HORNELL— ANATOMY OF PLACUNA 89 



lobes diflfer greatly in arrangement, whilst the posterior or rectal sinus is now unpaired ; 

 the posterior aorta has been suppressed ; the hinder extension of the right kidney has 

 developed at the expense of its neighbour on the left ; the palps are asymmetric and the 

 mouth and labia awry ; the alimentary canal tends towards the right side, whilst the 

 crystalline style, after its emergence from the visceral mass, lies embedded for the rest 

 of its course within the right mantle lobe. The right member of both the retractor 

 and the levator pair of muscles of the foot has disappeared and the nerve ganglia, as 

 may be expected, share, though in modified degree, in the general asymmetry of the 

 animal. 



The meaning of this asymmetry is explicable if we consider the peculiar mode of 

 life and habitat adopted by Placuna. 



In Lamellibranchs possessing a byssus, by which they are enabled to affix to stones 

 and rocks, and in siphonate forms which burrow in sand or mud, the body is 

 maintained normally in a more or less vertical position. In Placuna, where both 

 byssus and siphons are absent, the animal lies prone on the convex left valve upon the 

 muddy bottom of those shallow waters of the Indies which it chooses as its habitat. 

 It thus lies with the body in a horizontal position, with its organs more or less 

 suspended from the flat right valve which is uppermost. In the same way its ally, 

 Anomia, assumes a more or less horizontal position in life, as in it the byssus, which 

 here protrudes in a calcified form from the right valve, anchors the animal permanently 

 on its right side to some solid object, rock, stone, or another shell. 



Now, in those forms of Lamellibranchs where the axis of the body is regularly 

 maintained throughout life either approximately vertical in Ijurrowing forms 

 {Cardiu7n, My a, Solen), or at right angles to the plane of attachment in the case of 

 fixed forms, as in Area and Mytilus, the primitive bilateral symmetry is maintained, 

 whereas in Anomia and Placuna, which maintain a horizontal axis in their habitat, 

 asymmetry reaches a maximum. Intermediate forms of Lamellibranchs, with the axis 

 of the body maintained habitually in some position between these extremes, reveal an 

 asymmetry usually proportionate to the extent of the inclination of the axis of the 

 body from the vertical. Thus, in the pearl oyster {Margaritifera vulgaris), where the 

 inclination from the vertical is but slight, asymmetry is correspondingly small, and 

 may be said lo be limited to the valves of the shell. In these latter forms the 

 conditions which produce asymmetry are mechanical, and are in the nature of stresses 

 comparable with those which account mechanically for the asymmetry of typical 

 gastropod structure. 



But mechanical stresses will not account for the asymmetry of organs as seen in 

 Placuna, otherwise instead of the organs tending to gravitate to the right or upper 

 side of the body as the animal rests on the bottom, they would tend by gravitation to 

 embed themselves in the lower or left mantle. The cause is rather to be sought in the 

 obvious advantage of keeping the organs raised as far as possible above or beyond the 



