MULTICELLULAR ANIMALS 



139 



PROSTOMIUM 



SEGMENT 10 



SEGMENT 14 

 SEGMENT 15 



CLITELLUM 

 'iEGMENTS 31-37 



MOUTH 



SPERMATHECAL 

 OPENINGS 



OVIDUCT OPENING 

 SPERM DUCT 

 OPENING 



■SEGMENT 



larger space required by reproductive and excretory organs, fill the 

 ccelom and its divisions (Fig. 



65). 



The distinctive feature of the 

 Annelida is the division of the 

 body into segments or meta- 

 meres. Metamerism is most 

 highly developed in this phy- 

 lum; it continues to be a char- 

 acteristic of the structure of 

 more advanced animals up to 

 and including Man. The earth- 

 worm, while not a typical an- 

 nelid, is one with which most 

 are somewhat familiar and 

 serves to illustrate metamerism 

 for our purposes here. An ex- 

 amination of the anatomy of an 

 earthworm shows it to consist 

 of a series of rings (Fig. 83). 

 In the posterior region these 

 rings are practically alike, each 

 containing the same organs 

 similarly arranged and each 

 separated from adjacent meta- 

 meres by the septae which ex- 

 tend across the coelom. The 

 body therefore consists of a 

 series of secondary individu- 

 als, or partial individuals, 

 each closely resembling adja- 

 cent members. In an ideal segmental animal all metameres would 

 be identical and each would be provided with appendages for loco- 



ANUS 



Fig. 83. — External appearance of the com- 

 mon earthworm. 



