la.P- T>- 



Fig. 15. — Astaoim fluriatilis. — A diagrammatic transverse section of 

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

 circulation of the blood { x 3). orb. IS, the anterior or lower, and 

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

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

 bg, branohiostegite ; em,, extensor muscles of abdomen ; ep, epi- 

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

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

 thoracic ganglion ; /i, heart ; A^, hind-gut; iaa, 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 ; 2>^^- 1^< 

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

 »a,8ternal artery ; aaa, superior abdominal artery ; sc, sternal canal ; 

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

 the direction of the blood flow. 



