6 4 



EDWARD L. RICE. 



Concerning the process of elongation and flexure by which the 

 embryonic papillae are metamorphosed into the definitive filaments 

 of the adult gill in all forms except the very primitive Proto- 

 branchia the authors are again in the fullest agreement, so far as 

 their attention has been turned to this later development ; and 

 this plan of development has been generally accepted as charac- 

 teristic of the whole class. 



Stages of Gill Development in Mytilus. 

 The following tabulation gives the size of the animal and the 

 number of filaments present in the gills at certain of the more 

 important stages in gill development. The figures can be taken 

 only as approximate, as there is considerable individual variation ; 

 but they probably represent fairly accurate averages as they are 

 selected from a much larger tabulation. 



Stage. 



Earliest stage in which gills were observed. 

 Flexure of filaments of inner gill. 

 First appearance of outer gill. 

 Flexure of filaments of outer gill. 



Size. 



0.30 mm. 

 0.75 mm. 

 I.40 mm. 

 I.60 mm. 



Number of Filaments. 



Inner Gill. 



3 



12 

 20 

 25 



Outer Gill. 



O 

 O 



*5 



Order of Succession of Gill Filaments In Mytilus. 



Lacaze-Duthiers describes the order of the development of the 

 filaments of the inner gill as from front to back, the most anterior 

 being the first to develop ; in the outer gill the development is 

 described as extending in both directions, so that the first devel- 

 oped filaments would come to lie in the middle of the gill, with 

 late developed filaments at both ends. My observations are in 

 accord with those of Lacaze-Duthiers as regards the inner gill ; 

 but, in the case of the outer gill, amendment is required. 



In very early stages, in which the filaments of the outer gill 

 are mere papillae, the longest papilla is not the most anterior, 

 but is even posterior to the middle of the series. In one speci- 

 men with ten such papillae, the seventh from the front is the 



1 Theoretically but a single filament should be present. No stage has been actually 

 observed with less than three or four. Apparently several filaments appear almost 

 simultaneously. 



