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 
