578 
BULLETIN OF THE BUREAU OF FISHERIES 
water are satisfactory). Another is that scallops frequently cease to react after 
repeated stimulation. 
Marine mollusks have been grouped according to their reaction to changes in 
light intensity into those which react to increases and decreases, those which react to 
increases only, and those which react to decreases only. It seems generally agreed 
that scallops belong with those that react to decreases only. With normally acting 
individuals a shadow cast on the eyes causes quick complete or partial closing. Using 
small objects to produce local shadows, Rawitz (1888) found that the shadow must 
fall on several eyes to produce a reaction. Wenrich (1916), using instead local illumi- 
nation through slits or small holes in a disk, found that cutting off the light from as 
few as two eyes (the smallest number tested) produced definite, although sometimes 
local, reaction. 
It has been held that, in addition to being sensitive to decrease in illumination 
(shadows), scallops were sensitive to movement of objects. To test this Wenrich 
(1916) placed individuals which had been found to react to decreases in illumination, 
but not to increases, in a glass dish in one end of a box at the opposite end of 
which was black paper. Against this black background small white cards of various 
sizes were moved upward to a level with the scallops. It was believed, by this means 
and proper precautions to prevent uncontrolled light changes, that the only change in 
illumination of the scallop eyes was increase (to which, as above noted, the scallops 
had been found not to react) and, therefore, that reactions were attributable not to 
changes in illumination but to perception of the movement of the object. Unfailingly 
the animals gave immediate and vigorous responses by closing the valves or by con- 
traction movements of the velar folds and tentacles. The reactions occurred with both 
slow and rapid movements and also when downward or horizontal. The smallest 
effective white card was 1.5 centimeters square at a distance of 35 centimeters (the 
distance used throughout these experiments). 
Uexkull (1912) placed a scallop in one aquarium and a starfish — its principal 
enemy — in an adjacent aquarium. There was no response until the starfish moved, 
when the scallop instead of closing extended its tentacles in the direction of the 
starfish. 
According to my observations, movements, particularly sudden movements, of 
an object within a few feet of freshly caught scallops and in their line of vision (even 
when, as in Wenrich’s experiments, an increase in illumination is involved), caused 
complete or partial closing. When a scallop reopens, long tentacles of the adjacent 
sector may or may not follow the object as it moves. 
Probably there is sufficient evidence that the ocelli are organs of vision, but a 
better knowledge of their functioning and usefulness to the scallop is much to be 
desired. 
The very extensible tentacles of the middle or ophthalmic fold have been sup- 
posed (Uexkull, 1912) to be endowed with both chemical and tactile sensitivity. 
.Dakin (1910) found that the introduction of a chemically irritating substance, as 
one obtained from starfish, caused scallops to swim and concluded that the perception 
was by the “sensory tentacles.” The writer does not find that this chemical sensi- 
tivity has been traced definitely to these tentacles. Tactile sensitivity is readily 
demonstrated. 
Sections of the margin of the mantle, cut from a living scallop by the author and 
placed in a dish of salt water, soon relaxed with some extension of the tentacles of 
the middle fold. Touching one of these tentacles caused contraction not only of the 
