TYPE ANNELIDA. 203 



these are developed, is destitute of nephridia in the adult, 

 and contains primarily the supraoesophageal ganglion of the 

 nervous system (Fig. 97, ce), the ganglia of the trunk meta- 

 meres (w) lying ventrally to the digestive tube ; while the tail 

 segment bears the anal opening and usually presents other 

 characteristics which distinguish it from the preceding seg- 

 ments. It is rare, however, leaving aside this antero-pos- 

 terior differentiation, that a perfect metameric condition is 

 found in any Annelid. Secondary changes may interfere 

 with the similarity of all the metameres ; a suppression of 

 parts usually present in some of the segments may occur, as, 

 for instance, where the reproductive organs are confined to 

 one or two metameres, or again there may occur a differentia- 

 tion of the anterior appendages for a special function where- 

 by a marked dissimilarity between the anterior and posterior 

 metameres is produced. Finally, owing to peculiar habits of 

 life, the metamerism may be almost or completely lost, being 

 indicated only, perhaps, by one set of organs, such as the 

 nerve-ganglia, or else only evident in the larval stages. Para- 

 sitism or a fixed or tubicolous habit of life are among the 

 principal causes of this degeneration, examples of which will 

 be seen later. 



In consequence of this degeneration some Annelids pre- 

 sent a metamerism of a lower grade than that found in such 

 forms as the Nemerteans. Other peculiarities of structure 

 occur, however, which serve, together with the indications of 

 metamerism, to mark out the Annelid type. One of these 

 peculiarities is the occurrence in nearly all forms of a series 

 of nerve-ganglia along the ventral nerve-cords ; this feature 

 is of course a part of the metamerism, but it is not usually 

 marked in the metamerism of the nervous system seen in 

 lower forms. In these scattered ganglion-cells occur all along 

 the nerve-cords, which extend backwards from the brain, 

 while in the Annelids these scattered cells are associated 

 together to form metameric ganglia. Another peculiarity is 

 found in the structure of the nephridia. These are no longer 

 in all cases rows of perforated cells closed at the inner end 

 by a flame-cell, but may consist of more or less convoluted 

 tubes lined by ciliated epithelium and open as a rule into 



