Biology of Pachygrapsus crassipes — HlATT 
169 
stimulating a number of facets in rapid suc¬ 
cession imparts an impulse to the central ner¬ 
vous system which results in a more extensive 
reaction than that resulting from the perception 
of stationary objects. Slight movements within 
a few feet of the animals elicit instantaneous 
responses; whereas considerable movement at a 
distance is required to obtain a response of 
similar magnitude. The greatest degree of visual 
activity in the faceted eye is achieved not only 
at short distances but, in addition, on that area 
of the eye which has the least curvature, since 
it is obvious that the greatest number of light 
rays emanating from an object will strike com¬ 
paratively more ommatidia if the faceted surface 
is flattened. My observations coincided with 
the predicted response founded on the fore¬ 
going principles. Figure 12 is presented to 
demonstrate the curvature and extent of the 
faceted surface of the eyes of P. crassipes in 
several planes of vision. It is seen that the 
least faceted surface occurs on the medial por¬ 
tion of the eye (D); this is to be expected be¬ 
cause the opposite eye receives visual stimuli 
from the identical angle. The dorsal surface 
(A) is strongly convex and slightly faceted. 
These data indicate that vision dorsad would 
be poor, a fact substantiated on numerous occa¬ 
sions while observing crabs from a vantage 
point directly above. Space does not permit a 
detailed presentation of these observations. The 
posterior region of the eye (E) is likewise 
strongly convex and provided with relatively 
few facets. Vision in a posterior direction does 
not appear to be acute, but it is difficult to 
effect successful observations because the pos¬ 
terior end of the crab is usually directed toward 
a sheltering area in the habitat. It would seem 
that the compensatory movements of the eyes 
(rotation of the eyestalk to direct highly faceted 
areas of the eye to new positions), which will 
be discussed below, serve to span the posterior 
direction effectively. 
The anterior portion (C) of the faceted sur¬ 
face is much greater in comparative area and 
exhibits less convexity than the regions dis¬ 
cussed above. Vision in the forward direction, 
therefore, would be expected to be good. This 
supposition has proved valid many times. The 
slightest movement of a person at a distance up 
to 30 yards elicits immediate response. Com¬ 
pensatory movements probably increase visual 
acuity in the forward direction because the ani¬ 
mals seem somewhat more responsive to visual 
stimuli than one would expect from the number 
of facets located in the anterior portion of the 
eye. The outer or lateral (B) surface of the 
eyestalk is comparatively flat and completely 
covered by ommatidia. Observations indicate 
that visual acuity is most pronounced in the 
lateral direction, a predicted result based upon 
an examination of the morphological features 
of the eye. Further, it appears highly significant 
that the most effective visual surface lies in 
the plane of normal locomotion. 
The compensatory movements of the eye- 
stalks, such as those described for other Crus¬ 
tacea (Bethe, 1897; Cowles, 1908), occur in 
P. crassipes when they are tilted either from 
left to right or in an anteroposterior direction. 
c 
Fig. 12. Comparative amount of curvature and 
faceted surface on various aspects of the eye of P. 
crassipes. A, dorsal surface; B, outer or lateral sur¬ 
face; C, anterior surface; D, medial surface; E, pos¬ 
terior surface. 
