XI. EARTHWORM 



The Earthworm, of which there are a great many species widely 

 distributed in the soil of practically every region of the globe, 

 belongs to a phylum of segmented animals known as the Annelida. 

 Due to the fact that the Earthworm possesses a number of struc- 

 tural features which are of considerable importance in interpret- 

 ing those of still higher types of animal life, it is an especially 

 valuable form for study. These structural features may be 

 enumerated as follows : 



(1) The Earthworm is a triploblastic animal ; the three primary 

 germ layers, ectoderm, mesoderm, and endoderm, being present 

 as in higher animals, and in contrast to diploblastic animals like 

 Hydra. 



(2) The Earthworm possesses a body cavity, or coelom, lying 

 between the body wall and the tubular alimentary canal. Thus, 

 the body plan may be described as a tube within a tube. This type 

 of structure is present in higher forms, but it is not found in the 

 Coelenterates, in which the body may be said to consist of a single 

 tube. 



(3) The Earthworm shows a definite segmentation, or metamer- 

 ism of the body ; that is to say, the body is composed of a large 

 number of distinct segments which are arranged in a linear series. 

 Varying degrees of segmentation are present in most of the 

 higher forms of animals. 



(4) The Earthworm shows a two-sided, or bilateral, symmetry. 

 As a rule, the organs in such a case are paired : one situated on 

 the right side of the body and one on the left side. Accordingly 

 there is only one plane which will divide the animal into symmetri- 

 cal halves. Bilateral symmetry is even more pronounced in the 

 higher animal types. 



(5) The Earthworm possesses a number of highly developed 

 organ systems for performing various vital functions, such as 

 nutrition, transportation, excretion, etc. These arise by a group- 

 ing of certain tissues, and are characteristic of all the higher 

 organisms. 



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