60 



aeration comes into contact with the constantly changing 

 water supply of the mantle cavity, and is only separated 

 therefrom by the very delicate tissue of the gill filaments. 



Now, each filament is not a plain sheet, but is crinkled 

 or folded in such a way as to form half cylinders, which 

 alternate with one another as do the leaflets themselves, 

 on each side of the tissue which forms the axis of the 

 filament. This may be understood from the figure 

 where the crinkled filament is shown from the externa] 

 surface (fig. 62, /,, /._,. /.,). This sheet of alternating 

 semicylindricaJ folds does not extend along the full 

 length of the supporting- pillar, but only along- the 

 anterior two-thirds in the case of all the filaments on the 

 internal side of the gill, while those on the external side 

 bear the aerating folds all along their length, and 

 consequently there are more of these folds here (fig. (it), 

 L ± ). The number of aerating folds also varies, of course, 

 with the size of the leaflet. 



.Now, if we consider each hlament as an (dement 

 of the first order, each of these is crinkled into 

 alternating elements of the second order, these again, in 

 the same way, into elements of the third order, and so on 

 until the eighth series of (dements is readied, these 

 ultimate folds being microscopic. Accessory gill Leaflets 

 are situated on the vertical septum interior to the efferent 

 vessel, and occur in pairs between successive leaflets (tig. 

 GO, Lmcc.) all along the gill. Probably they originally 

 were attached in these Leaflets, and now have moved 

 slightly away. They equal one of the folds of the true 

 leaflets in size. In shape they are triangular, the apex 

 ol I he leaflet pointing downwards. Each is crinkled into 

 five or six pairs of secondary elements, whose further 

 structure exactly resembles that of the ordinary secondary 

 element of the gill. 



