a kind of 

 calcareous varnish derived from the external surface of the mantle 

 generally, which, being deposited layer by layer over the whole inte- 

 rior of the previously existing shell, progressively adds to its weight 

 and solidity. There is, however, a remarkable difference in character 

 between the material secreted by the marginal fringe and that fur- 

 nished by the general surface of the mantle membrane. The former 

 we have found more or less covered by glands appointed for the pur- 

 pose, situated in the circumference of the mantle ; but as these glands 

 do not exist elsewhere, no colouring matter is ever mixed with the 

 layers that increase the thickness of the shell, so that the latter 

 always remain of a delicate whitish hue, and form the well known 

 iridescent material usually distinguished by the name of nacre or 

 mother-of-pearl." (General Outline, p. 385.) 



The process by which shells attain their beautiful markings is thus 

 described by Mr. Jones : "The external surface is exclusively depo- 

 sited by the margin of the mantle, which contains in its substance 

 certain coloured spots, which are found to be of a glandular character, 

 and to owe their peculiar character to a pigment they secrete, which 

 is mixed with the calcareous matter ; coloured lines are therefore found 

 on the exterior of the shell wherever these glandular organs exist. 

 Where the deposition of colour is kept up throughout the process of 

 enlargement, the lines are unbroken and perfect ; but where the 



