AN EARTHWORM 73 



spermatozoa pass in from the spermathecse which at once fertilize 

 the eggs. The cylindrical membrane then passes completely off 

 the worm and its two ends close together. It forms thus a yel- 

 lowish, spindle-shaped capsule about as large as a small pea, and 

 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 backwards 

 through the septum to the next somite, in which the greater 

 portion of it lies, and through the wall of which it communi- 

 cates with the outside. The distal, middle, and proximal por- 

 tions of the tube differ from one another. The distal portion 

 (that next to the nephrostome) is very slender, the middle por- 

 tion is much thicker and has glandular walls, and the proximal 

 portion is a dilated tube which probably acts as a urinary bladder. 



Notice the four slight projections in the body-cavity on the 

 ventral side of each somite. These are the setigerous glands; 

 they secrete the setae. 



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

 grammatically the reproductive organs and two or three 



I pairs of nephridia lying in their proper somites, and 

 label all. 

 Crush the sperm-sacs of a fresh worm, that 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 spermatozoan. 



