59 



the branchial vessel and lies along the anterior margin, 

 or whether it enters more dorsally. 



The ventricle is very different in appearance from the 

 auricle. It is roughly spherical, with very thick spongy 

 walls, but the cavity is so much reduced by crossing 

 muscles that the consistency of the whole is very like that 

 of a sponge. Injections into the ventricle hardly ever 

 succeed, because most of the injecting fluid oozes out at 

 the point of insertion of the syringe. 



Arterial System. 



From the ventral pole of the ventricle a single vessel 

 arises, the Aorta (figs. 35, 36, Ao.). This is of very short 

 length for it divides almost immediately into two branches, 

 the anterior or Cephalic aorta (figs. 35, 36, A.c), and the 

 posterior or Visceral Aorta (figs. 35, 36, A. vis.). The 

 Anterior Aorta gives off immediately a small vessel 

 (fig. 36, a J) which sends branches to the oesophagus and 

 the columellar part of the spire and then plunges below 

 the floor of the mantle cavity into the large sinus in which 

 lie the oesophagus, salivary glands and proboscis. The 

 aorta lies at first at the left side but soon crosses over the 

 oesophagus and runs under the salivary glands. Just 

 after entering the body cavity, it gives off a vessel on the 

 right side which passes to the columellar muscle (fig. 36, 

 A. col.) and branches on its surface. The next large 

 vessel leaves the under surface of the Cephalic Aorta 

 (fig. 36, A. pall.) and passes through the muscles to 

 reach the surface of the mantle. It divides into two main 

 trunks, of which the ventral one reaches the surface on 

 the under side of the animal, and the dorsal vessel just 

 below the osphradium (fig. 35, A. pall/). 



This dorsal pallial artery supplies the siphon, and 



E 



