468 THE AQUARIAN NATURALIST. 



this case quite free and unconnected, except in the 

 immediate vicinity of the hinge. The borders of the 

 mantle are thickened, and surrounded with a delicate 

 fringe of retractile filaments ; they, moreover, present 

 a decided glandular appearance, and secrete colouring 

 matter of various tints, similar to those seen upon 

 the exterior of the shell; the glandular margins of 

 the mantle form, in fact, the apparatus whereby the 

 extension, as well as the ornamentation, of the shell 

 is effected. 



Between the layers of the mantle are seen the 

 branchiae or gills, always consisting of four delicate 

 leaves, composed of parallel fibres of exquisite struc- 

 ture, attached to the circumference of the body by 

 their fixed extremities, but elsewhere perfectly free, 

 so as to float loosely in the water, which, when the 

 shells are slightly opened, finds free admission to 

 them. 



The mouth is situated between the two inner 

 laminse of the branchiae, in a kind of hood, formed by 

 the union of the gills in the vicinity of the hinge : it 

 is a wide orifice, without any kind of dental appa- 

 ratus, but bordered by four thin and membranous 

 lips, a pair of which is situated on each side of the 

 aperture. 



The valves are opened by the elasticity of a com- 

 pressible ligament interposed between them, and are 

 closed by the contraction of a powerful muscle, that 

 passes directly from one to the other, while around 

 this the rest of the body is disposed. In the neigh- 

 bourhood of the oral aperture is placed a retractile 

 fleshy organ, which, although in the Scallop it exhibits 



