XVI 



ANATOMY 



455 



which may be divided into four regions. The mouth (m) leads 

 into the oesophagus (oe), which gradually enlarges into the 

 stomach (st) situated just before the bend ; a constriction just at 

 the bend separates the stomach from the intestine (int), and this 

 leads into the rectum (r), which terminates in the anus (an). 

 The first three divisions of the alimentary canal are ciliated, but 

 the rectum is not ; the walls of the stomach also contain ilan- 



e- 



Fig. 229. The dorsal surface of Ph. australis Has., looking down on the head. The ten- 

 tacles are cut away on the left side, and the innermost are shortened on the right 

 side to show the arrangement ; in reality they are of the same length throughout. 

 a, Mouth ; b, anus ; c, pore of left nephridium ; d, epistome ; e, break in the inner 

 series of tentacles. The drawing is to some extent diagrammatic, and is consider- 

 ably enlarged. (After Benham.) 



dular cells, but there are no special glands opening into any part 

 of the tract. 



One of the most interesting features of Phoronis is the 

 presence of a closed system of blood-vessels containing red blood. 

 There are two main blood-vessels ; one, lying in the rectal chamber 

 between the two limbs of the U-shaped alimentary canal, has 

 been named the afferent vessel. Just below the diaphragm this 

 splits into two, and each branch, after piercing this partition, 

 runs in a spiral course along the base of the crown of tentacles, 

 giving off a single blood-vessel into each tentacle. At its base 

 each tentacular vessel opens not only into the above-mentioned 



