Biology of Pacbygrapsus crassipes —Hi ATT 
157 
schism appear in the literature. Pearson (1908, 
on Cancer pagurus) and Hay (1905, on Colli- 
nectes sapidus) contend that the epimeral frac¬ 
ture appears first at the anterior end of the 
groove near the oral region and extends pos¬ 
tered to connect each end of the gap between 
the posterior edge of the elevated carapace and 
the first abdominal tergite. Churchill (1918, 
on C. sapidus) refutes Hay and suggests that 
the converse is true. This study on P. crassipes 
definitely corroborates Churchill’s findings. 
Many of the crabs which succumbed during the 
passive phase of the molt show the fracture in 
several stages of its splitting process; each frac¬ 
ture is oriented from the posterior to the anterior 
direction. Indeed, one may gently elevate the 
carapace and observe the actual splitting an¬ 
teriorly. 
Resorption of lime salts from the sternal 
integument is not apparent externally until the 
onset of the active phase of ecdysis. A split 
between the fused post-oral cephalothoracic and 
the fourth thoracic sterna, together with a schism 
along the mid-sternal line at the junction of the 
right and left sterna, is visible at the time the 
limbs are withdrawn. Slight pressure on these 
desclerotized areas prior to the molt likewise 
serves to detect the impending molting act. 
When the elevated carapace reaches an angle 
of approximately 30° as a result of increased 
body expansion, the pereiopods are moved 
slightly, simulating walking movements, and a 
depression appears to one side of the cardiac 
area in the new integument. This initial depres¬ 
sion disappears, but concurrently a new depres¬ 
sion is developed on the opposite side of the 
cardiac area; each individual depression persists 
approximately 10 seconds and recurs rhyth¬ 
mically for a period of about 7 minutes in an 
average-sized female crab of 24.6 millimeters 
in width. A smaller crab with a carapace 
breadth of 7.2 millimeters was observed to com¬ 
plete its active phase of exuviation in 3 minutes 
and 10 seconds. Drach (1939) reports that the 
pulsations in M. squinado vary from 10 to 15 
minutes. He further states that the alternating 
depressions are the result of muscle contraction 
in the coxopodites and basipodites of the 
thoracic appendages, and that these particular 
muscular movements are the principal ones 
occurring during the active phase. The muscles 
mentioned take origin on the endopleurites and 
endosternites, neither of which has appreciable 
rigidity at ecdysis; consequently, these newly 
formed skeletal elements offer less resistance 
to the contraction of the muscles than their 
insertions on the new integument of the coxo¬ 
podites and basipodites which are still en- 
sheathed by the old integument. Inasmuch as 
the endoskeleton and exoskeleton are continuous 
integumental structures, one would expect that 
particular part of the carapace (cardiac area) 
to be moved concurrently with the adjacent 
pleural wall. Since all of the muscles inserting 
on the coxa and basis of the various pereiopods 
take their origins in the same area of the pleural 
wall, and since the pleural wall is bound to 
the cardiac area of the carapace by a non- 
sclerotized portion of the integument, it is 
obvious that the depression occurring in the 
cardiac area during ecdysis will take place in 
that area only regardless of which legs are 
being withdrawn from the old integument. The 
hydrostatic pressure within the haemocoelic 
cavity forces the depression to disappear when 
the muscles on that particular side become 
quiescent. 
The resultant accomplishment of these pul¬ 
sations mentioned above is the withdrawal of 
the fourth and fifth pereiopods from the old 
integument. When the coxae and basipodites 
of the remaining legs are withdrawn, a rotation 
of the freed segments occurs in a manner sug¬ 
gestive of an attempt to release the distal podo- 
meres from their sheaths. As the podomeres 
are successively withdrawn they quickly become 
distended with haemocoelic fluid and are thereby 
stiffened to assist in exerting a tug on those 
remaining in the old sheath. 
The fifth pair of walking legs is the first 
pair to be withdrawn because withdrawal activ¬ 
ity starts there, and also, perhaps, because they 
are the shortest pair. However, all undergo 
simultaneous exuvial progress which ultimately 
