70 INVERTEBRATE ZOOLOGY 



Observe first the large alimentary canal which passes straight 

 through the animal; also several pairs of conspicuous white 

 bodies a short distance from the anterior end, which -are the 

 sperm-sacs. If the specimen has been freshly killed, the red 

 blood vessels will also be seen. Study and identify in detail 

 the following systems of organs : 



The circulatory system. The earthworm has two circulatory 

 fluids, a red one and a colorless one. The latter consists of 

 a plasma in which float amoeboid blood cells. It is present 

 only in the body-cavity and circulates throughout the body, 

 being driven by the movements of the animal from one somite 

 to another through small openings in the septa ; it will, of 

 course, not be visible in a dissection. The red blood consists 

 of a red plasma in which float colorless blood cells and it circu- 

 lates in a system of closed blood-tubes. The most important 

 of these blood vessels are five longitudinal and numerous 

 circular vessels. Observe the dorsal longitudinal vessel in the 

 median line, above the alimentary canal. It is contractile 

 and propels the blood towards the head. Push aside the 

 intestine and observe just beneath it the ventral vessel, which 

 runs parallel to the dorsal one. Notice that these vessels 

 break into small branches at their anterior ends. The other 

 three longitudinal vessels are very small and not easily seen 

 except in microscopic sections. They lie one beneath and the 

 other two to the right and left of the nerve cord in the 

 mid-ventral line. 



The circular or commissural blood vessels connect the dorsal and 

 the ventral vessels and have a paired and segmental arrange- 

 ment. They are not all of equal size. Observe several large 

 pairs near the forward end of the animal which pass directly 

 between the dorsal and the ventral vessels. They are, like the 

 dorsal vessel,- contractile and are sometimes called the hearts. 

 In which somites are they? Find the commissural vessels poste- 

 rior to them. These are much smaller and do not, in most 



