228 



ZOOLOGY 



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 outside into 

 rings — as the name (Annelida) implies. The internal organs 

 also show metamerism, but in various ways. These organs may 

 pass directly through, with slight segmental modification, as the 

 digestive tube and ventral nerve cord; they may be repeated 

 independently in each segment, as the setas or nephridial tubules ; 

 or they may be represented in only one or a limited number of 



n. c. 



d n. f. 



Fig. 102. 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; r./., nerve fibres; np., nephridia; p, prostomium; ph., pharynx; 

 s, sets; sn„ segmental nerves; v.g. ventral chain of ganglia; v.t., ventral blood vessel. Notice that 

 only a portion of the blood vascular system is shown, and this appears unsectioned in the figure; 

 also that nervous structures and nepridia are only partially represented. 



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

 identify tht 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? 



segments, as the brain or the reproductive organs. The seg- 

 ments are not therefore exactly equivalent, yet the agreement 

 between successive segments is sufficient to merit the term 

 homonomous (see §125). The number of segments varies from 

 a few to hundreds. The body is from four or five to many times 



