CONCHIFERA. 



377 



layer is secreted, and in many cases painted with gorgeous hues, 

 as will be explained more at large hereafter. 



Fig. 176. 



Between the lobes of the mantle are seen the branchiae (b, g) 9 

 always consisting of four delicate leaves, composed of radiating 

 fibres of exquisite structure, and generally attached to the circum- 

 ference of the body by their fixed extremities, but elsewhere per- 

 fectly free, so as to float loosely in the water, which finds free 

 admission to them. The mouth (/) is situated between the two 

 inner laminae 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 mem- 

 branous lips (k) placed on each side of the aperture. 



The valves, which are opened by the elasticity of a compressible 

 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,y ; the convolutions of the 

 intestine may be traced occasionally (w, o) ; and the termination 

 of the rectum, m, is visible externally, situated upon that side of 

 the adductor muscle which is opposite to the mouth. In the 

 neighbourhood of the oral aperture is placed a retractile fleshy 



