GILL DEVELOPMENT IN MYTILUS. 



6 9 



what diagrammatic representation of the highly specialized cili- 

 ated epithelium characteristic of the outer side of the filament, 

 i. e., the side turned away from the cavity of the gill. Thus in 

 the inner gill (Fig. 5, a) the outside of each limb of the bent 

 filament shows this characteristic ciliation ; the papilla, or unbent 

 filament, of the outer gill is ciliated on only one side, and that 

 the side corresponding to the outside of the direct limb. In the 

 later filaments (Fig. 4) the conditions are very different. In the 



A 



Fig. 4. Isolated filament anlagen from posterior tip of ctenidium of Mytilus. In 

 all cases, anlage of outer gill at right. A and B from specimen of 2.25 mm. length ; 

 C from one of 2.00 mm. ; D from one of 2.30 mm. ; E from another specimen of 2. 25 

 mm. Magnification 140. 



two youngest stages represented (Figs. 4, A ; 4, B) the tissue is 

 distinctly embryonic with no differentiation ; but in the three older 

 stages (Figs. 4, C ; 4, D ; 4, E) the ciliated epithelium is clearly 

 distinguishable, and is located on both sides of the anlage, show- 

 ing the equivalence of the latter to both limbs of the flexed 

 filament. 



Another difference between early and late filament formation 

 is found in the relations of the upper end of the reflexed filament 

 limb. In the earlier filaments, the reflexed limb, developed at 

 the free end of the direct limb, is primarily free. In Mytilus it 

 retains this freedom ; l but in many lamellibranchs the upper 

 ends of reflexed filament limbs are secondarily fused with the 



1 Even in Mytilus there is a fusion of the upper ends of the reflexed filament limbs 

 with one another to form a somewhat definite border to the reflexed lamella of the 

 gill. 



