SPIKAL VALVE IN THE GENUS EAIA. 51 



artery is distinguished as the rectum ; it is supplied by a special branch of the aorta, 

 the small inferior mesenteric artery; its mucous membrane is smoother than that of the 

 rest of the intestine ; and the spiral valve does not extend into it. With its dorsal wall 

 is connected the characteristic rectal gland. 



The pylorus is usually much contracted, and is very often produced into the cavity of 

 the intestine as a short, thick-walled tube (PI. X. fig. 8, and PI. XI. fig. 3), thus forming 

 a very perfect pyloric valve. 



§ 4. A comparison of a large number of intestines of Baki shows that the spiral 

 valve exhibits certain features of great constancy, and others, again, which vary almost 

 indefinitely. The constant features are : — 



1. The position of the anterior end of the valve. 



2. The course of its outer or attached edge. 



The variable features are : — 



1. The length, in relation to the width of the intestine, of the attached edge, upon 



which depend: — 



a. The number of turns. 



h. The position of the posterior end of the valve. 



2. The course and the length of the inner or free edge, upon which depend: — 



a. The direction of the successive turns of the valve, 



b. The width of the valve. 



c. The area. 



d. The resistance offered to the passage of food. 



3. The character of the mucous membrane. 



§ 5. The spiral valve commences immediately posterior to the pylorus, on the left 

 (inner) and slightly towards the dorsal side of the bursa entiana. From this point its 

 attached edge, represented in the diagrams (PI. X. figs. 2 & 5, and PI. XI. figs. 2, 4,7,& 9) 

 by the thick line, sweeps sharply backwards and ventralwards for about a quarter of a 

 turn, along the left side of the bursa. It then, almost suddenly, changes its direction, 

 passing into a slight backward inclination, from left to right, across the ventral side of 

 the intestine, curving round the right side, and finally passing, more obliquely, from 

 right to left across its dorsal side, thus completing the first turn. 



With the second turn the " pitch " of the spiral line becomes again less steep, and 

 continues to be tolerably uniform as far as the last turn but one, when it once more 

 becomes slightly steeper. 



In consequence of this the compartments or vertical spaces between successive turns, 

 from the second to the last but one. are approximately equal, while the space between 



