706 



THE POPULAR SCIENCE MONTHLY. 



In the leech we meet with some variations in the arrangement of 

 the nervous system, of a kind analogous to changes subsequently to 

 be spoken of as occurring in higher forms of life. The nervous sys- 

 tem becomes more concentrated. There is no longer a ganglion for 

 each segment, but one for every three or four segments of the animal ; 

 and the two ventral cords approximate so closely as to be almost 

 fused into one. In the common medicinal leech, for instance, there is 

 a bilobed ganglion (a) above the mouth, which receives fibres from 

 the tactile lips, and also ten distinct filaments from as many pigment- 

 spots (b b) or ocelli, situated round the margin of this upper lip. 



c-^= 



m 



^ 

 ^<i\ 



mm. 



Fig. 2. Nervous System op thb Medicinal Leech. 



From this bilobed ganglion, which corresponds with the brain proper 

 of higher animals, a cord descends on each side of the oesophagus, 

 and the two unite in a heart-shaped supra-oesophageal ganglion (c), 

 from which afferent nerves are given off to the muscles whose busi- 

 ness it is to move its three saw-like jaws, as well as to the muscles of 

 the oral sucker. This lower ganglion in part corresponds with the 

 " medulla oblongata " of vertebrate animals. It is continuous with 

 the double ventral cord, on which twenty equidistant rhomboidal 

 ganglia are developed. Each of these ganglia gives off two nerves 



