Annulata. 95 



are several, usually five, branches that connect the dorsal 

 and ventral blood tubes ; they arch around the gullet on 

 each side, hence are designated the "aortic arches"; in 

 some forms they have a series of enlargements, presenting 

 a necklace-like appearance. These enlargements contract 

 and dilate rhythmically, hence they are sometimes called 

 " hearts," but they probably have no greater share in the 

 work of propelling the blood than the other blood tubes. 

 Connected with these main blood tubes are branches by 

 which blood is supplied to the body walls, to the walls of 

 the digestive tube, to the partitions between the segments, 

 to the kidneys, and all the organs of the body. 



One earthworm common in the central states has two 

 dorsal blood tubes (hence the narne, Diplocardia). This is 

 a large worm whose girdle extends from the I3th to the 

 1 8th segment. It has two gizzards. 



How the Earthworm Breathes. The earthworm breathes 

 by means of the skin, there being no special organs 

 of respiration. The body wall is richly supplied with a 

 fine network of blood tubes. These are separated from 

 the external air by a thin membrane only. This thin 

 and delicate covering is always moist, and through it an 

 interchange is continually taking place between the blood 

 within and the air without ; oxygen is being absorbed into 

 the blood, while carbon dioxid and other waste matters 

 are passing in the opposite direction. The worm cannot 

 live long in a warm, dry air, for, when the skin cannot be 

 kept moist, respiration is stopped and the worm is suffo- 

 cated. They can endure immersion in water for some 

 time, but it seems injurious to them. They often are 

 found crawling about in large numbers after a heavy rain. 



The Excretory System of the Earthworm. Part of the 

 waste matter, the carbon dioxid, is thrown off by the skin, 



