LameUibranchia . 



37 



side. Each gill consists of an axis that is partly attached and of 

 two series of filaments. En the most primitive forms (Proto- 

 branchia) the series of filaments are divergent, but in the majority 

 (Filibranchia, Eulamellib ranch id) they are nearly parallel, with 

 each individual filament bent into a proximal (descending) and a 

 distal (ascending) portion. Further complication results from the 



Fig. 28. 



Gill of Mytilus edviis. 



A. Part of four filaments showing ciliated interfilamentar junctions (c.j). 



B, Diagram of a single filament showing the two lamellae connected at 

 intervals by interlamellar junctions (il.j) and the position of the interfilamentar 

 ciliated junctions (ep). 



Frum the Encyclopaedia Britannica (by permission of Messrs. A. & C. Black). 



formation of junctions between adjacent filaments, so that each 

 series of filaments becomes a double lamella, from the develop- 

 ment of interlamellar connexions and from extensive folding of 

 the lamellae themselves. In one group (Septibranchia) the gills 

 are reduced to a muscular septum. 



Four orders may be recognized, Protobranchia. Filibranchia, 

 Eulamellib ranchia, and Septibranchia. 



