476 CYPRINID^. 



jerk ; this alternating process was repeated at unequal in- 

 tervals during the whole time my specimens were under 

 examination, but at shorter intervals 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 in- 

 terrupt 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 3 inches 

 of superincumbent fluid. Animal small in proportion to its 

 shell, occupying when dead barely a third of the space en- 

 closed 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 per- 

 mitting a single drop of fluid to escape. Locomotion 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 capa- 

 ble of creating by the alternate opening and closing of its 

 valves." 



