DISSECTION. 63 



wrse septa dividing the body-cavity into segments. Carefully 

 pin out the flaps right and left ivith fine pins, stretching them 

 well as you do so. 



It will save much time in determining the positions of 

 some of the important organs if the pins are placed in de- 

 finite segments ; say the tenth, fifteenth, and twentieth. 



A. TheCcelom. 



The coelom, or body-cavity, is a large space, within which 

 the alimentary canal and other viscera lie. Ic is divided into 

 segments by transverse septa, which connect the alimentary 

 canal with the body-wall. The septa correspond to the ex- 

 ternal annuli throughout the whole length of the body, except 

 in two regions : (1) in front of the fourth segment, where they 

 are absent ; (2) in the region of the gizzard, where they are 

 slightly irregular in arrangement. The coelom contains a 

 fluid with numerous amoeboid corpuscles. 



B. The Digestive System. 



The alimentary canal is a straight tube running from the 

 mouth to the anus, and is divisible into the following regions. 



1. The buccal cavity is small, and is situated in the first 



three segments. The opening of the mouth in the 

 ventral part of the first segment has soft lips and 

 no teeth. 



2. The pharynx, which follows the buccal cavity, has thick 



musculiir walls connected with the body-wall by 

 strands of muscle which run obliquely backwards, 

 perforating the anterior septa. The pharynx 

 extends backwards about as far as the sixth or 

 seventh annulus when the animal is in an average 

 condition of extension ; but it lies entirely in front 

 of the septum dividing the fifth segment from the 

 sixth (fig. 23). 



3. The oesophagus is a straight tube extending from 



the pharynx to the thirteenth or fourteenth seg- 

 ment. When the animal is not fully extended the 



