ANNELIDA. 



365 



Organs of locomotion. Special organs of locomotion may either 

 have the form of bristle-bearing unjointed appendages (parapodia) 

 on each ring of the body (' 'In* /<>i>,,<lu), or of terminal suckers 

 (Hirudinea). In the first case each segment may possess a dorsal 

 and ventral pair of appendages (the neuropodia and notopodia), 

 which, however, are sometimes replaced by simple seta? embedded 

 in dermal pits. 



Alimentary canal. The mouth is placed on the ventral surface 

 at the anterior end of the body, and leads into a muscular pharynx, 

 which is often provided with a powerful armature and can be 

 protruded like a proboscis. This is followed by the gastric region 

 of the gut, which occupies the greatest portion of the length of the 

 body, and is either regularly constricted in correspondence with the 

 segments, or possesses lateral diverticula ; it is only coiled in excep- 

 tional cases. The amis is usually 

 dorsal at the hinder end of the 

 body. 



The nervous system consists of 



a cerebral or supra - cesophageal 



ganglion, which is derived from the 



apical plate of the larval prte-oral 



lobe, of an cesophageal ring, and of 



a ventral cord or ganglionic chain, 



the two halves of which lie moi-e 



or less approached to each other in 



the median line. The ventral cord 



arises from two lateral nerve cords, 



which probably correspond to the 



lateral nerve trunks of the Ne- 



mertines. These two cords are continuous with the cesophageal 



commissures, and, like the latter, are uniformly covered with 



ganglionic cells. This form of the nervous system may persist, 



as may also its ectodermal position (Archiannelida, Protodrilus) 



(tig. 295). In most Annelida, however, this is only a transitory 



condition ; for at a later stage the lateral cords become separated 



from the ectoderm, come together in the median line, and acquire 



a segmentation corresponding to the metaineres of the body. The 



nerves of the sense organs arise from the cerebral ganglion ; the 



other nerves pass out from the parts of the ventral cord 01-, us 



the case may be, from the ganglia of the ventral chain and from 



the longitudinal commissures between the latter. There is in 



' , 



s s a 



FIG. 295. Transverse section through 

 the body of Protodrilus (after B. Hat- 

 schek). S S, The two lateral trunks of 

 the nervous system; G, ganglionic 

 layer of the same ; D, alimentary 

 canal ; JV, nephridium ; M, muscles ; 

 Ov, ova. 



