1 66 



THE GILL IN PELECYPODA 



1. The simplest form of gill {Nucula, Lcda, Solenomya, etc.) is 

 that which consists (Fig. 7G, A, compare Pig. 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 anotlier, nor are the two leaves of each gill 

 united at any point. (ProtohraTichiata.) 



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 cavity 



Fig. 76. — Morphology of the branchiae of Pelecypoda, seen diagranimatically in section : 

 A, Protobranchiata ; B, Filibranchiata ; C, EulameUibranchiata ; D, Septi- 

 branchiata; e, e, external row of filaments ; i, i, internal row of filaments ; e', ex- 

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

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



(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 l)eing 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 tlie 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. (Filibranchiata.) 

 3. In the Pectinidae, Avictdidae, and 

 Ostreidae a fiurther development takes 

 The filaments of each gill are 

 reflected in the same way as in the Fili- 

 Iranchiata, but the part thus reflected may become completely 



Fj. 



;. 77. — Four gill lila- 

 ments of Mytilus, 

 highly magnified ; cj, 

 ciliary junctions ; /, place, 

 filament. (After Peck. * 



