74 INVERTEBRATE ZOOLOGY 



With a sharp knife or curved scissors carefully remove a 

 nephridium from the animal's body. Mount it on a slide and 

 examine it under a microscope. 



Exercise 6. Draw it and label its three divisions. 



The nervous system is essentially similar to that in arthropods. 

 Remove the sperm-sacs and observe the nerve cord as it lies in 

 the mid-ventral line. Note the slight swellings, which 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, together with the brain, 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 ganglia. What does 

 this signify as to the primitive condition of the system in 

 the ancestors of the earthworm? Note accurately the lateral 

 branches that leave the cord; 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 

 positions 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 

 and it 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 



