206 GENERAL ZOOLOGY 



in Ascaris. This is of small consequence, however, for 

 Lumbricus has a well-developed circulatory system of the 

 closed* type, with arteries, veins, capillaries, and con- 

 tractile vessels. The blood is forced forward by con- 

 tractions in the walls of the large dorsal vessel, flows 

 through the arteries to capillaries among the internal or- 

 gans and in the skin, then returns to the dorsal vessel 

 through veins. The blood acquires nourishing substances 

 from the digestive system and is aerated in the skin capillar- 

 ies. Like that of vertebrates, it carries both nourishment 

 and gases. Respiration is carried on wholly through the 

 skin, waste gases being eliminated from the blood and re- 

 placed by oxygen. The earthworm's blood contains 

 haemoglobin, and is hence an admirable carrier of carbon 

 dioxide or oxygen. 



An earthworm is well provided with excretory organs, for 

 nearly every one of the hundred or more metameres con- 

 tains a pair of nephridia. These are slender tubes which 

 coil about in the ccelom, and are therefore well situated to 

 absorb and eliminate liquid waste products. A nephridium 

 begins with a closed funnel-like expansion in one metamere, 

 passes back through a septum into the next, coils back and 

 forth three times, and opens to the exterior through a little 

 pore in the body wall. 



Self -protection. Lumbricus is much sought for food by 

 robins, shrews, and other animals. It is perfectly helpless 

 when captured, but escapes from many of its enemies by 

 remaining underground except at night. When some ani- 

 mal tries to pull it from the burrow, all the setae are pro- 

 truded and cling with great tenacity. If the body breaks 

 in two under the strain no particular harm results. To be 

 sure, the portion remaining with the enemy is speedily de- 

 voured and digested, but that in the burrow soon grows the 

 parts needed to make a complete body and is "as good as 



* In a closed circulatory system the blood is always within the vessels 

 and passes through a regular course; in an open system it flows in part 

 through vessels, but also enters large irregular spaces, or sinuses. 



