MOLL use AS YSTEMA TIC RE VIE W 



15 



without siphons ; Monomyarian. Foot small ; bodyand shell asymmetrical. Attached. 

 Branchial filaments entirely free {Anumia, Placuna). Fam. Arcidse : the branchial 

 filaments of each row connected by ciliated discs ; Dimyarian. No siphons. Foot 

 large {Area, Pectunculus). Fam. Trigoniidse : ctenidia like those of the Arcidw ; 



Fig. 22.— Mytilus edulis (after Meyer ami Mobius), left aspect, with extended foot attaching a 

 byssus thread ; d, byssus threads ; a, exhalent aperture (anal siphon) ; h, fringed edge of the in- 

 halant mantle aperture ; c, object to which the animal is attached. 



Dimyarian. No siphons {Trigonia). Fam. Mytilidse (excluding Aviculidce) : cten- 

 idia connected by means of non - vascularised trabecule. The anterior adductor 

 is smaller than the posterior (Heteromyarian). With siphons. Foot long. {Mytihis 

 [Fig. 22], Modiola, Lithodomus [boring mussel], Modiolaria). 



Order 3. Pseudolamellibranchia. 



The consecutive ctenidial filaments of each row are connected by means either 

 of ciliated discs or of vascularised trabeculiK ; and the ascending and descending 



