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EDWARD L. RICE. 



mode. The change may be noticed in specimens but a little 

 older than the latest stage figured by Lacaze-Duthiers. The 

 ctenidium has grown beyond the rather small body mass and the 

 posterior adductor, and extends free into the mantle chamber 

 (Fig i). The posterior part of the ctenidium is free from the 

 mantle except for a light attachment due to the interlocking of 





Fig. i. Posterior tip of ctenidium of Mytilus of 2.5 mm. length, viewed from in- 

 ner side. Outer gill omitted except for outline represented by dotted line. Magnifi- 

 cation 170. 



cilia. This suggests the connection of neighboring gill filaments 

 (interfilamentary connections) ; but here there is merely diffuse 

 ciliation and no specialized ciliated disks. 



At the posterior end the gill axis (Fig. I, a) is slightly curved 

 upward ; and around this curved end the developing filaments are 

 closely crowded and assume a radial arrangement. The anterior 

 filaments are attached to the ventral side of the axis and hang 

 down ventrally ; toward the end the position becomes more and 

 more oblique, and then horizontal ; while the youngest filament 

 anlagen actually project upward from the dorsal side of the axis. 

 Here all stages in filament development may be observed, ar- 





