No. 2.] FILAMENTS IN THE LAMELLIBRANCH GILL. 75 



Fig. 3 represents diagrammatically the surface of a single fold 

 of this species, each filament being represented by a simple line. 

 At the gill margin (lower side of diagram) there are eight fila- 

 ments, including the limiting filaments (Fig. 3, a and //) ; in the 

 upper part of the gill (and diagram) there are thirty. Of these 

 thirty filaments, twenty-one unite to form the two limiting fila- 

 ments (Fig. 3, a 

 and Ji) and the 

 two adjacent in- 

 termediate fila- 

 ments (Fig. 3, b 

 and g), which 



occupy the re- \/ V 



entrant folds. 



The remaining 

 four intermedi- 

 ate filaments 

 (Fig. 3, c, d, e, 

 and/"), those of 

 the projecting 

 fold, are formed 

 by the union of 

 only nine fila- 

 ments. The ir- 

 regularity of the 

 fusion is also 

 well shown by a. 



V 



V 



\l 





\ 



\ 



\ 



<* 



f g- 



k 



tlllS dia~ram. FIG. 3. Diagrammatic view of surface of fold of inner gill of Cardinal 



. editle, illustrating fusion of filaments. Filaments represented by 



iNOte especially lines. Lower edge of diagram represents free margin of gill, a and 



Hilt" ODP limit A, limiting filaments ; b-g, intermediate filaments. 



ing filament is entirely simple, while the other is the resultant 

 of the fusion of seven simple filaments. 



The fusion is usually almost exclusively limited to a some- 

 what narrow zone in the near vicinity of the free margin of the 

 gill. In this zone may also be noted a gradual reduction in the 

 folding of the gill as the margin is neared. In those lamelli- 

 branchs whose outer gill is provided with a dorsal appendage, 

 there is a second zone of fusion along the transition line from 



