arlJi 



FiGr, 15. — Asfacus JiurlatUis. — A diagrammatic transverse section 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 

 arh'. 12, the posterior or upper arthrobranchia of the twelfth 

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

 hg, branchiostegite ; em, extensor muscles of abdomen ; ej), epi- 

 meral wall of thoracic cavity ; ev, efferent branchial vessel ; fm, 

 flexor muscles of abdomen ; fp, floor of pericardium ; gn. 6, fifth 

 thoracic ganglion ; k. heart ; ////.hind-gut ; /rt-r/, inferior abdominal 

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

 liver ; mj), indicates the position of the mesophragm by which the 

 sternal canal is bounded laterally ; jt?, pericardial sinus ; pdb. 12 y 

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

 *■(-/, sternal artery ; ma, superior abdominal artery ; »e, sternal canal ; 

 t, testis ; XII.. sternum of twelfth somite. The arrows indicate 

 the direction of the blood flow. 



