216 THE LAMELLIBRANCHIA 
the most common, and is found, for example, in the Unionidae 
(Fig. 242), Tellina (Fig. 190, f), and Cardiuwm (Fig. 243). The 
anterior pointed end may be so much elongated, for example, in 
Poromya, as to give the foot the appearance of a cylindrical tentacle, 
sometimes slightly swollen at its free extremity (many Lucinidae, 
Fig. 238, III), or of a long cylinder directed forward and sometimes 
ending i in a swelling of constant shape (Solen, Mycetopus). In some 
cases the foot may secondar ily acquire an enlarged free extremity 
with a creeping surface, e.g. in Galeomma, Lepton, and certain species 
of Erycina. In Spondylus it ends in a pedunculated globular 
appendage. Some Anatinacea, such as Pholadomya and Halicardia, 
have an accessory foot-like organ, the so-called opisthopodium, on 
the posterior extremity of the visceral mass. In J/ytilus, also, a 
distinct posterior carinated projection may be seen behind the 
extensible pedal cylinder; this has been called the ‘“ Punch’s 
hump.” There is, on the other hand, an anterior and dorsal 
tongue-shaped projection on the foot of Zapes decussatus. 
Finally, the foot may become rudimentary through disuse in 
genera with restricted or no locomotory powers. This is especially 
the case in boring Lamellibranchs with extensive fusion of the 
mantle edges, such as Pholas and Teredo (Fig. 247, VI), and in such 
forms as are fixed by a byssus or by the substance of the shell, 
such as Pecten (Fig. 235, f), Ostraea, Aetheria, ete. 
The foot, then, is the locomotory organ, as it is in other 
Mollusca. Its special function is to grope in the shifting soil and 
to slowly drag along the animal by its successive contractions and 
extensions, its anterior extremity being supported or fixed. These 
movements of the foot are due to turgescence, produced by the 
afflux of blood into the pedal sinuses, and its subsequent contrac- 
tion by means of the retractor muscles. 
The foot is never provided with an aquiferous pore through 
which, as was for a long time believed, water can penetrate into 
the circulatory system. “But it very frequently presents a more or 
less posterior orifice in the middle line corr esponding to the ventral 
pedal pore of Gastropoda (Fig. 144, I) and leading into a cavity 
known as the byssogenous cavity (Fig. 197, I), into which certain 
unicellular glands situated in the foot discharge their secretion. 
This secretion, passing between the epithelial cells of the byssogenous 
cavity, hardens on contact with the water and forms threads of 
conchiolin, which unite to form the trunk of the byssus (Fig. 197, 
IV). This structure serves to attach the animal, but the fixation is 
not necessarily permanent ; the old byssus may be abandoned and 
a new one formed (Arca, Mytilus, Avicula, Dreissensia, etc.). The 
byssogenous organ is poorly developed in the Protobranchia, i 
which group the byssogenous cavity is situated far back (Fig. 204, 
VIII), and a functional byssus is absent. When it attains to its 
