THE LAMELLIBRANCHIA 



257 



provided with a highly developed byssogenous apparatus. The 

 order comprises five sub-orders the Anomiacea, Arcacea, Mytilacea, 

 Pectinacea, Dimyacea. 



SUB-ORDER 1. ANOMIACEA. 



Very asymmetrical animals with a single large posterior adductor 

 muscle. The heart is not contained in the pericardium, lies dorsad of the 



Fio. 232. 



A portion of the gill of Mytilus, showing the filaments : the median part is cut out. a.b.r, 

 afferent branchial vessel ; e.b.v, efferent branchial vessel : i.f.j, interfilatnentar ciliated junction ; 

 i.l.j, interlamellar connective tissue junction. (After Bonnet.) 



rectum, projects into the pallial cavity, and gives off a single and anterior 

 aorta. The reflected borders of the inner gill-plates of either side are 

 fused together in the middle line. The gonads open into the kidneys, 

 and the right gonad extends into the mantle. The shell is thin and the 

 animal fixed. 



Family ANOMIIDAE, Adams. Foot small. The inferior (right) valve 

 of the adult is perforated to admit of the passage of the byssus. Genera 

 Anomia, Linnaeus ; byssus large and calcined ; British. Placuna, 

 Bruguiere ; byssus atrophied in the adult Hypotrema, d'Orbigny. 

 Carolia, Cantraine. Ephippium, Bolten. Placunanomia, Broderip. 



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