ORGANS OF MOVEMENT IN THE PELECYPODA. 69 



this line indicates that the animal possesses retractile siphons. 

 The depth of the sinus does not, however, indicate the extent of 

 the contractibilit^^ of the siphons, but simply their retractile 

 movements ; the ver^^ contractile siphons of M3'a being much 

 longer, and those of certain Tellinidae three or four times the 

 length of the shell. The small siphons of Sphterium and 

 Dreissena cause no inflection or sinus of the pallial line. When 

 the siphonal sinus exists, its form is characteristic of genera and 

 species. 



In Lucina and other bivalves there are furrowed impressions 

 caused by the viscera ; the absence of polish and outline will 

 distinguish these from muscular scars. 



Occasionally the foot muscles are attached to prominent 

 apophyses proceeding from the beak cavity ; of this nature are 

 the prominent falciform processes or "teeth" of Pholas and 

 Teredo. In Cucullsea the posterior adductor attachment is 

 bounded by a carina or ridge. 



It is by the forcible contraction of the adductors that the so- 

 called swimming of Pecten and Lima is accomplished. The 

 motion is really a series of rapid dartings, uncertain in direc- 

 tion. Dr. Fischer remarks that, on account of the peculiar 

 form of the valves of Jouanettia, the hinge-line being short 

 and the shells gaping apart anteriorly and posteriorly, the 

 contraction of the anterior adductors must increase the posterior 

 gap and vice versa^ the action of the two pairs thus becoming 

 measurably antagonistic. A similar effect is produced in 

 Zirphgea, and indeed, in all the numerous species which are 

 termed gapers. 



Dr. Lockwood has placed on record * the fact that Modiola 

 plicatula when placed in an uncongenial situation, can use its 

 foot, in escaping, with as much facility and in same manner as a 

 gastropod ; not only traversing a part of the bottom of his 

 aquarium, but actually gliding up its perpendicular wall to a 

 distance of six inches. Dr. Aug. A. Gould observed Mi/tilus 

 edulis (iii, 40 i, climb the side of a glass jar by the aid of its foot 

 and byssus, thus accomplishing three inches in a single night. 

 To do this, the animal first stretched out its finger-like foot to 

 its greatest extent, and attached a fibre to the glass, and then 

 withdrew its body within the shell as much as possible, by which 

 the whole was raised about three-fourths of an inch. Numerous 

 fibres were then fixed in a radiating manner until sufficiently 

 secure, and then the whole of the pencil of radiating fibres 

 forming the preceding attachment was thrown off in a mass, at 

 the foot, the ends being still held together by a sort of knot. 



* Am. Naturalist, iv, 331. 



