ANNEL1 DA. 



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 (Clicetopoda), or of terminal suckers 

 (Hirudi/nea). 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 setse 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 anus is usually 

 dorsal at the hinder end of the 

 body. 



The nervous system consists of 

 a cerebral or supra - oasophageal 

 ganglion, which is derived from the 

 apical plate of the larval prae-oral 

 lobe, of an oesophageal ring, and of 

 a ventral cord or ganglionic chain, 

 the two halves of which lie more 

 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) 

 (fig. 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 metameres 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 or, e,s 

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

 the longitudinal commissures between the latter. There is in 



s a. 



FIG. 295. Transverse section through 

 the body of Protodrihu (after B. Hat- 

 schek). <S S, The two lateral trunks of 

 the nervous system ; G, ganglionic 

 layer of the same : Z>, alimentary 

 canal ; 2V, nephridium ; M, muscles ; 

 Or, ova. 



