THE GILLS. 99 



Each gill consists of two lamellae, continuous along their 

 ventral borders. Each lamella is formed of very numerous 

 vertical gill-filaments 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 sur- 

 faces of the filaments with each other. 



The two lamellae of each gill are further connected at inter- 

 vals by thick vertical ridges, parallel to the vertical filaments. 

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

 about twenty filaments ; in the outer gill they are more 

 numerous, occurring at about every seventh filament. 



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 fine of origin 

 from the body- wall. This fine 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 in front 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 differs in its 



attachment at different parts of its length. 



Turn the inner gill back, so as to expose the dorsal border 

 of its inner lamella. 



