i66 



THE GILL IN PELECYPODA 



CHAP. 



1. The simplest form of gill (^Nucula^ Leda^ Solenomya^ etc.) 

 is that which consists (Fig. 76, A, compare Fig. 100, p. 201) of 

 two rows of very short, broad, not reflected filaments, the rows 

 being placed in such a way that they incline at right angles to 

 one another from a common longitudinal axis. The filaments 

 are not connected with one another, nor are the two leaves of 

 each gill united at any point. (^Protobranchiata.^ 



2. In the Anomiidae^ Arcadae, Trigoniidae^ and Mytilidae 

 each gill consists of two plates or rows of much longer filaments, 

 which consequently occupy a much larger space in the mantle 



A B c D 



Fig, 76. — Morphology of the branchiae of Pelecypoda, seen diagrammatically in sec- 

 tion: A, Protobranchiata ; B, Filibranchiata ; C, Eulamellibranchiata; D, Septi- 

 hranchiata; e, e, external row of filaments; i, i, internal row of filaments; e', 

 external row or plate folded back ; i', internal row folded back ; /, foot ; m, mantle ; 

 s, septum; v, visceral mass. (From A. Lang.) 



cavity (Fig. 76, B). . Unable to extend beyond the limits of the 

 mantle, filaments are reflected or doubled back upon one another, 

 those of the external plate being reflected towards the outside, 

 those of the internal plate towards the inside. 

 Each separate filament is not connected with 

 the filament next adjacent, except by surface 

 cilia situated on small projections on the 

 sides of the filaments, and interlocking with 

 the cilia of the adjacent filament. The two 

 superposed plates or leaves of the gill may or 

 may not be united by cords running between 

 the two parts of a filament. (^F ilihranchiata.') 

 Fig. 77.— Four gill fiia- 3. In the Pectinidae^ Aviculidae, and 

 ments of Mytiius Qstreidae a further development takes 



highly magnified; c; rr.! />! ^ £ \ -n 



ciliary junctions; /, place. Ihc hlamcuts 01 cach gill ai'e 



filament. (After Peck.) reflected in the Same way as in the Fili- 



hranchiata^ but the part thus reflected may become completely 



