PHYLOGENY OF THE PELECYPODA. 339 



them. Lying on the bottom with tentacles extended, tlie scallop suddenly folds the 

 guard tentacles back so that they lie closely against the outer border of the perpendicu- 

 lar mantle wall. The valves are then closed ])y a quick action of the adductor muscle 

 and water is ibrcibly expelled. The first water expelled is driven out posteriorly in the 

 direction of the arrow a, PI. xxviii, fig. 8, 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; bnt the act of swimming is more complicated than this 

 would indicate. When the valves have closed to a slight extent the borders of the two 

 thick, i)erpendicidar mantle walls come in contact and then no more water passes out as 

 indicated by the arrow a, but instead, during further closure of the valves, it is forcibly 

 ejected from the lower border of one ear, where the mantle wall is low and thin, as in- 

 dicated by the arrow h, PI. xxviii, fig. 8. The water expelled at the point h is the 

 most forcible cm-rent 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 posteriori}' ; 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, PI. xxviii, 

 fig. 8 a; being the strongest current, it impels the animal in the direction of the arrow 

 e. This striking difference is noticeable, 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 curi'cnts and successive claps, illustrated in PI. xxviii, figs. 8-8 a, is therefore 

 to drive the animal in the direction of the free borders of the valves or postei'iorly. 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 a scal- 

 lop can scuttle over the bottom of a dish in a sidelong direction by successively expell- 

 ing 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 viilue of this jerk combined with the momentum in a poste- 

 rior direction that it maintains its position on the surface of the water and also the high 

 angle to the surface which it presents in sAvimming. 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 tliis action. 



In studying the shell we find a feature correlated with the swimming habit in the in- 

 complete closure of the valves on the eared areas; so that when the free borders of the 

 valves are in immediate contact there is still sufficient space for excurrent action at these 

 areas, and the valves close completely at each clap, for there is in swimming a sharp 

 clicking noise which coidd be caused only by the striking together of the valves. Scal- 

 lops are said to migrate in bands, being seen swimming in lai-ge numbers at some sea- 

 sons of the year. They are among the most active of all Pelecypods and a careful study 

 of theii- habits would doubtless prove fruitful and highly interesting. 



As stated, in young scallops, the outflowing water which has served the function of 

 aeration is jjassed out from the shell at a definite point on the dorsal border as indicated 

 by the arrow in PI. xxviir, fig. 2. This is equally marked in character and more notice- 



