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, adm, ventral 



abdominal muscles; if, leg; bm, ventral nerve-cord; d, intestine; dim, dorsal 

 muscles of abdomen; ep. wall of thorax; /;, 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 A natomy. ) 



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 

 {of. br. v; see also Fig. 127). Spaces in the gill-filaments 

 Q 



