254 ELEMENTS OF COMPARATIVE ZOOLOGY. 



receives a branchial vein, coming from the gill; while in 

 front it gives off an anterior aorta, which runs forward. 



Look on the side of the gill nearest the mantle and see 

 the branchial artery. Trace it towards the middle line 

 and find the branchial heart, just behind the branchial 

 vein. This receives the blood from the postcavae already 

 noticed, and also from a precava which comes from in 

 front through the kidney, but is not so easily traced. The 

 course of the circulation may be briefly described as fol- 

 lows: The blood is forced to all parts of the body by the 

 systemic heart. After supplying these regions it collects 

 in the pre- and postcavas and is brought to the branchial 

 hearts, which pump it through the branchial arteries to 

 the gills. From the gills it returns to the systemic heart 

 by way of the branchial vein to repeat its circuit. Sketch 

 all parts made out. 



Carefully trace the intestine backwards from the vent, 

 removing the systemic heart and the remains of the kid- 

 neys. Just behind the level of the systemic heart it will 

 be found to enter the thick- walled, muscular stomach. 

 This stomach gives off, behind, a large, thin-walled blind 

 sac, which extends far back into the body mass. Close to 

 where the intestine leaves the stomach the oesophagus 

 enters it. Trace the oesophagus forward to the region of 

 the neck, ~but not farther at present. In its course it can 

 be followed through the liver. Sketch the alimentary 

 tract as if viewed from the side, inserting intestine, ink- 

 sac, stomach, blind sac, liver, and oesophagus, leaving 

 room for the anterior end of the latter to be inserted later. 



With a single stroke of a sharp scalpel split the head 

 longitudinally, making the cut as nearly as possible in the 

 median plane. In the section thus made the anterior end 



