FRESH- WATER MUSSEL. 377 



Internal appearance. When the right half of the shell 

 is folded back, the anterior and posterior closing muscles 

 having been carefully cut close to the gently raised valve, 

 the mantle folds are seen lining the shell, and forming 

 posteriorly the ventral inhalant and dorsal exhalant lips. 

 The ventral lips have papillary processes. Internal to the 

 mantle there are two gill-plates on each side ; projecting 

 from between these is the foot, muscular ventrally, softer 

 dorsally ; the median dorsal pericardium is just beneath the 

 ligament ; the ventricle shines through its walls, and the 

 dark-coloured kidneys are seen through its floor. Below 

 the anterior adductor muscle is the large mouth, bordered 

 beneath by two lip processes (labial palps) on each side. 

 These resemble the gills in appearance, and are probably 

 modified portions of the gills. The anus is above the 

 posterior closing muscle. The whole space between the 

 two mantle flaps is called the mantle cavity, and it is divided 

 by a slight partition at the bases of the gills into a large 

 ventral infra-branchial chamber, and a small dorsal supra- 

 branchial chamber which ends at the exhalant orifice. This 

 mantle cavity is, of course, really outside the body. 



On the surface of the valves of the shell a few small 

 pearls may be seen ; they are formed by the enclosure of 

 some minute grains of sand in the prismatic layer. The 

 following muscles are inserted on the shell, and leave 

 impressions :- 



(a) The anterior adductor. 



(b) The posterior adductor. 



(c) The anterior retractor of the foot continues with (a}. 



(d) The protractor of the foot a little below (a). 



(e) The posterior retractor of the foot continues with (b}. 



As the shell grows, the insertion of the muscles and the attachment of 

 the mantle change, and the traces of this shifting are visible. 



Skin. There is much ciliated epithelium about Anodonta, 

 especially on the internal surface of the mantle, on the gills, 

 and on the labial palps ; and little pieces cut from an 

 animal incompletely dead (e.g. from the oyster swallowed 

 half-alive) have by means of their cilia a slight power of 

 motion. The skin of the foot is not ciliated but glandular : 

 on the mantle edge sensitive and glandular cells are abund- 

 ant, but usually in inverse ratio to one another. 



