86 THE PEESH WATER MUSSEL 



vertical and horizontal bars, united to form a kind of trellis- 

 work with very small meshes, the greater number of which 

 are blocked up by fusion of the deeper surfaces of the bars 

 with one another. 



The two lamellae of each gill are further united at intervals 

 by thick vertical ridges, parallel to the vertical bars. In the 

 case of the inner gill these ridges are at intervals of about 

 twenty bars ; in the outer gill they are more numerous, 

 occurring at about every seventh bar. 



1. The outer gill. 



a. The outer lamella of the outer gill is attached along 



the whole length of its dorsal border to the inner 

 surface of the mantle, close to its line of origin 

 from the body-wall. This line of attachment of 

 the gill runs obliquely across the body, its anterior 

 end being near the dorsal surface a little below 

 and behind the umbo, while its posterior end is 

 below and behind the posterior adductor. 



b. The inner lamella of the outer gill is attached 



along its whole length to the outer lamella of 

 the inner gill, and to the body-wall. 



Turn the outer gill back, so as to see its attachment to the 

 inner gill. 



2. The inner gill. 



a. The outer lamella of the inner gill is attached along 



its whole length, as just noticed, to the inner 

 lamella of the outer gill. 



b. The inner lamella of the inner gill varies in its 



attachment at different parts of its length. 



Turn the inner gill hack so as to expose the dorsal border 

 of its inner lamella. 



i. The anterior part of the dorsal border, about a 

 quarter of its total length, is attached to the 

 side of the visceral mass. 



ii. The middle portion has a free edge, and extends 

 back as far as the hinder border of the foot. 



