The Crayfish. 31 



groove into two parts— an anterior cephalic and a posterior 

 thoracic region. 



If one of the abdominal segments be examined, it will 

 be seen to consist of a ring of calcified cuticle oval in 

 section. The dorsal part of this is the tergum, the ventral 

 part the sternum; there •'is a ventro-lateral flap projecting 

 downwards, and partly concealing the limbs known as the 

 plmron. Finally, there is at the junction of the pleuron and 

 the sternum a tract of limited dimensions, termed the epimeron. 



tho 



cefi 



ail aio 



Fig. 14. — The Crayfish {Astactts Jiuviatilis), 



r, rostrum ; cef, cephalic shield ; tho, thorax ; cg^, cervical groove, between head and 

 thorax ; ab^ abdomen ; br, branchiostegite ; an, antennee ; a 9-T4> thoracic appendages. 



This last lies just above the joints for the articulation of the 

 limb on each side. 



In the cephalo-thoracic region precisely the same parts can 

 be distinguished, but their several proportions are different. 

 A section through the thorax is shown in Fig. 25, p. 48. The 

 tergum and the sternum are more limited ; while the pleura 

 are great laterally descending plates, forming a covering for 

 the subjacent gills, and are hence termed the branchiostegites ; 

 finally, the epimera are long plates of thinnish and imperfectly 

 calcified membrane. In addition, this region of the body 

 possesses what is called the endosternal skeleton ; this is formed 

 of ingrowths of both the sterna and the epimera ; the nervous 

 system is sheltered by the framework thus formed. 



