LAMELLIBRANCHIATA. 



329 



enormously enlarged and the posterior region of the body is 

 peculiarly elongated and uncovered by the rudimentary shell ; so 

 that the whole animal acquires a vermiform appearance, the shell- 

 bearing anterior part of the body constituting the head (Teredo, 

 Fig. 271). 



In cases in which there is no fusion the hind end of the edges of 

 the mantle lobes often presents two slight contiguous excavations 

 (Anodonta, Fig. 254), the ventral of which is bordered by numerous 

 papillae. When the two halves of the mantle are applied together 

 these excavations form, with the corresponding structures of the 

 opposite side, two slit-like openings placed one above the other. 



VA 



KrS 



HS 



JS 



J) 



F 



FIG. 254. Anatomy of Unio pictorum (after C. Grobben). A region of mantle lobes bounding 

 the cloacal or exbalent orifice ; Af anus ; Cy cerebral ganglion ; D intestine ; E region of 

 mantle lobes bounding the inhalent or branchial oritice ; F foot; G generative organs; 

 HA posterior aorta ; Hk ventricle ; IIS posterior adductor muscle ; K branchiae ; A'c.S' crys- 

 talline style ; L liver ; M stomach ; Mg splanchnic ganglion ; MS labial palp ; Mt mantle ; 

 N kidney ; mouth ; P pericardial gland ; Pg pedal ganglion ; Va anterior aorta ; Vh auricle ; 

 1'S anterior adductor muscle. 



The dorsal of these two openings functions as the cloacal (exhalent) 

 opening, the ventral as the branchial (inhalent). So that in such 

 cases, though there is no actual fusion, functionally two siphons are 

 present as in the siphoned forms. 



The most important of the muscles attached to the shell are 

 (1) those of the edge of the mantle: these are attached to the 

 pallia! line of the shell, and serve to retract the edges of the mantle; 

 the siphonal muscles which serve for the retraction of the siphons 

 are a specialized portion of this system, and arise from the pallial 

 indentation. (2) The adductors which pass from valve to valve of 



