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, /) 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: Ostrca, 

 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. 



