10 ARTHUR E. SHIPLEY. 



insertion of the retractor muscles Avhich are attached to this 

 part of the body. 



The Alimentary Canal. 



The digestive tube may be divided into three parts : (1) the 

 oesophagus, which extends from the mouth to the beginning of 

 the coiled intestine; (2) the intestine which forms a close, 

 fairly regular coil with from ten to sixteen turns ; in its coiled 

 state it is almost 10 mm. long; (3) the rectum, which is a 

 straight tube passing from the anterior end of the coil to the 

 anus. 



In spirit specimens the whole of the alimentary canal is 

 white in colour, and is usually full of fine sand. A spindle- 

 muscle serves to support and keep in position the coiled 

 intestine and rectum. This muscle arises from the extreme 

 posterior end of the body wall, and passes forward along the 

 axis of the coiled intestine and then parallel with the rectum, 

 to be inserted into the body wall a little in front of the anus 

 (fig. 3). It gives off during its course numerous fibres, which 

 are inserted into the walls of the intestine and rectum. In 

 addition to the spindle- muscle the intestine is held in position 

 by a thin muscle, which arises from the ventral surface of the 

 body and is inserted into the anterior end of the coil. 



The position of the mouth has been described above. It is 

 a crescentiform slit, lying between the lip and the convex side 

 of the tentacular crown (fig. 6), It is lined with a continua- 

 tion of the columnar ciliated cells which cover the inside of the 

 lip and the ciliated grooves of the tentacles. The walls of the 

 oesophagus are produced inwards into a series of from six to 

 eight ridges, which reduce the lumen of the oesophagus to a 

 star-shaped tube. The grooves between these ridges are 

 continuous with the grooves on the outside of the tentacles 

 (fig. 9). The whole is beset with short thick-set cilia. 

 Surrounding the oesophagus are a few muscle-fibres arranged 

 circularly. For about half its length this first part of the 

 alimentary canal lies betweeA the retractor muscles, which in 

 this region of the body have been reduced to two bundles of 



