1890.] Autotomy in the Crab. 141 
decalcified in Perenyi’s fluid, it readily separates there just as in 
autotomy. 
Sections of this region of the second segment show that the 
exoskeleton presents a definite plane of discontinuity in the lam- 
ellar structure at the proximal groove ( , Fig. 2), this plane being 
at right angles to the length of the limb, but bending abruptly 
in the outer part of the exoskeleton, so that after autotomy the 
exoskeleton of the stump of the limb is somewhat rounded off at 
its outer edge (x, Fig. 3.) This plane of discontinuity is quite 
different from the pores that penetrate the exoskeleton, and con- 
nect with the peculiar scale-like “ hairs” (4, Fig. 3.) Extending 
from this exoskeleton part of the “plane of rupture,” there is a 
definite structure in the soft part of the limb forming a double 
annular curtain ( 2, r, Fig. 4), extending inward from the epidermis 
to the central nerve and blood vessels (# and 4), and thus divid- 
ing the cavity of the second segment into a proximal and a 
distal part,—nearly separated from one another. This curtain is 
a membranous structure of which the distal part is more delicate, 
the proximal stouter and of a double nature. This proximal 
membrane is seen on section to remain upon the stump of the 
leg, as the conspicuous membrane mentioned above, after normal 
autotomy. 
There is thus a definite “ plane of rupture,” or preformed me- 
chanism consisting of a modification (7) in the exoskeleton (ex), 
and of a membranous ingrowth ({, 7), which together account 
for the peculiar surface presented after autotomy takes place. 
The explanation of the gradual acquisition by the crabs of this 
highly complex and perfected form of autotomy by natural 
selection presents difficulties which may, I judge, be lessened if 
we can show reasons for supposing that the mechanisms involved 
have their homologues in other animals, and have risen to their 
perfect expression in the crabs in connection with change of 
function. 
Leon Fredericq has shown that the chelz are thrown off easily, 
by autotomy, in the craw-fish, while the other legs are either 
thrown off with difficulty, in the lobster, or not at all, in his ex- 
periments on the crayfish. Examinations of these limbs showed 

