ANNULOSA : ANNELIDA. 257 



ANNELIDA. 



uV 



CLASS II. ANNELIDA ( = Annulata}. The Annelida are 

 vermiform animals, distinguished from the preceding by the 

 possession of distinct external segmentation ; the nervous system 

 is composed of a ventral, double, gangliated cord, with an ossopha- 

 geal collar and prce-cesophageal ganglion. 



This class comprises elongated worm-like animals, in which 

 the integument is always soft, and the body is more or less 

 distinctly segmented, each segment usually corresponding with 

 a single pair of ganglia in the ventral cord. All the segments 

 are similar to one another except those at the anterior and 

 posterior extremities of the body. Each segment may also be 

 provided with a pair of lateral appendages, but these are never 

 articulated to the body, and are never so modified in the region 

 of the head as to be converted into masticatory organs. 



In the higher Annelida each segment (fig. 126) consists of 



Fig. 126. Diagrammatic transverse section of an Annelide. d Dorsal arc; v Ventral 

 arc ; n Branchiae ; a Notopodium, or dorsal oar ; ^Neuropodium, or ventral oar, both 

 carrying setae and a jointed cirrhus (c). 



two arches, termed, from their position, respectively the " dor- 

 sal arc " and the " ventral arc ; " and each bears two lateral 

 processes, or " foot-tubercles " (parapodia), one on each side. 

 Each " foot-tubercle " is typically double, being composed of 

 an upper process, called the " notopodium," or " dorsal oar," 

 and a lower process termed the " neuropodium," or " ventral 

 oar ; " but these may be fused together. The foot-tubercles, 



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