MORPHOLOGY OF THE HELICINIDiE. 775 



obtain by reconstruction of sections. The ventricle is continued 

 forward into a short and wide aorta, which immediately after 

 passing through the pericardial wall — which it does at about the 

 level of the hinder end of the left columellar muscle — divides 

 into three principal branches. The one passes to the right 

 towards the stomach, and, entering the visceral mass, divides 

 into a number of branches which are distributed to the stomach, 

 the intestine, the right lobe of the liver, the gonad, and the 

 hypobranchial gland and gonaducts. The left branch runs 

 forward for a short distance and then turns downwaxxl into the 

 visceral mass and chiefly supplies the left lobe of the liver. 

 A branch is directed towards the posterior part of the left 

 columellar muscle. The third vessel is an almost direct forward 

 continuation of the aorta and runs up in the dorsal region of the 

 body towards the resophagus ; it passes above this organ and runs 

 over the surface of the radular sac, to which organ it becomes 

 firmly attached at about the level of the hind end of the 

 pharyngeal bulb. Here it passes into a number of lacunar 

 passages, supplying the pharyngeal bulb and the oesophageal 

 pouches, and eventually makes communication with the large 

 blood-space surrounding the nerve-centres of the head. This 

 space in turn communicates freely with the lacunfe surrounding 

 the pedal nerve-chords. 



In this labyrinth of blood-channels I have not been able to 

 recognize those by which the blood is collected and brought back 

 from the various organs to the organ of respiration, the mantle. 

 The principal hfemocoelic spaces or blood-sinuses are the 

 following: (1) a pedal sinus, surrounding the pedal nerve-cords ; 

 (2) a suboesophageal sinus, undei-lying the buccal mass and 

 oesophagus ; this is continued back into (3) a circum-intestinal 

 sinus, in which lie the coils of the intestine, and the posterior 

 pai't of the radular sac ; (4) a peri-gastric sinus surrounding the 

 stomach ; (5) a i-ecto-genital sinus, running the whole length of 

 the rectum and gonaduct. The last named is evidently the 

 pulmonary vein of Isenkrahe (" langs des Darmes zieht sich die 

 Lungenvene hin "), but it does not carry back blood from the 

 mantle to the auricle. On the contrary, it is easy to see that 

 blood passes from it to the numerous fine blood-vessels or 

 rather blood -spaces, for they have no definite walls, in the roof of 

 the pulmonary chamber. The efierent pallial vein that collects 

 blood from the mantle and returns it to the heart is on the opposite 

 or left side of the mantle-cavity. It is a direct continuation of the 

 auricle and can easily be traced forward in the left corner of 

 the mantle-cavity, running along the upper border of the left 

 columellar muscle (PI. XXXII. fig. 16, v. pal.) nearly as far as 

 the thick muscular anterior border of the mantle. It receives 

 nvimerous vessels from the mantle, especially in the anterior 

 part of its course. The blood from the intestinal and perigastric 

 sinuses does not pass straight to the mantle, but is collected into a 

 large sinus provided with definite walls (Pis. XXXIII. & XXXIV. 



