NATURAL HISTORY OF THE BAY SCALLOP 
597 
SWIMMING 
A swimming scallop has been well likened to a bellows ; the valves corresponding 
to the bellows boards, the velar folds or curtain to the leather sides or “bellows,” and 
the anterior and posterior velar openings to the nozzle. The valves are opened and for- 
cibly closed. As the closing starts the velar folds of the two mantle lobes are brought 
together to form a wall or curtain which prevents egress of the water except dorso- 
anteriorly and dorso-posteriorly through the gaps near the hinge. (See fig. 18.) The 
jets through these openings send the scallop in the opposite direction; that is, ven- 
trally with the free margin of the valves in advance and tilted upward even when 
progression is horizontal, as along the surface, as though the animal were biting its 
way through the water. 
Nearly all accounts agree on the means of progression. As to the manner, how- 
ever, there is some difference. Jackson (1890) stated that the jets are alternately 
through the dorso-anterior and dorso-posterior openings, thus causing an alternate 
rotation through 90° or more and resulting in an extremely zigzag course. This does 
Figure 18. — Diagrammatic sketches of scallop swimming: a. As seen from above and at right 
angles to the plane of the valves and showing, in dotted lines and arrows, the water channels 
as bounded centrally by visceral mass, branchial axes, adductor muscle, etc., and peripherally 
by velar folds; b, anterior view at end of “power stroke’’ (shell closing), showing tilting of shell 
(angle not determined); c, anterior view, early part of “power stroke”; L, left valve; 0, open- 
ing for emission of propulsive jets; P, direction of progression; R, right valve; V, velar folds 
united to form a wall or curtain which is continuous except near each end of hinge; W, pro- 
pulsive water jet 
not agree with my observations. As previously noted the account of Jackson seems 
to have been unhesitatingly followed by American writers. 
Jackson, who worked with P. irradians, stated that early in the shell closing 
there is a ventral egress of water before the edges of the velar folds come together. 
Anthony (1906) and Dakin (1909), working with Eurpoean scallops, stated that the 
juxtaposition is timed to prevent such water movement. This is somewhat difficult 
to determine. Apparently at times, with our species, there is such egress, at least 
at the first clap or power “stroke,” for which the valves may be more widely separated 
than for later ones. 
Steering according to Dakin is effected by a partial closing of one or the other of 
the velar gaps, according to Buddenbrock and to Uexkull also, and more accurately, 
by slight local separations of the edges of the velar folds. 
Besides normal swimming there have been described various sorts of scallop 
movement. 
Under special circumstances the scallop forces out the water ventrally and darts 
hinge foremost along the bottom. This has been described by Anthony (1906), 
Dakin (1909), Belding (1910), Buddenbrock (1911), and Uexkull (1912). Belding, 
