EARTHWORMS, LEECHES, AND SEAWORMS 81 



closed tubes. The red coloring matter, litEiiiojlobin, is 

 dissolved in the hquid itself and is not containeii 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 alimcntarj' 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 chjsc oljservation suc- 

 cessive wavelike contractions may be seen U> pass rapidly 

 from the posterior enil of this tube forwai'ds. Lying be- 

 neath the alimentary canal is a similar tube, the centred 

 blood vessel, which carries the ljlo(jd it receives from the 

 anterior end of the dorsal vessel, i^ostcrioily. Tliere are 

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

 around the gullet in the 7th, 8th, 9th, 10th, and 11th seg- 

 ments. These tujjes, 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 tuties 

 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 arc everywhere ti-aversed by a net- 

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

 face, so that they are separated from tlie air l^y only a very 

 thin membrane. Oxygen is therefore easily alisorlied from 

 the air through all parts of the skin and conversely carl:)onic 

 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 



herrick's zool. — 6 



