REPTARY AND SUBREPTATORY FOOT. 
195 
locomotory and muscular, being apparently homogeneous in structure 
altliough the mode of pigmentation is not uniform, even in closely 
allied species. In Helix aspersa trifasciation is perceptible owing to 
the greater pigmentation of the side areas, but in Helix pomatia the 
mid area is the most darkly coloured, the sides being usually (piite 
unpigmented and very distinctly defined thereby. 
In the Pelecypods the foot arises ventrally from the body, and in 
Nucula and otlier archaic s})ecies forms a 
fiat sole or creeping disc, by the expansion 
of the deep longitudinal cleft therein; such 
forms are distinguished as Reptary, and 
present a curious similarity to Helix pomatia 
and certain other species, in which the foot 
is longitudinally folded up in the same 
manner when the animal is retiring wdthin its shell. 
j\Iost of our species are, however, Subreptatory, possessing a laterally 
compressed, very extensile and fiexible linguiform or axe-shaped 
organ, with annular muscular bands to assist in its extension and 
longitudinal muscles for its retraction, being, however, still expansible 
as a fiattened crawling disc, although the longitudinal groove charac- 
terizing the primitive Reptary foot has become lost. The degTee of 
Fig. 386. — Drcissetisia polyiuorpha (Pall), 
Fig. 385. — Sphcerium rhncola (Leach), Canal, Northampton, 
showing the Subreptatory burrowing or Collected by Mr. L. E. Adams, B.A., 
crawling foot. Showing the digiiiform and vestigial foot. 
development is in correlation with the habits and locomotory powers 
of the mollusk, as it may be modified to form an efficient burrowing 
organ and tend to assume a more anterior position or become somewhat 
digitiform and vestigial, as in Dreissensia. The tip is often more 
deeply coloured and of a reddish tint, owung to the presence of 
Tetronerythrine, a substance which corresponds with the hfemoglobin 
of Planm-his and the higher animals and serves for cutaneous respira- 
tion by its great affinity for oxygen. 
The tissue of the protruded foot becomes rapidly firm and rigid by 
blood pressure or, as some believe, by inception of the surrounding 
Fig. nuc/cus (h.) 
(after H. A. Adams), showing 
the longitudinally grooved or 
primitiv’e Reptary foot. 
