Jan., 1903,] Development of the Gill in Mytilus. 51 



PRELIMINARY REPORT ON THE DEVELOPMENT 

 OF THE GILL IN MYTILUS. 



Edward h. Rice. 



(Abs/nui.) 



The earl}- development of the gill of this form was very thor- 

 oughh' worked out by Lacaze-Duthiers in 1856. To his account 

 of the development of the earlier filaments the present writer has 

 nothing to add. As described, a papilla is formed, growing down- 

 ward from the gill axis, and is reflexed on itself, giving rise to 

 the familiar U-shaped filament. Later filaments follow a ver}- 

 different scheme, there being no such bending of an originally 

 simple filament. At the posterior end of the curiously bent gill 

 axis a series of thin tran.sverse ridges are developed. At first the 

 edge of each ridge is entire ; but growth is verj^ earl}- checked in 

 the center, so that the ridge is divided into two flat, rounded 

 lobes, corresponding respectively to a filament of the outer and 

 one of the inner gill plate. As the lobe elongates it becomes per- 

 forated at its proximal end, thus being resolved into the two 

 branches of a U-shaped filament identical in form with those first 

 developed. This mode of development of the later filaments has 

 been observed in Modiola, Area, Anomia, and Mya ; the earlier 

 filaments have been studied only in Mya, where they correspond 

 with Mytilus. 



An interesting parallel is seen in the development of the inter- 

 lamellar connections. The interlamellar connection, in its finished 

 form, is a simple bar, containing a blood channel, and connecting 

 the two branches of one and the same filament. In an earl}- stage 

 the two branches are connected by a continuous plate of tissue 

 extending from the bend of the filament upward for a short dis- 

 tance. This stage is exactly comparable with the adult conditions 

 in Area and Modiola. Later a perforation appears in the plate, 

 and the portion above the perforation is transformed into the 

 characteristic bar-like connection. 



As }■ et the examination of sections is too little advanced to allow 

 any detailed statement concerning the mode of perforation in 

 either case. 



Ohio Weslevan University. 



