460 SANGUIFEROUS SYSTEM. 



arteries, which extend backwards ramifying along the anterior 

 edge of the mantle, and give off branches to the anterior 

 adductor muscle, (134. i. i.) and to the sensitive and vascular 

 apparatus around the mouth. The distribution of the ante- 

 rior aorta is nearly the same where it arises by a single 

 trunk from a single median ventricle, as here where it has a 

 double origin, and the same observation applies to the 

 general distribution of the posterior aorta, even when it ori- 

 ginates as a mere branch from the anterior or single systemic 

 aorta, as in the oyster. 



The posterior aorta most frequently arises by a single 

 trunk from the posterior or anal extremity of a lengthened 

 median fusiform ventricle, and continues backwards along 

 the median plain, above the intestine, to near the anus or 

 the vent, where it bifurcates to form the two great posterior 

 palleal arteries, which follow the course of the posterior and 

 inferior margins of the mantle. In the bicordate forms of 

 this class, as the area nose, the posterior, like the anterior 

 aorta, commences by a double origin from the right and left 

 bulbus arteriosus, (134. e. e.), and the two trunks soon anas- 

 tomose to form the usual median posterior dorsal artery (134. 

 ff.) In its course towards the anus along the middle of the 

 back, the posterior aorta gives off numerous small lateral 

 branches, (134. ^7.) like intercostal^, which spread chiefly on 

 the muscular parts of this portion of the abdomen, and 

 proceeding, with the colon, over the great posterior adductor 

 muscle, which it also supplies, it divides above the anus 

 (134. /.) into the right and left posterior palleal arteries 

 (134. h. h.) Near to their origin, the posterior palleal arteries 

 give off each an anal branch, (134. k. k.) which two branches 

 encompass and supply the rectum and continue their course, ra- 

 mifying along the curvature of the posterior adductor muscle. 

 The great palleal trunks (134. h. h.) extend along the poste- 

 rior and inferior margins of the mantle, sending off numerous 

 branches to its appendices, and ramifying extensively over 

 that expanded and ciliated covering of the respiratory cavity. 

 From the capillaries of this almost respiratory membrane, 

 the blood is collected by the palleal veins, (134. a. a. 120. B. 

 /.) and transmitted directly to the auricle, (134. c. c. 320. B. 

 h.) without passing through the gills with the venous blood 

 of the system. The systemic veins follow nearly the course 



