40 
LAND AND FRESH-WATER MOLLTJSKS. 
moved in an undulating manner, often from side 
to side, and appears to act as a tentaculum, as 
well as an organ of motion. The siphonal tube, 
which is at the same time extended and kept so, 
is short, cylindrical, truncate, and changes but 
little ; sometimes, however, it is nearly elliptical; 
it never extends beyond a sixth of the length of 
the shell. A current is seen passing out of it, 
and minute dark particles frequently escape. In 
this manner the animal advances with consider¬ 
able speed by jerks. At other times it ascends 
to the surface, where it proceeds in the same 
manner, with the shell reversed. The animal 
can advance equally with the shell inclined to 
either side; it can creep in any direction, on a 
level or inclined surface, and ascend or descend a 
perpendicular plane. On opening an individual 
in which young were seen, I found six lodged 
there. They were very large, much compressed, 
elliptical, with the umbones nearly central; the 
colour white; the surface beautifully glossed, 
and almost perfectly smooth. On being re¬ 
moved and placed in the water, some of them 
soon began to move in the same manner as 
the adults, but with the shell inclined on one 
sideA 
Pisidium Henslowianum (PI. IV., fig. 19), 
named after Professor Henslow, is very closely 
allied to the last, which may be a variety of this. 
