261 = 
aquaria. The foot is bent back under the valve on which the specimen 
is lying, the flaps spread, the shell turned nearly on its dorsal border, 
and the expanded disk planted on the bottom of the dish, Fig. 5. The 
posterior foot-muscles then contract with a sudden jerk. If the foot 
remains firm on the disk, this results in turning the shell end for end 
and frequently throwing it some inches. If the shell overbalances the 
foot, as more frequently happens, the foot shoots anteriorly with the 
flaps spread, with a speed peculiar to this movement. Starting with 
a direct push on the bottom of the dish and followed by that upon 
the water, such a movement usually results in a very considerable 
leap; the shell being thrown posteriorly. These leaps follow each 
other so rapidly, that it frequently happens that the shell hardly touches 
bottom before another leap is made. In this case the protruded foot 
is bent dorsally and then makes an antero-ventral sweep, the flaps 
being spread at the instant the sweep begins and shut together when 
the foot is at its greatest protrusion. 
That such movements as these are performed by animals buried 
in mud, seems doubtful, but a simple protrusion of the foot with the 
flaps spread would be of service in backing, and is probably used. 
It is very probable that the leaping movements are performed with 
effect when an animal, for one cause or another, finds itself stranded 
on some hard, smooth surface where the movements of burrowing are 
not effective. Nothing directly comparable to these movements have 
been observed for Nucula but slow thrusts, with the flaps spread, 
have been seen. 
The muscular system by means of which the movements are per- 
formed is quite complicated. Ramifying through the body in various 
directions are small bundles of muscle fibres, the more prominent of 
which run from one side to the other. 
Fig. 6. Yoldia limatula 
- seen from the right side, with 
the right shell-valve and 
mantle-lobe removed, Drawn 
to show the muscles that 
attach the body to the shell, 
aa anterior adductor muscle; 
afm anterior foot -muscles; 
pa posterior adductor muscle: 
pfm posterior foot-muscle; x 
line of muscles that are at- 
tached to the shell ventral to 
the genital mass and liver; 
y a larger muscle, apparently 
in the same series with =, 
The foot of Yoldia, Fig. 6, is attached to the shell by three pairs 
of anterior foot-muscles, by a large pair of posterior foot muscles, 
