16 GENERAL BIOLOGY. 



1. The body-wall consisting of three layers : 



(a) The cuticle. 



(b) The hypodermis. 



(c) The muscular layer. 



2. The space between the body-wall and the 



alimentary canal, the body-cavity or ccelom. 



3. The thin muscular partitions (dissepiments) 



which divide the ccelom into a series of 



compartments. 



II. Cut through the dissepiments on the left side close 

 to the body-ivall ; spread out the body -wall and pin 

 it to the wax (putting the pins in obliquely). Sim- 

 ilarly for the right side. Note : 



1. The alimentary canal : a straight tube extend- 



ing throughout the whole length of the 

 body. 



2. The nephridia : delicate, whitish bodies at- 



tached by one end to the posterior side of 

 a dissepiment, and by the other end to the 

 ventral body- wall ; one on each side of the 

 alimentary canal in each somite. 



3. The seminal vesicles : three pairs of white 



bodies lying partly at the sides of the 

 alimentary canal and partly above it, be- 

 tween the 9th and the 12th somites. 



4. The dorsal blood-vessel : a long tube lying 



upon the dorsal side of the alimentary 

 canal. It gives off : 

 (a) The aortic arches: five pairs of large 



