59 



a small oval body formed by the thickening of the circular muscular coat. Its 

 lumen is no larger than that of the artery. An injection mass cannot be driven 

 past these "hearts" when they are contracted. Their function undoubtedly is to 

 prevent the transmission of an excessive blood pressure to the head and fins , 

 for it is evident that , during the contraction of the mantle , the viscera are 

 subjected to great pressure. If the peripheral hearts contract in a degree coordi- 

 nate with the contraction of the mantle, the blood pressure in the extrapallial 

 vessels is not disturbed even by the most violent contractions of the mantle. 



The Veins. (Plate II fig. 8). The distribution of the smaller veins is in general 

 very similar to that of the corresponding arteries and usually the smaller branches 

 of the arteries, veins, and nerves correspond closely to one another but the larger 

 venous trunks have an arrangement totally different from that of the arteries. 

 The arterial system is largely median and anterior or antero-dorsal in reference 

 to the alimentary canal ; the venous system is largely bilateral and is posterior 

 or postero-ventral to the alimentary canal. Moreover the venous system is charac- 

 terized by the great size of its vessels which form large sinuses and by the 

 specialization of a portion of its central vessels in connection with the excretory 

 organs, the nephridia. These organs are formed by the modification of the walls 

 of a large sinus which is shaped like the letter V and is enclosed in the ne- 

 phridial sac. The nephridial sinus receives the median anterior vena cava and 

 five or six paired veins : the posterior vena cava , the anterior pallial veins , the 

 splanchnic , the rectal , and in the female the nidamental veins. 



The posterior vena cava (VII Plate II figs. 8 and 9) is a large, pear-shaped 

 vessel whose larger end opens into the nephridial sinus, and which is formed 

 near the peripheral heart of the lateral pallial artery by three veins arising 

 from the fin and from the mantle behind and in front of its point of origin. 

 It crosses the lower surface of the visceral mass, on the outer side of the cor- 

 responding artery , and enters the corresponding arm of the nephridial sinus 

 near the end of the posterior aorta. 



The anterior vena cava and its tributaries. Each arm has two longitudinal 

 veins ; one in its central canal with the brachial artery and nerve and one on 

 its inner surface between the two rows of suckers. These twenty brachial veins , 

 meet in a large, thin, oval brachial sinus (7 6 ) below the pharynx. This sinus 

 opens near the dorsal end of the pharynx into the outer buccal sinus (9 6 ) which 

 surrounds the pharynx and extends from the base of the peristomial membrane, 

 from which it receives blood , to the entrance of the skull where it opens into 

 the inner buccal sinus (8 6 ). It contains the rotators of the pharynx and the 

 retractor mandibuli. The inner sinus (8 6 ) is separated from the outer by the 



