NATURAL HISTORY OF THE BAY SCALLOP 
579 
tentacles but of the whole excised portion — an action not caused, in my observation, 
by touching the tentacles of the velar fold. However, after a portion of the mantle 
was so cut as to separate the middle velar folds from the tissue containing the circum- 
pallial nerve, there appeared to be no contraction of the piece as a whole or of the 
parts. Touching a tentacle did not even cause contraction of that tentacle. 
The third fold of the mantle margin, sometimes referred to as velum, but 
here termed "velar fold” or "flap,” is the most prominent and distinct of the three. 
(Figs 3 and 4.) It is wide and well supplied with muscles, is brightly marked (as 
with yellow, black, and white), and, near its free margin, bears alternately large and 
small tentacles (the guard tentacles) in a somewhat zigzag row. The velar folds 
play an important part in swimming and presumably in feeding. The free margins 
may be brought together so that a continuous wall or curtain is formed. When the 
scallop lies at ease with the valves well separated, they are extended toward each 
other, nearly at right angles to the plane of the valves, meeting or nearly meeting 
close to the posterior ribs. 
The so-called guard tentacles, which are not considerably extensible, are directed 
in a convex arc toward those of the other flap to form a screen through which the 
food and water is drawn. If an indrawn object, such as a large carmine grain, hits 
a tentacle, the tentacle makes a peculiar flicking motion, but the shell is not closed 
nor the object otherwise prevented from entering. Moreover, a touch with a prod, 
at least at times, does not cause the valves to close. While the valves remain apart, 
whether the opening is wide with the velar folds extending up and down, or narrow 
with these folds horizontal, a more or less complete screen of these "guard” tentacles 
is maintained. With important tactile function apparently wanting, a chemical, 
olfactory, or taste function is strongly suggested. 
To test this a starfish was crushed in a mortar and a cloudy liquid irritating to 
scallops obtained. A freshly caught scallop was placed in a rectangular glass dish. 
When the scallop had opened wide its shell and arranged the mantle margin with the 
guard tentacles across the inhalent opening and well separated from the tentacles of 
the middle fold, the irritating cloudy liquid was gently introduced directly to the guard 
tentacles by a special pipette with bent tip. Repeatedly and unfailingly as the cloudy 
liquid came to the guard tentacles, these were sharply withdrawn and the shell vio- 
lently closed. Beyond question the tentacles of the middle fold did not enter into 
the response and it seemed clear that the exciting substance had not entered the 
pallial cavity when the reaction began. Pipetting the same liquid against individual 
extended tentacles of the middle fold caused no reaction beyond some contraction of 
the tentacles touched, as when water was used — evidently a tactile response. Squirt- 
ing sea water against the guard tentacles failed to induce shell closing. These exper- 
iments seem to show that the guard tentacles possess an olfactory, gustatory, or some 
chemical sensitivity. 
Anterio-dorsally and posterio-dorsally, near the ears, the velar folds are narrowed 
with some abruptness and are without tentacles. When the valves are apart these 
folds are so extended that they touch, or nearly touch, at the ventral limit of the pos- 
terio-dorsal narrowing, bounding a well marked exhalant opening. For the young 
this 'arrangement of the folds has been termed a pseudo-syphon. The opening 
formed by the anterio-dorsal narrowing may be confluent with the large inhalant 
opening (extending from the ventral limit of the exhalant) or separated. The ciliary 
current through it is inhalant (and the only inhalant current when the shell is closed), 
