AN EARTHWORM 59 



in from the sperm receptacles and at once fertilize the eggs. The 

 cylindrical membrane then passes completely off the worm and 

 its two ends close together. It forms thus a yellowish, spindle- 

 shaped capsule about as large as a small pea, which is called the 

 cocoon. In it the young animals are born. 



Excretory Organs. These are the kidneys of the animal. They 

 consist of a pair of coiled tubules, called nephridia, which lie near 

 the lateral and ventral wall of the body cavity in each somite, 

 except the first three or four and the last one. Each nephridium 

 has two openings, a funnel-shaped, ciliated opening into the body 

 cavity, called the nephrostome, and one through the body wall 

 to the outside. The former in each case is attached to the anterior 

 side of a septum. The tube passes backward through the septum 

 to the next somite, in which the greater portion of it lies, and 

 through the wall of which it communicates with the outside. The 

 distal, middle, and proximal portions of the tube differ from one 

 another. The distal portion (that next to the nephrostome) is 

 very slender ; the middle portion is much thicker and has glandular 

 walls ; and the proximal portion is a dilated tube which probably 

 acts as a urinary bladder. 



Exercise 4. Make a sketch of somites 8 to 20, representing diagram- 

 matically the reproductive organs and two or three pairs of ne- 

 phridia lying in their proper somites, and label all. 



Crush the sperm sacs of a fresh worm, which has not been 

 in alcohol, mount some of the milky fluid in it, and examine it 

 under a compound microscope. Notice the sperm spheres and 

 spermatozoa. 



Exercise 5. Draw a sperm sphere and a spermatozoon. 



With a sharp knife or curved scissors carefully remove as much 

 of a nephridium as possible from the animal's body. Mount it on 

 a slide and examine it under a microscope. 



Exercise 6. Draw it and label its divisions so far as observed. 



The nervous system is essentially similar to that in arthropods. 

 It consists of a double cord joined by segmental, paired ganglia ; 

 the two cords are so closely bound together that they appear as 



