262 CARDITID^. 



Zoster a marina. I have also seen them at Balta clus- 

 tering in vast profusion round the base of the pegtop- 

 shaped plants of Himanthalia lorea, or the common 

 strapweed; and my friend Mr. Norman brought me 

 one day a small hand-net nearly full of them, which he 

 had captured by sweeping the seaweeds at low water on 

 the Whalsey Skerries. There must have been millions 

 in this heap. The animal is a fearless or hardy little 

 creature, and very active. When put into a saucer of 

 sea-water, it immediatelv stretched out its white and 

 slender foot, attaching the point to the surface of the 

 vessel, and walked about with tolerable rapidity. I 

 have observed it in a rock-pool, after having thus shifted 

 its quarters, drawing in the foot and mooring itself to a 

 stone, or leaf of a seaweed, by its strong but almost 

 transparent byssus. Under a microscope the gills may 

 be seen through the shell, when it is of a lighter colour 

 and therefore more transparent than usual, to flap like 

 those of a fish. It swims freely in an inverted position, 

 the beaks of the shell downwards, and the point of the 

 foot expanded. The motion of swimming seems to be 

 effected by a series of muscular contractions parallel with 

 the line of progress, the point only of the foot being so 

 employed. The gills are then distinctly visible through 

 the open folds of the mantle, and consist of two pairs ; 

 the outer pair are rather larger and higher than the 

 inner pair. The mantle appears to have two folds, the 

 inside one being the smaller. When the animal is 

 creeping, the foot is folded inwards for its whole length. 

 It is then filled with water through a wide and open 

 slit in the heel or base, and becomes of a tubular or 

 cylindrical shape. Several individuals being confined 

 in the same vessel, after restlessly and actively moving 

 about for some time, assembled in groups, young and 



