fp- 



gn.6. 



FlG. 15. AstacH* Jiuvlatilis.A diagrammatic transverse section of 

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

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

 arV. 12, the posterior or upper arthrobranchia of the twelfth 

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

 bff, branchiostegite ; em., extensor muscles of abdomen ; fp, epi- 

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

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

 thoracic ganglion ; h. heart ; Jig, hind-gut ; ma, inferior abdominal 

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

 liver ; mp, indicates the position of the mesophragm by which the 

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

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

 m,sternal artery ; saa, superior abdominal artery ; sc, sternal canal ; 

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

 the direction of the blood flow. 



