CHAPTER IX 



THE CHAETOPODOUS WORMS THE ARCHIANNELIDA ANATOMY OF 



NEREIS, AS TYPICAL OF THE POLYCHAETA 



THOSE animals which possess lateral bundles of bristles (techni- 

 cally termed " chaetae ") for use in locomotion constitute the 

 group of " Bristle-worms," or Chaetopoda. The body of these 

 animals is made up of a preoral lobe or prostomium, and a number 

 of more or less distinct segments following one another in a line, 

 and repeating one another in their internal and external structure. 

 The Chaetopoda embrace the following smaller groups or Orders : 

 I. Archiannelida, II. Polychaeta, III. Myzostomaria, IV. 

 Oligochaeta. The Archiannelida, although without the charac- 

 teristic chaetae, are yet anatomically so similar to the true 

 Chaetopoda that they must be included in the group, just as 

 certain fishes are classed as " Vertebrata," although they do not 

 possess vertebrae. The old term Annelida is sometimes used to 

 include the above-mentioned groups, together with the Gephyrea l 

 and the Hirudinea or leeches. 



Order I. Archiannelida. 



The Arehiannelida are very simple worms, but simplicity may 

 be, and very frequently is, the result of degeneration ; and it is 

 not always possible to determine whether a simple animal is 

 primitively, i.e. ancestrally simple, or whether it is secondarily 

 simplified. Hence the term Haplodrili has been employed by 

 Professor Lankester as the name of the group ; a term which does 

 not prejudge the question as to whether or not the worms are 



1 The Echiuroid Gephyrea (see p. 434) are by some authorities considered to be 

 a division of the Chaetopoda. 



VOL. II R 



