EARTHWORMS, LEECHES, AND SEAWORMS 81 



closed tubes. The red coloring matter, hcemoglobin, is 

 dissolved in the liquid itself and is not contained in cor- 

 puscles as in the blood of mammals. The blood of the 

 earthworm contains corpuscles, but they are colorless. 

 Running along the dorsal side of the alimentary canal is 

 a long, muscular tube that can be seen through the partly 

 transparent skin of a living worm as a dark red band. 

 This is the dorsal blood vessel. By close observation suc- 

 cessive wavelike contractions may be seen to pass rapidly 

 from the posterior end of this tube forwards. Lying be- 

 neath the aUmentary canal is a similar tube, the ventral 

 blood vessel, which carries the blood it receives from the 

 anterior end of the dorsal vessel, posteriorly. There are 

 also five pairs of short tubes that arch to the right and left 

 around the gullet in the 7th, 8th, 9th, 10th, and llth seg- 

 ments. These tubes, the aortic arches, connect the dorsal 

 vessel with the ventral vessel and since they are contractile 

 are often called the "hearts" of the worm. Smaller tubes 

 branch off from these main ones and extend to different 

 parts of the body. 



How the earthworm breathes. — The earthworm has no 

 lungs, gills, or other special organs of respiration. The thin 

 walls of the moist skin are everywhere traversed by a net- 

 work of minute blood vessels that lie just beneath the sur- 

 face, so that they are separated from the air by only a very 

 thin membrane. Oxygen is therefore easily absorbed from 

 the air through all parts of the skin and conversely carbonic 

 acid gas is given off by the blood and passes outward through 

 the skin. This is a very simple form of respiration, but 

 answers admirably for such an elongated, thin-skinned 

 animal. 



The excretory system. — As we have just explained, some 



herkick's zool. — 6 



