42 THE AMERICAN NATURALIST. (Not. XXXIX. 
quiescent and from their deep coloration are difficult to examine 
but sometimes a more transparent one permits certain organs to 
be traced. When freshly collected and examined in a watch-glass 
of pure sea-water many of them exhibit the greatest activity, . 
protruding the velum between the antero-ventral margins of the 
shell, expanding it somewhat like opening an umbrella, and swim- 
ming rapidly about by the violent movement of its cilia. While 
in motion the heavy shell is suspended below the expanded 
velum. Jarring the watch-glass will cause the animal to 
instantly withdraw its velum, at the same time snapping the 
valves of its shell together and dropping towards the bottom. 
Upon again assuming activity it may protude a long, slim, 
ciliated foot from the middle of its ventral surface, just behind 
the velum. The foot at this period is well developed and is a 
most capable organ by means of which the animal can creep 
rapidly about and forcibly flop its heavy shell from one side to 
the other. It can also bend up along the outside of the shell 
and perform feeling movements over all parts of the body within 
the shell. Its lower or posterior surface sometimes appears 
flattened or even grooved lengthwise and at a short distance 
frum the base of attachment to the body there is a heel-like pro- 
jection which is doubtless the position of the byssus gland. In 
the proximal part of the foot, 7. e., about the center of the animal, 
are right and left otocysts each containing about a dozen oto- 
conia. A little before and above these, but more superficial, are 
two lateral, black pigment-spots (eye-spots). Along each side, 
past the base of attachment of the foot to the body, lies a series 
of short gill-filaments, extending from the eye-spot backwards 
and downwards to near the posterior margin of the shell ; in the 
oldest free-swimming larvz there are eight, diminishing in size 
from before backwards, the last ones being mere knobs ; their 
lower ends are free, but their upper ends spring from one con- 
tinuous axis of origin, that, behind the foot, joins its mate of the 
opposite side near the margins of the mantle. The mouth and 
cesophagus lie in the median plane immediately below and behind 
the velum to which they are attached and with which they are 
protruded and withdrawn. In the quiescent animal the gullet 
lies between velum and foot, in the median sagittal plane as well 
