FRESH-WATER MUSSEL. 395 



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. 



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. There 

 are two teeth in front of the umbo in Unio, but not in 

 Anodonta. 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 ol 

 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. 



Muscular system. The shell is closed and kept closed 

 by the action of the two adductor muscles. When these 

 are relaxed under nervous control, the elasticity of the hinge 

 ligament opens the valves. The foot is a muscular protru- 

 sion of the ventral surface, under the control of three 

 muscles a retractor and a protractor anteriorly, and a 

 posterior retractor. Its upper portion contains some coils 

 of gut and the reproductive organs ; its lower region is very 

 muscular. The protrusion or extension of this locomotor 

 organ is mainly due to an inflow of blood, which is pre- 

 vented from returning by the contraction of a sphincter 

 muscle round -the veins. In moving, the animal literally 

 ploughs its way along the bottom of the pond or river pool. 



