128 ANATOMY OF THE LAMELLIBRANCHIATA. 



orbicular species of Venus. In My a, Thracia, Corbula, Pan- 

 dor a, &c, the muscular part is very small, projecting through 

 a small opening of the mantle. It is something like the hu- 

 man foot in Chama ; very long and attenuated in Loripes. I 

 It is club-shaped in Solen ; in Pholas, small, short and 

 rounded. The extremity of a cartilaginous body, to be de- 

 scribed hereafter, is contained within this organ ; and seems 

 to add to its elasticity and resilience. By the action of this 

 foot these animals can bore with great facility in the sand, 

 where some are found at considerable depth ; they can, like- 

 wise, accomplish a quick progression, by using it as a hook, 

 or pushing themselves forwards by its means ; they also swim 

 on the surface of the water, by expanding it into a concave 

 dish ; and climb perpendicular surfaces, by fixing its extre- 

 mity like a sucker. 2 Some species have the power of secret- 

 ing air into two sacs of the mantle, attached to the excretory 

 organs, by which their specific gravity is diminished, and 

 they readily change their situation at the ebb and flow of the 

 tides. The edge of the mantle is muscular ; in some genera 

 it is strongly adhesive to the shell ; in others loose, and ca- 

 pable of being considerably retracted by means of distinct 

 bundles of muscular fibres, attached to the valves at some dis- 

 tance from their edge, as is the case in Pecten, Pinna, &c. 

 In Lima, Pecten, Spondylus, &c, its margin is furnished with 

 long tentacles ; and in the two latter we see, at regular dis- 

 tances on this margin, small ocelli, looking in the fresh ani- 

 mal like so many emeralds, from their green colour and great 

 brilliancy. Each of these ocelli possesses a cornea, lens, cho- 

 roid, and nerve : they are without doubt organs of vision. 3 

 There is, likewise, in these animals, a muscular flap at the 

 edge of the mantle, apparently for the purpose of preventing 

 the escape of the water. When the syphons are developed, 

 a strong muscle takes its origin from the impression seen in 

 many shells at the posterior part of the internal surface, and 

 is inserted into them. The external fibres of these tubes are 

 circular, the internal longitudinal ; they are very contractile. 

 In the Anomia we find Jthe ordinary muscle of the Monomy- 

 aria ; also another which originates chiefly from the convex 

 valve, and is inserted into the operculum. This latter like- 

 wise receives a bundle of fibres from the articulating process 



« Poli. 



2 Bosc says the Venus genus comes to the surface, using one valve as a 

 boat and the other as a sail. Kirby, 'Bridgewater Treatise.' 



3 See Poli. 



