216 THE LAMELLIBRANCHIA 



the most common, and is found, for example, in the Unionidae 

 (Fig. 242), Tdlim (Fig. 190, f), and Cardium (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 in a swelling of constant shape (Solen, Mycetopus). In some 

 cases the foot may secondarily 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 Mytilus, 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 Tapes 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 Pliolas 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, /), Ostraea, Aetheria, etc. 



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 corresponding 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 (Area, Mytilus, Avicula, Dreissensia, etc.). The 

 byssogenous organ is poorly developed in the Protobranchia, in 

 which group the byssogenous cavity is situated far back (Fig. 204, 

 VIII), and a functional byssus is absent. When it attains to its 



