338 THE CRAYFISH less. 



eyes ; paired antennary arteries (aa), going to the anten- 

 nules, antennae, green glands, &c., and sending off branches 

 to the stomach ; and paired hepatic arteries, going to the 

 digestive glands. The posterior end of the heart gives off 

 two unpaired arteries practically united at their origin, the 

 dorsal abdominal artery (oaa), which passes backwards 

 above the intestine, sending branches to it and to the dorsal 

 muscles ; and the large sternal artery (sa), which passes 

 directly downwards, indifferently to right or left of the 

 intestine, passing between the connectives uniting the third 

 and fourth thoracic ganglia, and then turns forwards and 

 runs in the sternal canal, immediately beneath the nerve- 

 cord, and sends off branches to the legs, jaws, &c. At the 

 point where the sternal artery turns forwards it gives off the 

 median ventral abdominal artery (?'. a. a), which passes 

 backwards beneath the nerve-cord, and supplies the ventral 

 muscles, pleopods, &c. 



All these arteries branch extensively in the various organs 

 they supply, becoming divided into smaller and smaller off- 

 shoots, which finally end in microscopic vessels called 

 capillaries These latter end by open mouths which com- 

 municate with the blood-sinuses, spacious cavities lying 

 among the muscles and viscera, and all communicating 

 sooner or later with the sternal sinus (Fig. 83, a, B. 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 

 thorax the sternal sinus (Fig. 88, st. s) 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 {af. br. %>). Spaces in the gill-filaments place 

 the afferent in communication with the efferent branchial 

 vein {ef. br, v). which occupies the inner side of the gill- 



