ISOCAKDIA. 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. Branchiae 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 animalculse. 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 superincumbent 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 



