74 



RICE. 



[VOL. II. 



the theory of gill development advanced by Lacaze-Duthiers 1 

 and universally accepted, according to which the ascending limbs 

 of the filaments are essentially the reflected tips of the origi- 

 nally simple straight filaments. The larger number of filaments 



in the upper parts of the descending 

 lamellae could be explained by the 

 assumption of the presence of a large 

 number of abortive filaments. But this 

 explanation, unsatisfactory even for the 

 descending lamellae, cannot apply to 

 the ascending lamellae. 



A more detailed study of serial sec- 

 tions, and, better, of microscopic dis- 

 sections in which the lamellae are 

 separated and the folds spread out as 

 smooth as possible, shows clearly that 

 the filaments of the upper portion 

 gradually meet and fuse as the free 

 margin of the gill is approached. This 

 phenomenon is illustrated by Figs. 2 

 and 3. In the former we have a surface 

 view of a small portion of the inner gill 

 of Batissa tcnebrosa, extending upward 

 from the free edge. While the two 

 filaments at the left (Fig. 2, a and b] 

 are simple throughout, the next filament 

 (Fig. 2, c), indistinguishable from the 

 others at its lower end, is really formed 

 by the fusion of nine filaments. 



In this species such groups of fusing 

 filaments are found principally in the 

 projecting folds of the gill, very seldom 

 in the reentrant folds. In Cardium 

 ednlc, on the other hand, the fusion 

 is more marked in the reentrant than in the projecting folds. 



a. 



FIG. 2. Surface view of group of 

 filaments of inner gill of Batissa 

 tenebrosa. x 60. Lower edge 

 of figure represents the free mar- 

 gin of gill. Cilia and complex 

 interfilamentary c onnections 

 omiUr.l for ill,- sake of clearness. 

 n and /', simple filaments; c, 

 compound filament, formed by 

 I ..... n of nine simple filaments. 



1 I .;n a/u-Duthiers, II.de, " Memoir sur le developpement des branchies des Mol- 

 lusques Acephales Lamellibranches," Annales des Sciences Naturelles. Zoologie 

 Scr. iv, Tome v. 1856. 



