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BEING A WORM 



Preview. A typical worm ; external structure of the earthworm {Lum- 

 bricus terrestris) ; the digestive tract and its functions ; how blood circulates, 

 the blood and its functions ; organs of excretion ; the muscles and their work ; 

 reactions to stimuli ; the nervous system and its functions ; the reproductive 

 system and reproduction • Regeneration • Suggested readings. 



PREVIEW 



Passing from the simple two-layered development of the Hydra, in 

 which division of labor among the cells is slight, we come to the earth- 

 worm, another lowly animal, but one which represents the big idea 

 of a typical three-layered, segmented form. 



In Hydra, the egg develops into an adult form having two layers, 

 namely, edodertn and endoderm, but in the earthworm, a third 

 layer, the mesoderm, appears, w^iich is characteristic of all the higher 

 animals. These three germ layers are of great significance in the 

 study of animals, for all of the complex tissues of the body are derived 

 from them. 



Another reason why the earthworm is chosen for study is because it 

 represents a very simple type of segmented or metameric animal of 

 which a great variety is found not only among worms but also among 

 insects and crustaceans. Judging by the insects, segmented animals 

 are the most abundant and successful of all animals, since they out- 

 number all other species. The pages that follow will concern them- 

 selves chiefly with the "hows and whys" of the activity of the 

 common ''night crawler," some of which are: How far has division 

 of labor progressed ? What organ systems are well developed ? How 

 does co-ordinated movement take place, and how do worms become 

 aware of their surroundings ? 



A Typical Worm 

 External Structure of the Earthworm (Lumbricus terrestris) 



The body of the earthworm is divided into segmented parts, or 

 metameres, which in adult worms may number over one hundred. 

 The body tapers bluntly at each end, the anterior end being easily 



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