160 
PACIFIC SCIENCE, Vol. II, July, 1948 
stratum of the old. Yonge (1932) provided 
proof to substantiate the second possibility set 
forth by Herrick. Furthermore, he asserted that 
the internal membranous layer was dissolved 
by the action of cells which passed into it from 
the epithelium prior to and during ecdysis. 
Drach (1939) agreed with Yonge concerning 
the stratum involved but found no basis for 
the mode of gelatinization suggested by that 
investigator. After a meticulous histological 
study of the internal membranous layer during 
stages D 4 to D 4 , Drach found that the lympho¬ 
cytes were irregularly grouped and insufficient 
in numbers to effect a uniform transformation 
of this stratum into a jellified condition. He 
maintained, moreover, that the homogeneity of 
this jellification presupposed the intervention 
of some chemical agent secreted by the epithelial 
cells. In the Insecta an enzyme, chitinase, causes 
an analogous resorbing action. Wigglesworth 
(1933) stated that chitinase is a glandular secre¬ 
tion of the epidermis; therefore, it is not un¬ 
reasonable to assume that in the Crustacea a 
comparable enzyme is secreted by epithelial 
cells or glands which induces the gelatinization 
so significant to the success of exuviation. 
The relatively high exuvial frequency coupled 
with the extreme infrequency of observations 
of the molting act seem to present at least two 
implications. First, ecdysis must occur very 
rapidly, or second, ecdysis must take place 
during times when the animals are not under 
observation. It is the writer’s opinion that the 
initial hypothesis may be discounted (see p. 
155). The latter supposition, however, is ade¬ 
quately supported by data compiled from ani¬ 
mals confined to laboratory aquaria. Of the 141 
molts which occurred among captive crabs, 
all but three took place between 8:00 P.M. and 
5:00 A.M.; thus, 97.9 per cent of the molts 
occurred during nocturnal hours. Broekhuysen 
(1941) and MacKay (1942) also noted that 
most of their captive specimens underwent 
exuviation at night. Early suggestions which 
coupled molting with moon and tidal effects 
are contradictory, devoid of evidence, and un¬ 
tenable in the present investigation; therefore, 
these conjectural causes are discarded forthwith 
in favor of the evidence presented below. It has 
been pointed out previously that crabs in stages 
A 1 and A 2 are extremely susceptible to injury 
and possible fatality if attacked by predaceous 
species during the first 6 to 10 post-exuvial 
hours. The reduced vision of littoral predators, 
including cannibalistic P. crassipes and the ubi¬ 
quitous Norway rat (Rattus norvegicus ), is cer¬ 
tainly advantageous to the comparatively de¬ 
fenseless, recently molted crabs. The several noc¬ 
turnal hours following the molt undoubtedly 
provide substantial, but quantitatively unknown, 
protection for this species. Moreover, the ele¬ 
vated littoral position preferred by P. crassipes 
markedly exposes them to the predation of 
numerous gulls. Therefore, if molting were 
undergone during the day to any appreciable 
extent, the mandatory position in the tide 
pools, at least for a portion of the early, post- 
exuvial hours, would place these crabs in certain 
danger which would be magnified because of 
the subnormal locomotory powers of the soft 
crabs. 
Although considerable variability occurs in 
the time required for maximum post-exuvial 
expansion, an indication of typical expansion 
with respect to the time intervals involved may 
be secured from Figure 6, which illustrates these 
data derived from three distinct sizes of crabs 
selected to emphasize the comparative aspect. 
Pre-exuvial carapace breadths were: crab A, 6.3 
millimeters; crab B, 25.0 millimeters; and crab 
C, 35.8 millimeters. The first post-exuvial 
measurement of crab A was made immediately 
after the molt and revealed a 4.7 per cent size 
increment with respect to initial width at that 
time. Crabs B and C were not measured until 
2 hours after the completion of ecdysis. Regard¬ 
less of the initial carapace width, the greater 
part of post-exuvial expansion occurs prior to 
the third hour after ecdysis. Actually, consider¬ 
able expansion occurs before exuviation is com¬ 
pleted, because swelling of the body accom¬ 
panies the active phase of exuviation. However, 
the expansion is not measurable until ecdysis 
terminates. 
