Biology of Pachygrapsus crassipes —Hi ATT 
159 
ically, the casting off of the old apodemes is 
facilitated because they are generally directed 
toward the basal portion of the appendage; 
therefore, the withdrawal of the new appendages 
presents no special difficulty. A comparison 
between the apodemes of a skeleton and those 
of an exuvia shows that those apodemes not 
directed posteriorly are either resorbed in stages 
D 3 and D 4 or are secondarily oriented in a 
posterior direction after the sternal schism. 
The apodemes which partition the two sides 
of the thorax offer more difficulty to successful 
exuviation than those of the pereiopods. They 
consist of invaginations of the endosternites and 
pleural walls and serve as the attachment for 
the origins of the muscles of the basipodites 
and coxopodites. It was stated above that when 
the body is elevated above the old sternal floor, 
this elevation, in conjunction with anteropos¬ 
terior elongation, serves to disengage the apo¬ 
demes which are directed posteriorly after the 
sternal schism. A comparison of the endo- 
skeletal structure of a normal animal with that 
of an exuvia shows that the normal structure 
has been altered during ecdysis. This altera¬ 
tion is manifested in partial or total resorption 
in certain endoskeletal areas. Without this re¬ 
sorption, molting would not be possible because 
the transverse apodemes together with their 
ramifications would hinder the withdrawal of 
tissues without injury. The occurrence of the 
sternal schism in P. crassipes serves to reduce 
effectively the degree of resorption which would 
otherwise be requisite providing the sternum 
remained entire. 
The structural association of the phyllobran- 
chiate gills with the proximal portion of the 
gill-bearing thoracic appendages makes it im¬ 
perative that the withdrawal of the gills coincide 
with the onset of the active phase of ecdysis. 
Inasmuch as the brachyuran gill is composed 
of two series of lamellae inserted on a median 
raphe, the lamellae must flatten and be drawn 
through the median raphe over the orifice of 
insertion on the skeleton. The branchial aper¬ 
tures of the skeleton are small, but the edges 
are rounded so that the gills pass through them 
without great difficulty. Subsequent to the com¬ 
plete withdrawal of the new gill from its old 
integument, it remains wedged between the 
old and new epimeral walls until the body is 
elevated to a position above the old exuvia and 
until the proximal podomeres are moved to the 
upper level of the old pleural wall. It is evident 
that the abnormal torsion undergone by the 
gills during ecdysis might conceivably provide 
respiratory difficulties which would designate 
ecdysis as a critical interval in the life of the 
animals. Supportive evidence for the above 
statement was secured upon the compilation of 
mortality data for crabs confined to the labora¬ 
tory. Of a total of 78 fatalities among laboratory 
animals, 61 (78.2 per cent) occurred prior to 
complete withdrawal of the pereiopods. There¬ 
fore, these deaths took place when the gill 
lamellae were in an abnormal, contorted con¬ 
dition. 
One of the features of ecdysis which invites 
discussion is the origin of the mucilaginous 
material which covers the inner surface of the 
exuvia and the external surface of the new 
exoskeleton. Its apparent function of lubrica¬ 
tion of the frictional surfaces during ecdysis is 
manifestly significant. Its initial appearance in 
P. crassipes occurs when the rift between the 
posterior edge of the carapace and the first 
abdominal tergite shows a tenuous membrane 
stretched across it. With further separation be¬ 
tween the carapace and the first abdominal ter¬ 
gite the membrane fractures. There is little 
doubt that this mucilaginous layer is probably 
the same as that first described by Reaumur 
(1712), who recognized its apparent function. 
Vitzou (1882) suggested that the mucilage 
was secreted by the epithelium, in which case 
it would have had to penetrate the two new 
integumental strata prior to reaching its defini¬ 
tive location between the old and new integu¬ 
ments. Herrick (1896) subsequently proved 
Vitzou’s contention erroneous because this layer 
bore the impress of a mosaic of cells, and, in 
view of its cellular composition, postulated 
that it was either the first secreted portion of the 
new integument or the internal membranous 
