ISOCARDIA. 301 



peated at unequal intervals during the whole time my 

 specimens were under examination, but at shorter inter- 

 vals on receiving fresh supplies of sea- water, when, I 

 suppose, food (its quality 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 being 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 

 its shell, occupying when dead barely a third of the 

 space enclosed in the valves. Its 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. Locomotion is very confined ; it is capable, 

 with the assistance of its foot, which it uses in the same 

 manner (but in a much more limited degree) as the 

 Cardiacea, of fixing itself firmly in the sand, generally 

 choosing to have the umbones covered by it, and the 

 orifices of the tubes of the mantle nearly perpendicular. 

 Resting in this position on the margin of a sand-bank, 

 of which the surrounding soil is mud, at too great a 

 depth to be disturbed by storms, the Isocardia of our 

 Irish Sea patiently collects its food from the surround- 

 ing element, assisted in its choice by the current it is 

 capable of creating by the alternate opening and closing 

 of its valves. Some of the specimens that had been 



