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 of 
receiving considerable quantities of water. When the organ swells, 
* ‘General Outline of the Animal Kingdom,” p. 385. 
