Tissues in the Crustacean Limb. 133 
The ischiopodite is an exception to the above description, for the 
muscles are not arranged to act in opposition. There are two 
muscles in this segment, but they both have their origin on the same 
side of the limb, 7. e., on the ventral side of the fourth joint (Fig. 
37, e*). One muscle is slightly larger than the other, but both are 
smaller than any other muscles of the chela. The larger muscle is 
inserted partly on the morphologically anterior side and partly on 
the dorsal surface of the exoskeleton in the proximal part of the 
ischiopodite. The smaller muscle is inserted directly on the morpho- 
logically anterior wall of the segment. Both muscles, therefore, 
appear to function as extensors. A similar arrangement of muscles 
has been described by Fredericq (’92) in the ischiopodite of the crab. 
That a similar condition exists in the crayfish is not so clear, how- 
ever, for Reed (’04) states that the ischiopodite of the cheliped in the 
crayfish possesses ‘‘both an extensor and a flexor” (p. 310). 
2. The “Breaking Joint.”—a. Exoskeleton—By the breaking 
joint (Fig. 37, bk) is meant the region in which a fusion has taken 
place between the ischiopodite and the basipodite. Since it is at 
this region that the chela is amputated in autotomy and the new limb 
begins to regenerate, a more detailed description of the breaking 
plane or joint is essential for our present purpose. 
The external appearance of the breaking joint in the adult has 
been described by Herrick (’95) as a “fine hair line leading from 
the small spur next to the articular facet on the under side, round 
the anterior border to the upper side of the joint. It then bends 
forward and abruptly backward, crossing the small proximal end of 
the joint, to near its point of departure” (p. 10). This hair line 
represents the plane of union of the basipodite and ischiopodite into 
a solid compound segment. In the first three larval stages there is a 
movable articulation between these two segments and as described 
by Herrick, “there is no true fusion of the segments until after the 
fifth stage” (p. 102). It may be questioned, however, whether there 
is any longer a functional articulation of these segments even during 
the fourth and fifth stages of development. Although externally 
the joint may present the appearance of a free articulation as indi- 
eated in Herrick’s figures, the examination of longitudinal sections 
