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 

 he at the inner, and the latter at the outer ends of the 

 partitions or arthrophragms (fig. 39, 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. 



