VII 



MOLLUSC A OUTER ORGANISATION 



35 



Projecting into the mantle cavity, there is a large muscular process of 

 the body, the foot, which is directed downward and somewhat forward, 

 and can be protruded between the free edges of the mantle. This foot 

 is also laterally compressed. In certain cases which, though excep- 

 tional, deserve special mention, its free end is flattened, and it thus has 

 a flat sole. The outer surface of the trunk and mantle folds secretes 

 a bivalve shell which covers the whole body. One valve lies to the 

 right, the other to the left of the median plane, and the two are exactly 

 alike. Each valve repeats the outline of its own side of the trunk 

 with its mantle fold. 



The two valves articulate dorsally, and are open anteriorly, ventrally, 

 and posteriorly. Two strong muscles (adductors) run transversely 



FIG. 4o. Anatomy of Unio (Margaritana) margaritiferus, left side (after Leuckart and Nitsche). 

 o, Mouth ; Cg, cerebral ganglion ; MI, anterior adductor muscle ; a>, oesophagus ; I, digestive gland 

 (liver) ; HO, nephridial aperture : lo, apertures of the digestive gland in the stomach m ; Aa, anterior 

 aorta ; ?i, nephridium, the outline given in dotted lines ; I", heart ; r, hind-gut ; Ap, posterior 

 aorta ; J/ 2 . posterior adductor ; a, anus ; Vg, visceral ganglion ; Br, gill ; Bk, mantle cavity ; go, 

 gonads with genital duct goi ; Pg, pedal ganglion ; p, foot. The arrows indicate the direction of 

 the inhalent and exhalent streams of water. 



from one valve to the other. Their contraction serves to shut the 

 shell completely. One of these muscles lies anteriorly, the other 

 posteriorly. Their points of attachment produce impressions on the 

 inner surface of the shell, which are always distinctly visible when 

 the shell is removed. 



The mouth lies below the anterior adductor, between it and the 

 anterior base of the foot. The anus lies behind the posterior adductor. 

 There is no distinct head. Xear each side of the mouth, the body 

 carries two leaf -like processes, the oral lobes or labial palps. At the 

 line of insertion of the foot in the mantle cavity, a longitudinal 

 ridge rises on each side in the middle and posterior regions of the 

 body ; this carries two rows of long branchial leaflets. There is thus, 





