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 ameboid 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 
