XI 



PHYLUM ARTHROPODA 



511 



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 (itaa.), 

 which passes backwards beneath the nerve-cord, and supplies th* 

 ventral muscles, pleopods, &c. 



All these arteries branch extensively in the various organs they 

 supply, becoming divided into smaller and smaller offshoots, which 

 finally end in microscopic 

 vessels called capillaries. 

 These latter end by open 

 mouths which communicate 

 with the Uood-sinuscs (Fig. 

 405, s.), spacious cavities lying 

 among the muscles and vis- 

 cera, and all communicating, 

 mediately or immediately, 

 with the sternal sinus (st.s.), 

 a great median canal run- 

 ning 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 sends an offshoot to 

 each gill in the form of a 

 well-defined vessel, which 

 passes up the ou^er side of 

 the gill and is called the 

 afferent branchial vein (af.br. v.] 

 see also Fig. 404). , 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- 

 stem. The eighteen efferent branchial veins open into six 

 (>j-anchio-cardiac veins (br.c.v.), which pass dorsally in close contact 

 with th<- lateral wall of the thorax and open into the pericardial 



H'nus (pcd.s.). 

 The whole of this system of cavities is full of blood, and the 

 sart is rhythmically contractile. When it contracts, the blood 

 contained in it is prevented from entering the pericardial sinus by 

 the closure of the valves of the ostia, and therefore takes the only 

 other course open to it, viz., into the arteries. When the heart 

 relaxes, the blood in the arteries is prevented from regurgitating 



FIG. 404. Transverse section of thorax of Cray- 

 fish, diagrammatic, abm. ventral abdominal 

 muscles; bf, leg; bui, ventral nerve cord; </, 

 intestine ; dbm. dorsal muscles of abdomen ; 

 ep, wall of thorax ; h. heart ; k, gills ; M, gill- 

 cover ; /. liver; or. ovary ;' j)c. pericardial 

 sinus ; sa. sn, sternal artery ; vs. ventral sinus. 

 The arrow shows the direction of the blood - 

 current. (From Lang's Comparative Anatomy.) 



