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Fig. 15. — Asfacns ffuv'i/iiUis. — A dingrammatic transverse ?ection of 

 the thorax through the twelfth somite, showing the course of the 

 circulation of the blood ( x 3). arh. 12, the anterior or lower, and 

 arb'. 12, the posterior or upper arthr. branchia of the twelfth 

 somite ; nr, afferent branchial vessel ; her, branchio-cardiac vein ; 

 ht), brauchiostegite ; em. extensor muscles of abdomen; cjK epi- 

 meral wall of thoracic cavity ; cr. efferent branchial vessel ; fm, 

 flexor muscles of abdomen ; fp, floor of pericardiiim ; gn. 0, fifth 

 thoracic ganglion ; //.heart; ////.hind-gut; ;"?/«, inferior abdominal 

 artery, in cross section ; la, lateral valvular apertures of heart ; Ir, 

 liver : inj). indicates the position of the mesophragm by which the 

 sternal canal is bounded laterally ; p. pericardial sinus; pdb. 12, 

 podobranchia, and jilh. 12, pleurobranchia of the twelfth somite ; 

 5-rt,sternal artery ; xaii, superior abdominal artery ; .sr, sternal canal ; 

 f . testis ; xii.. sternum of twelfth somite. The ai-rows indicate 

 the direction of the blood flow. 



