or MASSACHUSETTS. 59 



\\.-ill. Tlic valves :nv (lien dosed by ;i o,nick adion of the adductor muscle 

 and \\:itiT is forcibly expelled. The first \\ater expelled is driven out pos- 

 rly in the direction of the arrow A (Fig. Ul), :md if this \vere the only 

 in- the main direr! ion in which a current is expelled the ;i ni ma I \\ould by 

 impact tit' \\ater be impelled in the' opposite direct inn or anteriorly; lint the 

 action of swimming is more complicated than this would indicate. \Vhen 

 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 \\aler passes 

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

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

 mantle \\all is lo\\ and thin, as indicated by the arrow II (Fig. 01). 



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 ( '. The valves open i|iiickly and clap ;igain. 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 

 a-ain thanked, and is forced out from the lower border of one ear, in the 

 direction of the arrow I") (Fig. ('>-} ; being the strongest current, it impels 

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

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

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

 ^veral 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 tins action. 



A Jew additional observations upon ihe swimming habit may not be 

 tuit id' (dace. Scallops acquire tin 1 power of swimming at an early age, 

 as they are able to swim in the manner described above soon afk'r they 

 attain 1 millimeter in si/.e. The swimming habit is adopted when the 

 scallop becomes less proficient in moving with (he font, owing to the 

 increasing weight of its body. 



Scallop- an- capable of movements in other directions than described 

 in the above paragraph. Specimens S to 10 millimeters in si/e. 

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

 together and dart bad< in a dorsal direction, evidently to gel 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 



