Biology of Pachygrapsus crassipes —HlATT 
189 
epimeron, and the wide arthrodial membrane 
between the coxa and ischium was removed 
(Fig. 16). By inserting forceps through the 
epimeral window the muscles inserting thereon 
could be grasped, and by tugging on each muscle 
it was possible to determine which was respon¬ 
sible for elevating the ischium—-the anterior 
levator basis. The posterior levator basis, which 
inserts on the posterodorsal edge of the basis, 
CO. 
Fig. 16. The basal podomeres and autotomizer 
muscle of the left fifth pereiopod of P. crassipes. The 
integument on the dorsal side of the coxa and the 
arthrodial membrane between the coxa and basis have 
been removed to show the insertion of the anterior 
levator basis. A.L.B., anterior levator basis'. ; BA., basis; 
CO., coxa; F.P., fracture plane; IS., ischium; M., merus; 
X, cut edge of coxal integument; Y, tendon of anterior 
levator basis. 
is short and arises from the proximal edge of 
the coxa, hence it is not easily grasped through 
the epimeral window. This leaves only the 
anterior levator basis available for such an 
operation. By grasping the tendon (the fibers 
themselves tear too easily) with the forceps 
and holding a finger on the upper side of the 
merus to serve as the resistance, it was possible 
to simulate autotilly by pulling on the tendon. 
The break occurred at the fracture plane and 
proceeded with a minimum of effort. When 
the appendage was pulled or twisted without 
also pulling on the autotomizer muscle, breaks 
occurred at the arthrodial membranes between 
the podomeres. The foregoing observations 
serve to emphasize two significant points con¬ 
cerning mutilation: (1) the weakest point in 
the leg is not the fracture plane, and (2) the 
autotomizer muscle must be stimulated to con¬ 
tract before self-amputation can occur. Fred- 
ericq’s application of a resistance (some force 
other than the crab’s own body; e.g., placing a 
finger on the dorsal edge of the merus) influ¬ 
enced him to believe that some external force 
was indispensable, and it is the type of amputa¬ 
tion which he described as autotomie. It has 
been shown above that this type of amputation 
is more aptly designated autotilly. Later work¬ 
ers discovered that the appendages could be 
severed without any form of external resistance. 
Amputated appendages were commonly 
found in the aquaria if the water was stagnant 
or contaminated with decaying food. Inasmuch 
as no external agents were available, the crab 
must have shed its appendage or appendages 
by autotilly or autotomy. P. crassipes does em¬ 
ploy autotilly and it is, perhaps, the method 
most generally used. Autotilly can be readily 
demonstrated by injuring the distal podomeres 
(with the exception of the dactylus). The limb 
anterior to the mutilated one is moved pos¬ 
teriorly and over the injured member; the 
injured member is straightened and moved 
upward while the anterior limb provides the 
resistance. The leg breaks at the fracture plane 
and autotilly occurs. It is accomplished hastily, 
the entire procedure occupying less than 2 
seconds in healthy individuals. 
Figure 15 illustrates the method whereby 
the morphological features of the coxa and 
basi-ischium, together with the autotomizer 
muscle, are alone responsible for the mechanics 
of autotomy. Autotomy may also be demon¬ 
strated on dead or preserved animals, although 
it is difficult to perform. Among living animals 
this method is perhaps the least utilized and 
has never been observed in this species; how¬ 
ever, considerable circumstantial evidence set 
forth below serves to denote its presence. The 
procedure follows: The autotomizer muscle con¬ 
tracts, forcing the appendage dorsally; the basis, 
of less diameter than the coxa, slips inside the 
latter structure, allowing protuberance Y on the 
ischium to contact protuberance X on the coxa. 
The meeting of these two structures provides 
the resistance required to sever the limb when 
the autotomizer muscle is further contracted. 
The fracture originates at the dorsal side and 
travels ventrally. 
