la. P- h. 



Fi(i. 15. — Astdciix furititUh. — A diagrammatic transverse section of 

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

 circulation of the blood ( x 15). urh. 12, the anterior or lower, and 

 itrli'. 12, the posterior or ujiper arthrobrancliia of the twelfth 

 somite ; (iv, afferent branchial vessel ; hcf, branchio-cardiac vein ; 

 hg, branchiostegite ; rm, extensor muscles of abdomen; cp, epi- 

 meral wall of thoracic cavity ; cv, efferent branchial vessel : fin, 

 flexor muscles of abdomen ; fp, floor of pericardium ; ;/«. 6', filth 

 thoracic ganglion ; //.heart; 7; //. hind-gut ; /r/r/, inferior abdominal 

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

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

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

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

 .<v/, sternal artery ; -w/rt, superior abdominal artery ; sc, sternal canal ; 

 t. testis ; XII.. steminn of twelfth somite. The arrows indicate 

 the direction of the blood flow. 



