6o PHYLUM ANNELIDA 



a single strand. Remove the sperm sacs and observe the nerve 

 cord as it Hes in the midventral Hne. Note the sHght sweUings ; 

 these are the segmental ganglia. Trace the nerve cord forward 

 to the region of the mouth, where it encircles the forward end of 

 the pharynx and joins the small brain. Observe the two ganglia 

 of which the brain is composed. Remove the forward portion 

 of the nervous system from the body. Mount it on a slide and 

 examine it under a microscope. Note the double nature of the 

 nerve cord and of the gangHa. Note accurately the lateral 

 branches that leave the cord ; note also the shape and branches 

 of the brain. 



Exercise 7. Draw the nervous system on a large scale, accurately 

 representing all the details. 



Study of a Cross Section. This is instructive because it shows 

 the relations of the organs to one another in their natural posi- 

 tions and also illustrates their finer structure. A properly stained 

 and mounted cross section of any portion of the body will serve 

 for this study. 



Observe first the integument ; it is made up of the cuticula on 

 the outside and the cellular hypodermis beneath it. The latter is 

 composed, in most parts of the body, of a single layer of cells 

 which secretes the cuticula. Note the numerous single-celled 

 glands in the hypodermis. If the section passes through a seta, 

 notice its method of attachment and its muscles. Beneath the 

 integument are the body muscles. Of these there are two sys- 

 tems, the circular and the longitudinal muscles. The former are 

 a narrow band just beneath the hypodermis. The latter are much 

 more extensive and project into the body cavity; they are ar- 

 ranged in groups and will be seen of course in cross section. Near 

 the center of the body cavity note the large alimentary canal. If 

 the section is in the region of the stomach intestine, note the 

 longitudinal fold in the dorsal intestinal wall which very largely 

 increases its surface. Observe the structure of the alimentary 

 canal ; its cavity, or lumen, is bounded by a thick mucous mem- 

 brane consisting of a single layer of very long, slender cells, 

 around which are two muscle layers, an inner circular and an 



