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MANUAL OF ELEMENTARY ZOOLOGY 



filament passing down one lamella and up the other, so 

 that the whole gill may be said to be composed of a 

 number of bent filaments fused side by side so as to form 

 two lamella;. The two lamellae of each gill are connected 

 at intervals by thick vertical ridges parallel to the filaments. 

 The lamellae diverge upwards, so that in transverse section 

 the two gills of each side have the form of a W. The 

 space into which each interlamellar space widens at the top 

 is known as an epibranchial space. The outer lamella of the 

 outer gill of each side is attached along the whole length of 



Fig. 225. — Diagrams of transverse sections through the swan mussel. 



A passes through the middle of the foot and shows the inner lamella of the inner 

 gill attached to the side of the foot ; B passes through the hinder part of the 

 foot and shows the inner lamella of the inner gill free ; C is taken behind the 

 foot and shows the inner lamella? of the inner gills joining in the middle line ; 

 D is further hack and shows the axes of the gills free. 



ax.g.. Axes of the gills ; cl.c, cloacal chamber ; efi.sp.^ epibranchial space \f., foot ; 

 i.g.1, inner lamella of inner gill ; !.g.2, outer lamella of inner gill ; i/.sf., inter- 

 lamellar space ; o.g-.i, inner lamella; of outer gill ; o.g.1, outer lamella of outer 

 gill ; m/., mantle lobe ; ml.c, mantle cavity. 



its upper border to the inner surface of the mantle, close to 

 the origin of the latter from the body-wall. The inner 

 lamella of the outer gill is attached along the whole length 

 of its upper edge to the outer lamella of the inner gill. 

 The line of their junction is thickened and may be called 

 the axis of the gills. The axis is attached for most 

 of its length to the ventral side of the body, but behind 

 becomes free. The inner lamella of the inner gill is 

 attached in front to the top of the foot, its middle portion has 

 a free edge, and behind the foot it is attached to its fellow 

 of the opposite side. The result of this arrangement of the 

 attachments of the gills is that the epibranchial spaces, which 



