158 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
mere infoldings of the cuticle, or apodemes ; and, as such, 
they are shed along with the other cuticular structures 
during the process of ecdysis. 
Without entering into unnecessary details, the gene- 
ral principle of the construction of the endophragmal 
skeleton may be stated as follows. Four apodemes are 
developed between every two somites, and as every 
apodeme is a fold of the cuticle, it follows that the 
anterior wall of each belongs to the somite in front, and 
the posterior wall to the somite behind. All four apodemes 
lie in the ventral half of the somite and form a single 
transverse series ; consequently there are two nearer 
the middle line, which are termed the endosternites, and 
two further off, which are the endopleurites. The former 
lie at the inner, and the latter at the outer ends of the 
partitions or arthrophragms (fig. 89, A, a, a’, fig. 42, aph), 
between the articular cavities for the basal joints of the 
limbs, and they spring partly from the latter and partly 
from the sternum and the epimera respectively. 
The endosternite (fig. 42, ens.) ascends vertically, with a 
slight inclination forwards, and its summit narrows and 
assumes the form of a pillar, with a flat, transversely 
elongated capital. , The inner prolongation of the capital 
is called the mesophragm (mph.), the outer the paraphragm 
(pph.). The mesophragms of the two endosternites of a 
somite usually unite by a median suture, and thus form 
a complete arch over the sternal canal (s.c.), which lies 
between the endosternites. 
