gill development in mytilus. 75 



Development of Interlamellar Connections in 

 Other Lamellibranchs. 

 All stages in the development of the interlamellar connections 

 in Mytilus have been repeatedly observed ; but the investigation 

 has not been extended to other genera. The similarity of the 

 structure of the interlamellar connections in certain, other forms, 

 e. g., Astarte, suggests similarity of development. But in the 

 majority of the lamellibranchs the structure of these connections 

 resembles Modiola rather than Mytilus. 



General Conclusions. 



There is a very striking parallel in Mytilus between the forma- 

 tion of the interlamellar connections (p. 71) and the separation 

 of the two limbs of the later gill filaments (p. 68). In each case 

 a fiat, plate- like organ becomes locally thinner and is finally per- 

 forated. The parallel may be extended, though less fully, to the 

 separation of the upper ends of the reflexed limbs of these later 

 filaments from the gill axis (p. 70). As regards the two latter 

 processes, at least, the parallel has been further extended to em- 

 brace several other widely divergent genera (p. 71), and is prob- 

 ably capable of general application. Moreover an essentially 

 similar process may be recognized in the formation of slits in the 

 primitive gill fold, as described by Hatschek ('80) and Sigerfoos 

 ('96) for Teredo, and by Ziegler ('85) for Cyclas. The general 

 process of perforation or separation in gill development appears 

 to be of very common occurrence among the lamellibranchs. 



The opposed process of fusion is equally characteristic. In 

 the Mytilus gill this process is reduced to a minimum, but it may 

 be noted in the fusion of the ends of the reflexed filament limbs. 

 Other forms, however, show a high degree of fusion in the 

 development of complicated inter-filamentary connections, repre- 

 sented in Mytilus by the simple ciliated disks ; and an entirely 

 different type of fusion has been noted in a former paper ('00) as 

 occurring in certain of the complexly folded gills. 1 And, finally, 



l This paper has been criticised by W. G. Ride wood (" On the Structure of the 

 Gills of the Lamellibranchia," Phil. Trans. Royal Soc. London, Ser. 3, Vol. 195, 

 1903), who holds that the phenomena there described should be interpreted as the 

 result of a splitting of filaments, not a fusion. I see no reason to change my previous 

 view. As regards the present argument, splitting, like fusion, would indicate plas- 

 ticity of the gill. 



