CHAPTER XL 



SEGMENTED WORMS OR ANNELIDA. 



Chief Classes : — ChjEtopoda, Discophora. 



The Annelids or Annulata include segmented " worms," in 

 most of which the segmentation of the body is visible 

 externally. There is usually a well-developed body cavity, 

 which communicates with the exterior by paired nephridia. 

 The nervous system consists typically of dorsal cerebral 

 ganglia, a commissural ring round the gullet, and a ventral 

 ganglionated chain. Not infrequently the nephridia func- 

 tion also as genital ducts. The development may be direct 

 or indirect, and then includes a larval Trochosphere stage. 



In habit, form, and structure, the Annelids exhibit much 

 diversity. The Chsetopods, represented on the one hand 

 by the familiar earthworm, and on the other by the marine 

 worms, best exhibit the structure upon which the Annelid 

 type is founded. With these, however, may be included the 

 aberrant Echiuridae, e.g. Echiurus and Bonellia. A few 

 primitive forms (Archi- Annelida), and the Myzostomata 

 (parasitic on Crinoids), may also be appended to the 

 Chstopod class. The leeches (Discophora) are probably 

 Annelids which have diverged in consequence of a peculiar 

 half-parasitic habit. Finally, some zoologists include 

 Sagitta (Chsetognatha) in this series as an Annelid with 

 three segments, and also the Rotifers (Rotatoria), whose 

 adult form somewhat resembles the Trochosphere larva of 

 many Annelids. 



Class Ch^etopoda. Worms with Bristles. 

 Segmented animals with seta developed in little skin-sacs, 



