vn CIRCULATORY ORGANS 377 



nerve-cord, as. a ventral thoracic artery sending off 

 branches to the legs, jaws, &c. At the point where the 

 sternal artery turns forward it also gives off the median 

 ventral abdominal artery (Fig. 93, 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 

 capillaries (p. 95). These latter end by open mouths, 

 which communicate with the blood-sinuses spacious 

 cavities lying among the muscles and viscera, and all 

 communicating, sooner or later, with the sternal sinus, 

 a great median canal running longitudinally along the 

 thorax and abdomen, and containing the ventral nerve- 

 cord and the ventral thoracic and abdominal arteries. 

 In the thorax the sternal sinus (Fig. 94, vs, and Fig. 96, 

 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. v). 

 Spaces in the gill-filaments place the afferent in com- 

 munication with the efferent branchial rein (ef. br. v.), 

 which occupies the inner side of the gill-stem. The 

 eighteen efferent branchial veins open into six branchio- 

 cardiac veins (br. c. v.} which pass dorsally in close contact 

 with the lateral wall of the thorax and open into the 

 pericardial sinus, some of them uniting before doing so. 



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

 and the heart 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 by the valves at their origins, and the 



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