1883.] NATURAL SCIENCES OF PHILADELPHIA. 217 



shells of bivalves, and allow them to become coated with mother- 

 of-pearl.' 



The mantle. — If the shell be carefully removed from the animal, 

 the form of the body is found to be like that of the shell, namely, 

 a depressed cone, and covered with a thin white membrane, the 

 mantle. The base of the mantle, or that part which comes in 

 contact with the aperture of the shell, is thickened and separated 

 from the body, so that a deep groove is found running around the 

 foot bounded externally by the internal surface of the mantle. 

 The deepest point of this groove is at that point where the mantle 

 and foot join. From this point, or the base of the groove 

 (looking at the animal from below), hangs the gill, between the 

 foot and the mantle, on the left side in A. Jiuviatilis, and on the 

 right in A. lacudris. The inferior portion of the external surface 

 of the mantle has a deposit of black pigment ; this band of black 

 pigment is not present in A. lacustrin. 



Organ of locomotion. — The only organ of locomotion is the 

 foot, which is an oval muscular disk. The shape is like that of the 

 aperture of the shell to which it belongs. The foot is formed of 

 muscular fibres which run in four different directions, and between 

 which the lacunae or blood-spaces are found. One system of mus- 

 cular fibres passes from before backward (longitudinal fibres) ; 

 another, perpendicular to these, passes from side to side (transverse 

 fibres). The other two systems are continuations of the muscle 

 that binds the body to the shell. These latter fibres pass perpen- 

 dicularly from the shell, and entering the foot, spread out fan-like 

 into it, so that some of the fibres are almost horizontal and 

 others almost perpendicular to the sole of the foot ; these may be 

 called lateral fibres. The animal holds itself to objects on which 

 it creeps, by the foot, which acts like a sucker. If the animal be 

 disturbed it draws the shell tightly downwards so that the soft parts 

 are completely covered by the shell and thus protected. The 

 movement of Ancylus is very slow. It never swims, as does, for 

 example, Limnseus^ on the surface of the water, as Gray and 



^ Ad intei'estiiig account of this process may be found in F. Hague, 

 Ueber d. natiirliche u. kiinstliche Bilduug der Perlen; and C. Th. von 

 Siebold, Ueber d, Perlenbildung chinesischer Siisswasser-Muscheln, als 

 Zusatz z. d. vorbergcheuden Aufsatz. Zeitschr. f. wiss. Zool., Bd. viii, 

 1857. 



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