492 CONCHIFERA. 



they moreover present a decidedly 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 by which the extension of the shell is effected ; and by them 

 its outer layer is secreted, and in many cases painted with gorgeous 

 hues, as will be explained more at length hereafter. 



Fig. 247. 



Pecten Jacobtea. 



(1290.) Between the lobes of the mantle are seen the branchiae (b, g), 

 always consisting of four delicate leaves, composed of radiating fibres 

 of exquisite structure, and generally attached to the circumference of 

 the body by their fixed extremities, but elsewhere perfectly free, so as 

 to float loosely in the water, which finds free admission to them. The 

 mouth (I) is situated between the two inner laminas of the branchiae, in 

 a kind of hood formed by the union of the gills at their origin ; it is a 

 simple orifice, without any kind of dental apparatus, but bordered by 

 four thin and membranous lips (&) placed on each side of the aperture. 



(1291.) The valves, which are opened by the elasticity of a com- 

 pressible ligament interposed between them at the hinge, are closed by 

 the contraction of a powerful muscle (c), which passes directly from one 

 to the other, and around this adductor muscle the viscera of the body 

 are disposed : the stomach, liver, and generative system are imbedded in 

 the mass (d, e,f) ; the convolutions of the intestine may be traced occa- 

 sionally (n, o) ; and the termination of the rectum (m) is visible exter- 

 nally, situated upon that side of the adductor muscle which is opposite 

 to the mouth. In the neighbourhood of the oral aperture is placed a 



