vii CIRCULATORY ORGANS 377 



the 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 back- 

 wards 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 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 sternal and ventral abdominal arteries. 

 Mn 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 (of. br. v). Spaces in the gill- 

 filaments place the afferent in communication with the 

 efferent branchial vein (ef. br. v\ which ocupies 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, 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 peri- 

 cardial 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 



