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 Uzio, but not in 
Anodonta. The following muscles are inserted on the shell, 
and leave impressions :— 
(a) The anterior adductor. 
(4) The posterior adduetor. 
(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 (4). 
As the shell grows, the insertion of the muscles and the attachment ot 
the mantle change, and the traces of this shifting are visible. 
Skin.—There is much ciliated epithelium about Azodonta, 
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, 
