306 VERMES. 



body ; the parapodia are distributed in pairs on the individual seg- 

 ments along the whole length of the body, and belong to the dorsal as 

 well as to the ventral surface, so that each segment bears a dorsal and 

 a ventral pair of parapodia. 



The internal organization of Worms is extraordinarily various. 

 In those flat and round worms which live in the chyme or the other 

 organic juices of the higher animals, as, for instance, the Cestoda and 

 the Acanthocephala, the whole of the digestive apparatus, including 

 the mouth and anus, may be wanting ; the nutrition in such cases 

 taking place by osmosis through the body-wall. When the alimen- 

 tary canal is present, the mouth is usually situated ventrally in the 

 anterior region of the body, while the anus is placed either terminally 

 at the posterior end of the body, or near it on the dorsal surface. 

 The alimentary canal is generally simple, and is only exceptionally- 

 divided into numerous portions corresponding to the special functions. 

 A muscular pharynx can most often be distinguished, also a well 

 developed stomach and a short rectum opening at the anus. 



The nervous system appears in its most simple form as an unpaired 

 ganglion or, when the two parts of which it is composed are 

 separated, as a pair of ganglia (fig. 76), which are placed near the 

 anterior pole of the body above the oesophagus and genetically may be 

 referred to the apical plate of Loven's Chsetopod larva. The nervous 

 system has more rarely the form of a nerve ring surrounding the 

 oesophagus and connected with groups of ganglion cells (Nemntoda). 

 The nerves given off from the supra-cesophageal ganglion are distri- 

 buted symmetrically forwards and laterally ; they supply the sense 

 organs, and form two strong lateral nervous trunks, which run back- 

 wards. In still higher types two larger ganglia appear, which are con- 

 nected by an inferior commissure (J^em&rtinea). In the Annelids with 

 degenerated rnetarneres (Gephyrea) there is in addition to the supra- 

 cesophageal ganglion (the brain) a ventral nerve cord connected with 

 the supra-cesophageal ganglion by an cesophageal ring. This nerve 

 cord is in all other Annelids divided into a series of paired ganglia, 

 which, in most cases, correspond to the segmentation. The lateral 

 nerve trunks approach each other in the middle line below the ali- 

 mentary canal, and constitute, together with their ganglia, which are 

 connected together by transverse commissures, a ventral chain of 

 ganglia, which is connected with the brain by the circurn-cesophageal 

 commissures, and is continued to the hind end of the body, giving off 

 in its course paired nerves to the right and left. 



The sense organs are represented by eyes, auditory apparatus, and 



