158 
PACIFIC SCIENCE, Vol. II, July, 1948 
results in complete consecutive withdrawal of 
all pereiopods from the fifth pair to the chelae. 
Special difficulties in withdrawal are presented 
by the chelae because of the enlarged distal 
podomeres. Special lines of absorption on the 
coxa, basi-ischium, and merus have been men¬ 
tioned previously (Fig. 3, R.). This resorbed 
line fractures to release a roughly triangular 
flap of integument. This flap is hinged at the 
junction between the merus and basi-ischium 
and the anterior portion of the line of absorp¬ 
tion on the merus. The lines of absorption on 
the basi-ischium and coxa fail to fracture but 
become very soft and pliable. 
The frequency with which the animals under¬ 
going ecdysis lose one or more limbs illustrates 
the critical nature of exuviation in the general 
welfare of the individual. A cursory examina¬ 
tion of several exuviae which contained chelae 
of crabs unable to withdraw them from the old 
exoskeleton revealed that the lines of absorp¬ 
tion were hard, with resorption insufficiently 
advanced to effect the fracture, which indicated 
that resorption of calcium salts in the old integu¬ 
ment was apparently impeded in some manner. 
The severence of appendages through the in¬ 
ability to withdraw them successfully has been 
recognized previously and designated "exuvial 
mutilation”; however, a more precise expression, 
'exuvial autotomy,” suggested by Drach (1939) 
seems more appropriate. 
After the successful withdrawal of the pos¬ 
terior four pairs of appendages, the old sternal 
integument is separated from the new. An 
elongation of the body, together with an upward 
and backward movement of the crab, combines 
to exert the necessary pull on the abdomen, 
which is subsequently withdrawn. The limbs 
are then directed forward. An ejection of 
amber fluid from the mouth occurs, after which 
a few seconds of quiescence ensues. After this 
brief period of inactivity, rapid movements of 
the limbs occur in a forward-backward motion. 
This activity is repeated two or three times, 
and serves to remove the buccal appendages 
and the chelae. The amber ejection is probably 
the material in the stomach which must be 
expelled with the old epithelium. The chelae 
are decidedly wrinkled as they are exsheathed 
because of the forced expulsion of the haemo- 
coeiic fluid to enable the bulbous distal end to 
be withdrawn through the small coxal opening. 
However, they become turgid immediately after 
ecdysis. 
An examination of the exuvia shows that 
the apodemes and branchial filaments are pre¬ 
served in approximately their original morpho¬ 
logical position. Since it is not possible to 
observe the exsheathing of the apodemes and 
gills this phenomenon must be interpreted 
through critical study of the morphological 
characteristics of the exoskeleton. 
The apodemes are invaginations of the in¬ 
tegument, each lamina being formed by two 
thicknesses of sclerotized integument united by 
their morphologically external surfaces. Prior 
to the impending molt new strata are secreted 
and these strata (the epicuticle and pigmented 
layer) of the new integument lie internal to 
the old integumental strata of the apodemes 
(Fig. 5); during the withdrawal of the ap¬ 
pendages the new apodeme is slipped off the old, 
which is rigidly attached to the exuvia. Mechan- 
Fig. 5. A diagrammatic section through an apodeme 
to show the integumental strata. The arrow indicates 
the direction of withdrawal at ecdysis. E., epicuticle; 
E/, new epicuticle; M.L., membranous layer; P.L., pig¬ 
mented layer; P.L/, new pigmented layer; PR.L., prin¬ 
cipal layer. 
