ANATOMY AND PHYSIOLOGY. 35 



of the lining membrane or " inner tunic " of Huxley. In fact, 

 the so-called vascular ramifications of the test, however minute 

 and divided, ought perhaps to be regarded as prolongations of 

 the pallial cavity, although it is quite possible that they carry 

 true vessels ; and, indeed, from the way they are connected 

 with the heart, this would seem almost probable. 



The heart (Plate III, fig. 4) is tubular, and is of consider- 

 able leno-th. In Ascidia it is attached to the lower border of 



O 



the stomach, one end extending some way up the dorsal region 

 towards the intestinal tube ; this may be called the dorsal 

 extremity ; the other, the ventral end, points in the direction 

 of the oesophagus. It lies between the mantle and the lining* 

 membrane, within a distinct chamber or pericardium, along 

 one side of which it is attached from end to end. The chamber 

 seems as if formed by a fold of the lining membrane ; and the 

 heart is probably coated with it in the manner of a peritoneum, 

 and is so attached to the wall of the chamber. 



A large trunk vessel passes from the dorsal extremity of the 

 heart, and immediately divides into three branches, one of 

 which advances between the mantle and the lining membrane 

 along the dorsal region at the back of the endostyle ; another 

 passes in the opposite direction down the dorsal margin to the 

 bottom of the branchial sac. These two form the great dorsal 

 branchial channel, and are equivalent to the ventral or thoracic 

 sinus of Milne-Edwards ; and they both communicate with 

 the dorsal extremities of the transverse channels of the bran- 

 chial sac. The third branch turns off at right angles to this 

 great dorsal channel, close to the point where it is united to 

 the heart, and, in company with another vessel, to be shortly 

 described, penetrates the mantle and goes to ramify in the 

 test. 



From the other or ventral extremity of the heart there are 

 two large trunk vessels given off, one to each side of the 

 stomach. These ramify over the digestive organs and supply 

 a minute network spread over both sides of the visceral mass ; 

 this network may be termed the visceral plexus. It is in direct 

 communication with a similar plexus of blood-channels or 

 sinuses that lies between the mantle and the lining membrane 

 of the right side ; and this latter is continuous with another 

 plexus similarly situated in the left side of the mantle ; these 

 together form what we shall call the pallial plexus. The trunk 

 branch that supplies the left side of the stomach and the 

 portion of the visceral plexus there situated divides into two 

 large stems, one of which inclines towards the intestine, the 

 other towards the oesophagus ; the former passes for some 



