THE EARTHWORM AND OTHER SEGMENTED WORMS 177 



Ch&topoda or bristle-foot worms. Many of the marine worms 

 live in tubes somewhat like certain fresh-water species. Serpula, 

 for example, builds itself a crooked tube of calcium carbonate and 

 fastens it to the rocks near shore. Into this tube the worm 

 quickly withdraws when an enemy threatens. 



Characteristics and Classification of the Annelida. Anne- 

 lids are segmented worms, the body consisting of a linear series 

 of more or less similar parts. Many of the internal organs are 

 also segmentally arranged, notably the blood vessels, excretory 

 organs, and nervous system. Setae are usually present. 



Most of the annelids belong to the two following classes : 



Class i. CH^ETOPODA. Annelids with setae. This class 

 may be subdivided into two subclasses : (i) the Polychata, like 

 Nereis (Fig. 100, B), with many setae and fleshy outgrowths, the 

 parapodia, and (2) the Oligochceta, like the earthworm, with few 

 setae and no parapodia. 



Class 2. HIRUDINEA. Leeches. Annelids without setae or 

 parapodia, but possessing anterior and posterior suckers. 



REFERENCES 



The Formation of Vegetable Mould, etc., by Charles Darwin. D. Appleton 



and Co., N. Y. City. 



General Biology, by G. N. Calkins. Henry Holt and Co., N. Y. City, 

 introduction to Zoology, by R. W. Hegner. The Macmillan Co., N. Y. 



City. 



