ANNULATA. 



221 



7. Development may be either direct or indirect. 



258. General Survey. The Annulata though conforming 

 to the type outlined above are very diverse in appearance, 

 habits and internal structure. While the Chaetopoda, the 

 class to which the forms studied in the laboratory belong, 

 are taken as the type, the leeches, which have no bristles but 

 possess suckers, are undoubtedly related, as is shown by their 

 development. The Rotifers and other forms are sometimes 

 included among the relatives of the Annulata. Metamerism 

 in animals is a most interesting phenomenon to zoologists. 

 This group is the first in which we have found true metamerism. 

 The body of the animals is more or less constricted on the 



d. m. 



n. c. 



n. f. 



CO. 



FIG. 100. Longitudinal section of anterior end of Dero. A, sagittal section; B, 

 frontal section to show anterior portion of nervous system, b, brain; co., nervous 

 collar about the mouth; c.v., contractile blood vessels ("hearts"); d, dissepiment; 

 d.m., dermo-muscular wall; d.v., dorsal blood vessel; m, mouth; n.c., nerve cells; n.f., 

 nerve fibres; np., nephridia; p, prostomium; ph., pharynx; s, setae; sn., segmental nerves; 

 v.g, ventral chain of ganglia; v.v., ventral blood vessel. Only a portion of the blood 

 vaseular system is shown, and this appears unsectioned in the figure. 



Questions on the figure. Compare this with the cross-section of Dero 

 and identify the parts. How do the four anterior segments differ from 

 the others figured? Does the ventral nerve cord continue the whole length 

 of such an animal as this? Which organs may be described as segmental 

 and which as continuous through the segments? 



