44 CONCHIFERA. 



very numerous, and prominently situated. They 

 may be well studied in the common Scallop {Pecten 

 opercularis), where they occur in great numbers, 

 placed among the tentacles on the borders of the 

 mantle. They are beautiful objects, as they gleam 

 with the radiance of precious stones. In other 

 genera the eyes are differently placed ; as near one 

 or the other extremity of the mantle (the fore part 

 in Pinna, the hind in Tellina), or around the 

 siphon -orifices, as in Pholas, Venus, and Solen. 

 In the Cockle {Cardiimi) the short siphons are 

 surrounded with an extraordinary number of tenta- 

 cles, capable of elongation and protrusion, each of 

 which bears an eye of diamond-like brilliancy."^ 



There is in many Conchifera a curious organ, 

 known, from some of its functions, as the foot. It 

 is a muscular mass, frequently of great size, and 

 capable of being pushed out from between the 

 mantle and valves to a long distance. It some- 

 times acts as a pushing pole, and at others as an 

 anchor. In the boring kinds, which live in sand 

 or mud, it is the organ by which they form 

 their perforations, and descend into them. Being 

 stretched to its utmost, the extremity, which is 

 generally pointed, is inserted into the soil ; then 

 being bent up in the form of a hook, the whole 

 organ is suddenly and forcibly contracted; the 

 hook maintains its hold of the mud, and the result 

 is that the whole animal with its shell is dragged 

 into the hole. The Cockles are able to perform 

 vigorous leaps by means of this well-developed 

 and versatile foot. 



In other species the foot is grooved ; and being 

 associated with a gland which has the power of 



* Siebold, Anat. Invert. § 187. 



