240 COLLEGE ZOOLOGY 



6. ANNELIDS IN GENERAL 



Three morphological characteristics of the ANNELIDA are espe- 

 cially worthy of notice: (i) metamerism, (2) the ccelom, and 

 (3) the trochophore stage in development. 



Metamerism. The segmentation of the body as exhibited 

 in annelids is called metamerism, and is here encountered for the 

 first time. This type of structure is of considerable interest, since 

 the most successful groups in the animal kingdom, the ARTHRO- 

 PODA and VERTEBRATA, have their parts metamerically arranged. 

 How this condition has been brought about is still doubtful, but 

 many theories have been proposed to account for it. According 

 to one view the body of a metameric animal has evolved from 

 that of a non-segmented animal by transverse fission. The in- 

 dividuals thus produced remained united end to end and gradu- 

 ally became integrated both morphologically and physiologi- 

 cally so that their individualities were united into one complex 

 individuality. Some zoologists maintain that the segmental 

 arrangement of organs such as nephridia, blood-vessels, and re- 

 productive organs has been caused by the division of a single 

 ancestral organ, and not by the formation of new organs as the 

 fission theory demands. 



True metamerism, as exhibited by annelids, should not be 

 confused with the pseudometamerism of the tapeworms (p. 163, 

 Fig. 107). The proglottides of the tapeworms are individuals 

 budded off from the posterior end and differing from one another 

 only in the degree of development. The tapeworm may be 

 considered a row of incomplete individuals. 



The Coelom. The ccelom has already been defined (p. 89) 

 as a cavity in the mesoderm lined by an epithelium; into 

 it the excretory organs open, and from its walls the reproductive 

 cells originate. The development of the ccelom is described 

 on page 89. 



The importance of the ccelom should be clearly understood, 

 since it has played a prominent role in the progressive develop- 



