OF MASSACHUSETTS. 59 



wall. The valves are then closed by a quick action of the adductor muscle 

 and water is forcibly expelled. The first water expelled is driven out pos- 

 teriorly in the direction of the arrow A (Fig. 61), and if this were the only 

 or the main direction in which a current is expelled the animal would by 

 impact of water be impelled in the opposite direction or anteriorly; but the 

 action of swimming is more complicated than this would indicate. When 

 the valves have closed to a slight extent the borders of the two thick, per- 

 pendicular mantle walls come in contact and then no more water passes 

 out as indicated by arrow A, but instead, during further closure of the 

 valves, it is forcibly ejected from the lower border of one ear, where the 

 mantle waU is low and thin, as indicated by the arrow B (Fig. 61). 



The water expelled at the point B is the most forceful current and 

 probably of the greatest volume; by its means the creature is impelled in 

 the direction of the arrow C. The valves open quickly and clap again. The 

 second time, as before, the first water is driven out posteriorly; but when 

 the mantle walls come in contact, the direction of the excurrent water is 

 again changed, and is forced out from the lower border of one ear, in the 

 direction of the arrow D (Fig. 62) ; being the strongest current, it impels 

 the animal in the direction of the arrow E. This striking difference is 

 noted, viz., that at successive claps the water is driven out from alternate 

 ears, first on one side and then on the other. The resultant action of the 

 several currents and successive claps, illustrated in Figs. 61, 62, is, therefore, 

 to drive the animal in the direction of the free borders of the valves, or 

 posteriorly. It is due to the alternate expulsion of the water first from 

 one ear and then from the other, as described, that the animal presents a 

 succession of zigzag jerks in swimming. The direction of the current 

 alternately to the two ears appears to be voluntary, as scallops can scuttle 

 over the bottom of a dish in a sidelong direction by successively expelling 

 the water at each clap from one and the same ear. The action of the first 

 current of water expelled posteriorly, before the mantle walls come in contact, 

 gives the animal an upward jerk, and it is in virtue of this jerk, combined 

 with the momentum in a posterior direction, that it maintains its position 

 on the surface of the water, and also the high angle to the surface which 

 it presents in swimming. The current driven out posteriorly in the initial 

 closure of the valves is so powerful that water may be squirted by adults 

 to the height of five inches or more from the surface by this action. 



A few additional observations upon the swimming habit may not be 

 out of place. Scallops acquire the power of swimming at an early age, 

 as they are able to swim in the manner described above soon after they 

 attain 1 millimeter in size. The swimming habit is adopted when the 

 scallop becomes less proficient in moving with the foot, owing to the 

 increasing weight of its body. 



Scallops are capable of movements in other directions than described 

 in the above paragraph. Specimens 8 to 10 millimeters in size, 

 when approached ventrally with the point of a pencil, snap their valves 

 together and dart back in a dorsal direction, evidently to get away from 

 the pencil, which they allow to get within reach of their tentacles before 

 moving. The water is expelled with a quick squirt from the ventral 



