LAMELLIBRANCHIATA. 



337 



filaments. The axis projects freely posteriorly, and the filaments are 

 directed transversely in Nucula and Leda, while in Solenomya one is 

 directed dorsally and the other ventrally. In the Filibranchiata the 



ft 



IV 



FIG. 262. Heart of Ostrea 

 magnified (after Poli, 

 from Pelseneer). 7 fused 

 auricles ; II afferent 

 veins ; III ventricle ; IV 

 aorta. 



FIG. 263. Ventral view ofNitcula (after 

 Deshayes, from Perrier). a anterior 

 adductor muscle ; b ctenidium ; d 

 posterior adductor; I labial palps; 

 m internal surface of mantle ; o pos- 

 terior appendage of palps ; p foot. 



branchial processes of the ctenidial axis have the form of filaments 

 (Fig. 264 B), which project ventrally into the mantle cavity ; more- 

 over, they are bent upon themselves in such a manner that the reflected 

 portion, or ascending limb, is external in the case of the filaments^of 



FIG. 264. Series of diagrams of transverse sections, illustrating the different arrangement of 

 the gills in Lamellibranchs. A, Protobranchiata. B, Filibranchiata. C, Eulamellibranchiata. 

 D, Septibranchiata. 1 mantle ; # body ; 3 foot ; suprabranchial cavity ; 5 branchial 

 (mantle) cavity ; e outer plate or filament of ctenidium descending limb ; e x ascending 

 limb of e ; i, i l the corresponding parts of the inner filaments of ctenidium; o pores in s the 

 branchial membrane of the Septibranchiata (from Lang). 



