PHYLUM ARTHROPODA 



225 



cavities lying among the muscles and viscera, and all com- 

 municating, mediately or immediately, with the sternal sinus 

 (st. s), a great median canal running longitudinally along the 

 thorax and abdomen, and containing the ventral nerve-cord 

 and the sternal and ventral abdominal arteries. In the 



FIG. 127. Transverse section of thorax of crayfish, diagrammatic, abm, ventral 

 abdominal muscles; bf, leg; bm, ventral nerve-cord; d, intestine; dbm, dorsal 

 muscles of abdomen; <?/, wall of thorax; h, heart; k, gills; kd, gill-cover; 

 /, liver; ov, ovary; pc, pericardial sinus; sa. sn, sternal artery; vs, ventral 

 sinus. The arrow shows the direction of the blood-current. (From Lang's 

 Comparative Anatomy} 



thorax the sternal sinus sends an offshoot to each gill in the 

 form of a well-defined vessel, which passes up the outer 

 side of the gill and is called the afferent branchial vein 

 (a/, br. v\ see also Fig. 127). Spaces in the gill-filaments 

 Q 



