170 
PACIFIC SCIENCE, Vol. II, July, 1948 
When the anterior end of the crab is tilted 
downward from the horizontal plane, the eye- 
stalks are gradually lowered to their normal 
reclining position. As the crab is tilted back¬ 
ward and upward the eyestalks move outward 
and come finally to describe a 45° angle with 
the sagittal plane of the body; in this position 
the eyestalks are farthest removed from the 
depressed position. This movement serves to 
bring the best visual surface, the lateral area 
(B), into a position to perceive in a horizontal 
forward direction; whereas, when the animal 
is held so that the longitudinal axis is parallel 
to the horizontal substrate, this lateral faceted 
region is directed laterally. Through the em¬ 
ployment of this rotary compensatory move¬ 
ment, the crab assumes an oblique position as 
it attains the top of a rock with steep sides; 
the dactyls of the ambulatory legs of the ad¬ 
vancing side are anchored on the top edge, 
while the legs on the downhill side are placed 
just below the top of the rock. Hence, the 
crab is oriented to facilitate a survey of the 
upper rock surface with the advance eye before 
exposing the entire body from above. Par¬ 
ticularly successful observations of this behavior 
pattern are encountered by approaching the 
rocky shore without undue disturbance. Crabs, 
previously located on the top surface, will 
crawl over the edge of a rock and leave visible 
to the observer only the dactyls and propodus 
of the ambulatory legs of one side, as though 
they were hanging from the top of the rock. 
By approaching with caution, following the 
initial disturbance of the animals on the upper 
rock surface, the crabs may be seen clinging to 
the edge of the rock in a position to observe 
movements on its top surface. There is little 
doubt that these compensatory movements con¬ 
tribute considerably to more successful vision 
which is, of course, closely associated with the 
general welfare of the animals in their com¬ 
paratively precarious surroundings. 
Repeated attempts were made to induce 
compensatory movements of the eyestalks by 
moving objects of several colors and sizes in 
arcs at varying distances in many planes, but 
all met with failure. It is difficult to compre¬ 
hend this lack of eye rotation inasmuch as it 
transcends the supposition that rotation of the 
eyes serves to achieve acuteness in perception. 
In most vertebrate animals similar experiments 
are successful in inducing compensatory rota¬ 
tion. 
Upon tilting a crab to the right from its 
normal horizontal position, the left or upper 
eye becomes lodged in its groove as if com¬ 
pletely withdrawn; and the right or lower eye 
moves to an erect position almost perpendicular 
to the carapace. The reverse behavior is charac¬ 
teristic when animals are tilted to the left. 
Crabs observed on steep rock walls in a normal 
sideways orientation display this differential 
eyestalk movement; the upper eye is depressed, 
directing the anterior surface laterally and the 
lateral, well-faceted surface posteriorly. The 
lower, erect eyestalk commands a better view 
than if it were in its normal position. Its posi¬ 
tion provides increased posterior vision without 
decreasing anterior vision to any appreciable 
extent, while the lateral vision remains normal. 
The compensatory reaction suggests an adapta¬ 
tion to increase posterior vision when the ani¬ 
mal’s back is not to the wall, as it were, while 
ascending steep rocks. 
Crabs which remain motionless in shallow 
pools or near the edges of deeper pools charac¬ 
teristically have the faceted surface of the eyes 
protruding above the water level. Perception 
of objects outside the pool would, of course, 
be enhanced because the bending of light rays 
upon entering the water is thereby obviated. 
Rather typical responses toward the preda¬ 
cious gulls are exhibited by these crabs. 
Throughout the observations of crabs in tide 
pools and the surrounding rocks, rapid move¬ 
ments for concealment were frequently noted. 
Gulls in flight close at hand were the stimulat¬ 
ing agent. On several occasions the shadow of a 
flying gull was sufficient to elicit a hiding re¬ 
sponse. Whether or not these responses to 
shadows were effected by the actual sight of 
