ISOCARDIA. 109 
at each end; at the posterior extremity forming two short 
siphons or tubes, ciliated at the orifices; colour yellowish- 
white; margin orange. Foot very muscular, broad, trian- 
gular, compressed, pointed, orange. Branchiz external, con- 
cealed between the mantle and the body. Body soft, com- 
pletely included within the valves. On being placed in a 
vessel of sea-water the valves of the shell gradually opened ; 
the feelers or ciliated fringe of the upper orifice moved slowly, 
as if in search of animalcule. Having remained in this situa- 
tion about ten minutes, water was ejected with considerable 
force from the lower orifice, which till now had remained 
motionless. The expulsion of the water appeared to be effected 
by a sudden contraction of the muscles, because this was 
never done without the valves closing at the same instant. 
After a few seconds the valves gradually returned to their 
open position, and remained quiescent as before till the water 
was again ejected with a jerk. This alternating process was 
repeated at unequal intervals during the whole time my spe- 
cimens were under examination, but at shorter intervals on 
receiving fresh supplies of sea-water, when I suppose the food, 
the quality of which I could not ascertain, was more abundant. 
The animal appears to be insensible both to sound and light, 
as the presence or absence of either did not at all interrupt 
its movements, but its sense of feeling appeared to be very 
delicate ; minute substances dropped into the orifice of the 
mantle instantly excited the animal, and a column of water, 
strongly directed, expelled them from the shell. With so much 
strength was the water in some instances ejected, that it rose 
above the surface of three inches of supermeumbent fluid. 
Animal small in proportion to the shell, occupying, when dead, 
barely a third of the space enclosed in the valves. The mantle 
is slightly attached to the shell and to the epidermis at the 
margin, and appears to be kept distended, and in contact with 
the interior of the valves, by the included water. The valves 
fit so closely that the animal can remain two days or more 
without permitting a single drop of fluid to escape. Loco- 
motion very confined. It is capable, with the assistance of 
its foot, which it uses in the same manner, but in a much 
