16 



MOLLUSCA. 



two groups are so essential, that a close connection between them 

 cannot be maintained. 



The usually strictly symmetrical body is laterally compressed and 

 of considerable extent, and is surrounded by two lateral mantle 

 lobes, which are continuous across the dorsal middle line and secrete 

 a right and left shell valve. To the sides of the mouth opening 

 are found two pairs of leaf- or tentacle-like buccal lobes the labial 

 palps. On the ventral surface a lai-ge, usually hatchet-shaped foot 

 (fig. 498, F) projects; and two pairs, rarely one pair, of large 

 lamellar gills are always placed in the mantle furrow between the 

 mantle and foot (fig. 498, K}. 



as 



FIG. 498. Anatomy of Unio pictorum (after C. Grobben). TS, anterior adductor muscle; 

 HS, posterior adductor muscle ; MS, labial palp ; F, foot ; Mt, Mantle ; , branchiw ; 

 Off, cerebral ganglion ; Pg, pedal ganglion ; My, splanchnic ganglion ; O, mouth ; 

 -V, stomach ; L, liver ; KrS, crystalline style ; D, intestine ; Af, anus ; G, generative 

 organs ; A, region of mantle lobes bounding the exhalent or cloaca! orifice ; E, region 

 of ditto bounding inhalent or branchial orifice ; N, kidney ; Tk, auricle ; Ilk, ventricle ; 

 VA, anterior aorta; HA, posterior aorta; P, pericardial gland (schematic). 



The hind end of the edges of each mantle lobe almost always 

 presents two slight, contiguous excavations (fig. 498, A and E\ the 

 ventral of which is bordered by numerous papilla?. When the 

 two halves of the mantle are above together, these excavations 

 form, with the corresponding structures of the opposite side, two 

 slit-like openings, placed one behind the other. The upper or 

 dorsal of these two openings functions as the cloacal, or exhalent 

 opening ; the lower or ventral as the inhalent opening. Through 

 the latter, with a slightly gaping shell, the water is driven by 

 the peculiar arrangement and action of the cilia on the inner 



