CHAPTER 12 

 THE EARTHWORM AND OTHER ANNULATA 



In the preceding chapters we have studied the structure and 

 functions of a familiar vertebrate animal, of the single-celled organ- 

 isms, and of one of the simpler types of many-celled animals. The 

 general biological problems that are most suitably illustrated by 

 these forms have also been examined. In the work beginning 

 with the present chapter, we shall extend this method to other 

 representative types among invertebrate animals. The phenomena 

 of metabolism, irritability, and reproduction will be further exam- 

 ined, but particular attention will be given to the structures and 

 functions of various animals as representatives of their respective 

 phyla, and to some of the general biological problems that each 

 type naturally suggests. The earthworm, the crayfish, and various 

 insects will be studied in detail; and other representatives of the 

 phyla concerned will be described for purposes of comparison. 

 Thus we shall complete a broad survey of the animal world before 

 proceeding to the concluding chapters, which deal with certain 

 problems of a more general nature, although two important phyla, 

 the Mollusca and the Echinodermata cannot receive special con- 

 sideration. 



The division of the phyla of the Animal Kingdom into Entero- 

 aela, or forms with no internal cavity other than the enteron, and 

 Coelomocwla, or those that have a coelome in addition to an enteron, 

 will be recalled {cf. Fig. 117, p. 240). In the Annulata, or seg- 

 mented worms, which are the subject of the present chapter, the 

 coelome is highly developed and the body is divided into segments, 

 the somites, or metameres. The same type of organization exists in 

 modified form among the Arthropoda and Chordata. The Annu- 

 lata constitute an important group from the structural standpoint, 

 because they exhibit the simplest and most extensive development 

 of these important features of organization. 



282 



