Tissues in the Crustacean Limb. 115 
the entire surface of the stump except at the centre, ‘where there 
is a rounded hole with a little torn tissue and blood exposed”. Reed 
states that in the lobster and the hermit crab “the opening through 
the membrane for the nerve and the blood vessel is in about the centre 
of the exposed surface when the leg is thrown off” (p. 309). In an 
earlier description of the lobster, the writer (705) also described this 
membrane as “extending almost entirely over the basipodite” and 
“perforated only at the centre by an artery, the blood sinus, and a 
large nerve” (p. 89). It was with considerable surprise, therefore, 
that it was found in the present study of serial sections of the chela 
that the above descriptions cannot, for the lobster at least, be regarded 
as strictly accurate, particularly in regard to the relation of the blood 
sinuses. 
In order to obtain a more thorough conception of these structural 
relations, graphic reconstructions of the breaking joint have been 
made from serial sections of the right chela of a fourth stage lobster. 
The tissue structures of the breaking joint at this stage are perhaps 
somewhat simpler than in the chela of an older animal, but their 
essential relations are, I believe, practically the same. It should 
also be stated that these reconstructions are in one detail not complete, 
since the connective tissue network in which the arteries and nerves 
are partly embedded as they pass along the walls of the large venous 
blood sinuses, is not represented in its entirety. Figs. 1 and 2, re- 
spectively, show the outer and inner, or morphologically posterior and 
anterior, parts of the breaking joint and adjacent segments. Tig. 3 
represents a median section between the two parts shown in Figs. 
1 and 2. The latter figures do not show quite half of each side. 
b. Blood Vessels and Nerves.—It may be observed in these figures 
that there are two nerves, two arteries, and a large venous sinus 
(divided into two separate channels) which pass through the region 
of the breaking joint. The two nerves n', n?, lie on the inner or 
medial side of the joint, the larger nerve (n') giving off a small 
branch as it leaves the basipodite. The main artery (a') passes 
along the outer wall. In the basipodite it gives off a small branch 
(a?) which takes an oblique course across toward the inner wall of 
the ischiopodite. The venous blood of the chela is carried toward 
