Biology of Pachygrapsus crassipes —Hi ATT 
175 
as depending entirely upon visual stimuli, be¬ 
cause without exception the diffusion of food 
juices through the water required several min¬ 
utes (Experiments 1, 3, 4, and 6). Furthermore, 
it seems, from Experiments 1, 3, 4, and 6, that 
the threshold of chemical stimulation for this 
species is relatively high because responses 
which suggested stimuli of chemical nature were 
invariably delayed several minutes after the 
introduction into the pool of some food sub¬ 
stance. In Experiment 6 the juices were visible 
in the water about the crab for several seconds 
before response was shown. 
The tactile sense of this species appears to 
be rather highly developed. Except in Experi¬ 
ment 5, the participation of this sense was 
clearly revealed. The apparent prodding by 
the chelae and distal podomeres of the remain¬ 
ing pereiopods discloses that these crabs rely 
greatly upon this sense in their food procure¬ 
ment. Moreover, it is employed equally well 
in and out of water. In addition to explaining 
the frequently observed backward approach to 
food under water on the basis of the orientation 
of the respiratory current, the elevation of the 
fifth pereiopods to be employed as testing 
structures brings forth the supporting role 
played by the tactile receptors. It is significant 
to note that this backward approach to food 
has never been observed out of water, suggest¬ 
ing that in this type of reaction under sub¬ 
merged conditions, tactile sensations are sub¬ 
ordinate to chemical ones. The data derived 
in Experiments 6 and 7 provide proof that food 
is identified by the dactyls, although the latter 
experiment seems to indicate that these struc¬ 
tures are not selective chemically. Experiment 7 
provides evidence that the dactyls do distinguish 
texture of substances; and inasmuch as their 
chemical reception is slight or wanting, the 
major role played by the dactyls in food procure¬ 
ment may possibly be concerned with texture 
differentiation. Food materials may be selected 
in this manner because they feel soft and fleshy. 
At any rate, the spongy texture of the art gum 
eraser was sufficient to release all the reactions 
attendant to food manipulation. 
Nocturnal foraging, the chief type involved in 
food procurement by this species, presents an 
entirely different aspect of comparative sensory 
participation. Apparently vision is completely 
in disuse at this time. Motion near crabs at 
night elicits no response; however, if the mov¬ 
ing object is slightly illuminated, instantaneous 
responses occur. Furthermore, the nocturnal 
behavior of this species tends to limit the utility 
of the chemical sense because the crabs emerge 
from the tide pools and forage high on the 
rock surfaces. Experiments 2, 5, and 6 indicate 
that odors elicit virtually no response; conse¬ 
quently, the chemical sense is reduced to the 
sense of taste alone, which would seem to serve 
no significant function in the actual searching 
for food. 
The tactile sense becomes exceedingly im¬ 
portant at night in lieu of the non-effective 
visual and chemical senses. Inasmuch as the 
major constituent of the diet is algae, primarily 
of the ulvaceous type, and since the foraging 
area is generally adjacent to diurnal refuges, 
the tactile sense apparently meets the require¬ 
ment of food procurement. Fishermen frequently 
leave sardines and other bait materials high on 
the rocks upon the termination of their activi¬ 
ties. These food caches are frequently located 
during the nocturnal wandering of the crabs, 
notwithstanding the fact that this food is often 
deposited in supralittoral areas which are sev¬ 
eral feet higher than the normal upper horizon 
of the diurnal range. Frequently, too, these food 
caches are unnoticed although crabs often wan¬ 
der near them. It seems reasonable to assume 
that those found were located accidentally, rather 
than through the medium of a sense of smell. 
Additional information concerning the tac¬ 
tile sense was derived through observations on 
normal and blinded animals. Two crabs, one 
with opaqued eyes and one in a normal con¬ 
dition, were released on a rock IV 2 feet from 
the edge of a tide pool; the rock sloped gently 
to the pool. The normal individual ran rapidly 
to the water, submerged, and concealed itself 
under a rock. The blinded crab wandered slowly 
