. BIVALVE MOLLUSCA. 319 



enlarges the diameter of the valve. The concentric strata thus 

 deposited remain distinguishable 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 author 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 interior 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 furnished by the general surface of the mantle membrane. 

 The former we have found more or less covered by glands appointed 

 for the purpose, 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."* 



The process by which shells attain their beautiful markings is 

 thus described by Mr. Rymer Jones : " The external surface is 

 exclusively deposited 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 coloured matter is furnished only at intervals, 

 spots and patches of irregular form and increasing in size with the 

 enlargement of the mantle are the consequence." 



Many bivalves move about and change from place to place by 

 means of an extensible fleshy organ called, from some of its functions, 

 a foot ; but in fact, it has less resemblance to a foot than to a large 

 tongue. It is a muscular mass, capable of being pushed out from 

 between the mantle lobes and the valves, and varies much in form ; 

 it resembles in turn a hatchet, a ventilator, a pole, an awl, a finger, 

 and a sort of whip. This foot is simple, forked, or fringed. In 

 some species the tissues of the foot are spongy, and capable ol 

 receiving considerable quantities of water. When the organ swells. 



* "(General Outline of the Animal Kingdom,'' p. 385. 



