XI 



PHYLUM ARTHROPODA 



553 



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 extends 

 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, sending off branches 

 to the legs, jaws, &c. At the point where the sternal artery turns 

 forwards it gives off the \^ 



median ventral abdominal 

 artery (uaa.), 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 offshoots, which finally 

 end in microscopic vessels 

 called capillaries. These latter 

 end by open mouths which 

 communicate with the blood- 

 sinuses (Fig. 439, s.), spacious 

 cavities lying among the 

 muscles and viscera, and all 

 communicating, mediately or 

 immediately, with the sternal 

 sinus (st.s.), a great median 

 canal running longitudinally 

 along the thorax and ab- 

 domen, and containing the 

 ventral nerve-cord and the 

 sternal and ventral abdom- 

 inal 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 outer side of 

 the gill and is called the afferent branchial vein (af.br.v. ; see also 

 Fig. 438). Spaces in the gill-filaments place the afferent in com- 

 munication 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 branchiocardiac veins (br.c.v.), which pass 

 dorsally in close contact with the lateral wall of the thorax 

 and open into the pericardial sinus (pcd.s.). 



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

 heart is rhythmically contractile. When it contracts, the blood 



vol. i N N 2* 



Fio. 488. — Transverse section of thorax of Cray- 

 fish, diagrammatic, abm. ventral abdominal 

 muscles ; bj\ leg ; bm. ventral nerve cord ; </. 

 intestine ; dirm . dorsal muscles of abdomen ; 

 ep. wall of thorax ; h. heart ; k. gills ; *•</. gill- 

 cover ; I. liver ; ov. ovary ; pc. pericardial 

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

 The arrows show the direction of the blood- 

 current. (From Lang's Comparative Anatomy.) 



