7o 



EDWARD L. RICE. 



mantle (outer gill) or body (inner gill). In the later type of de- 

 velopment the upper end of the reflexed limb is primarily united 

 with the gill axis (Fig. 4, D), and only secondarily attains its 

 freedom. The details of the process have not been studied. In 

 the specimen represented in Fig. 4, E, this separation had not 

 taken place, but the reflexed limbs were torn loose in the dis- 

 section as indicated by the rough edges. 



Fig. 5. Section through ctenidium of 

 Mytilus of 1.6 mm. length. Section passes 

 through posterior end of reduced foot, a, 

 filament of inner gill ; b, filament of outer 

 gill ; c, mantle ; d, intestine ; e, foot. 

 Magnification 140. 



Fig. 6. Section through ctenid- 

 ium of Mytilus of 1.6 mm. length. 

 Section passes through middle of 

 posterior adductor. a, filament of 

 inner gill ; b, filament of outer gill ; 

 c, mantle ; d, posterior adductor. 

 Magnification 140. 



At first sight the two modes of filament formation seem fun- 

 damentally distinct ; but the change from one to the other is 

 bridged by all intermediate gradations. A comparison of Figs. 

 4, 5, and 6 gives some suggestion of the transition in the case of 

 the inner gill ; and the process is identical for the outer gill. 



Figs. 5 and 6 are taken from the same animal, Fig. 6 being 

 more posterior, thus showing a later filament in a younger stage 

 of development. But it will be found by measurement that the 

 ratio of the length of the reflexed limb to the direct limb of the 

 filament of the inner gill is almost the same in the two cases, 

 thus showing a precocious flexure of the posterior filament. A 



