202 



INVERTEBRATE MORPHOLOGY. 



CHAPTEE X. 



TYPE ANNELIDA 



The type Annelida includes a series of forms among 

 which metamerism reaches a high grade of development. In 

 what may be considered a typical Annelid 

 (Fig. 97) a number of segments or meta- 

 meres succeed one another from the 

 head to the tail, each one resembling its 

 predecessor and its successor in all its 

 parts ; the nephridia, reproductive organs, 

 nerve-ganglia (n), and appendages, when 

 present, are repeated in each successive 

 segment, and each metamere is marked 

 off from its fellows, externally by a groove 

 surrounding the body and internally by 

 a partition or dissepiment extending 

 transversely across the coelom from the 

 body-wall to the digestive tube. This 

 latter structure and the blood vascular 

 tubes cannot well from the nature of 

 things be divided metamerically, but are 

 continuous from one end of the body to 

 the other, showing, however, in the meta- 

 meric pouches and intermetameric con- 

 Pm. 97,-DiAGEAM OF strictions of the digestive tract, and in 

 Gbnbkal Plan of an the metamerically arranged lateral vessels 

 Annelid. of the blood vascular system which encir- 



cle the digestive tube, indications of the di- 

 vision which has affected the other organs. 

 Two segments, however, the head 

 (pr) and the tail, usually present differ- 

 ences from the rest in their structure ■ 

 the head or anterior metamere bears sense-organs when 



a = anus. 



ce = cerebral ganglion. 

 TO = mouth. 

 n = ventral nerve-cord. 

 pr = prostomium. 



