262 



COLLEGE ZOOLOGY 



feed on minute organisms. Most of them burrow into the sand 

 or mud; a few bore cavities for themselves in calcareous rocks; 

 and still others are sessile, like the oyster. Some PELECYPODA 

 live commensally or parasitically on or in the bodies of ascidians, 

 sponges, and echinoderms. 



Classification. The PELECYPODA are divided into four orders 

 according to the structure of the gills. 



Order i. Protobranchia (Fig. 189, A). PELECYPODA with 

 plate-like gill filaments (e, i) which are not reflected; mantle 



FIG. 189. Morphology of the gills of PELECYPODA, seen diagrammatically 

 in section. A, PROTOBRANCHIA. B, FILIBRANCHIA. C, EULAMELLIBRANCHIA. 

 D, SEPTIBRANCHIA. e, e, external row of filaments; i, i, internal row of fila- 

 ments; e', external row or plate folded back; i', internal row folded back; 

 /, foot; m, mantle; s, septum; v, visceral mass. (From the Cambridge Natural 

 History, after Lang.) 



cavity not divided into two parts. Examples: Nucula, Leda, 

 Yoldia. 



Order 2. Filibranchia (Fig. 189, B). PELECYPODA with gill 

 filaments reflected and united by ciliary junctions. Examples: 

 Area, Mytilus, Modiola, Pecten. 



Order 3. Eulamellibranchia (Fig. 189, C). PELECYPODA 

 with gill filaments forming plates or lamellae. Examples: Ostrea, 

 Cyclas, Unio, Anodonta, Mactra, Venus, My a, Teredo (Fig. 190), 

 Solen. 



Order 4. Septibranchia (Fig. 189, D). PELECYPODA with 

 gills transformed into a muscular septum (s) and not functioning 

 as respiratory organs. Examples: Silenia, Cuspidaria. 



