CHAP. V.] THE NERVOUS SYSTEM OF VERMES. 



89 



along related channels in the great efferent bundles, the 

 fibres of which proceed to the contractile proboscis and 

 also to the muscles on one or both sides of the body. 

 Other departments of the nervous system may exist in 

 these animals, though as yet none have been detected. 



In the common Medicinal Leech the nervous system is 

 somewhat differently developed. The lateral ganglia of 

 the Nemertida3 are replaced by two small 

 upper ganglia (fig. 31, a), connected by 

 lateral commissures with a single lower 

 ganglion (c) ; and, as a consequence of this 

 coalescence of the two sub-oesophageal gan- 

 glia, we have, instead of the two lateral 

 cords of the Nemertidse, a double ventral 

 nervous cord traversing the whole length 

 of the body. The two cords approximate so 

 closely as to be almost fused into one, and 

 they bear a series of ganglia one for every 

 three or four of the segments into which the 

 body of the animal is obscurely divided. 



The bilobed ganglion above the oesopha- 

 gus, which is mainly sensory, receives fibres 

 from the tactile lips, together with ten dis- 

 tinct filaments from as many pigment- spots 

 or ocelli (b b), situated round the margin of 

 the upper lip. From this bilobed ganglion, 

 corresponding with the brain proper of 

 higher animals, a cord descends on each 

 side of the oesophagus, and the two join the 

 heart-shaped sub-oesophageal ganglion (c), 

 from which efferent nerves are given off to 



FIG. 31. Nervous System of the Medicinal Leech. (Owen.) a, Double supra- 

 iBSophageal ganglion connected by nerves with b, b, rudimentary ocelli ; c, the double 

 infra-oesophageal ganglionic mass, which is continuous with the double ventral 

 cord, bearing distinct compound ganglia at intervals. 





