GENERAL ANATOMY OF THE LEECH. 167 



(b.) The remaining ganglia, except the two last, give 

 off two nerves each on each side. 



(c.) The most posterior ganglion but one gives rise to 

 only one pair of nerves. 



(d.) The last ganglion is much larger than those which 

 immediately precede it, and gives off seven pairs of nerves. 



(iii.) Anterior to the first ventral ganglion the two 

 commissural fibres diverge from each other and bend up 

 around the anterior end of the pharynx to form the mouth- 

 ring. On the upper surface of the anterior end of the 

 pharynx they end in, 



(iv.) The supra-cesophageal ganglion, or brain, com- 

 posed of two halves meeting in the median line. 



(.) Each half of the brain gives rise to five optic 

 nerves, which pass to the eye-spots of that side of the body. 



(b.) The brain is also united to the three stomato-gastric 

 or sympathetic ganglia ; one on each of the three muscu- 

 lar lobes of the buccal pouch. One of these ganglia lies 

 in the median line in front of the brain, and is united at 

 each end to one-half the latter. The other two are upon 

 the sides, and each joins the corresponding half of the 

 brain. 



III. The Mouth. This may now be examined in the 

 dissected specimen. By bending up the upper lip the tri- 

 angular opening of the mouth may be seen at the bottom of 

 the cavity of the anterior sucker. The ventral slit is verti- 

 cal, and the two dorsal slits are inclined towards it so as to 

 form a Y. Open the buccal cavity by a cut along one of 

 the slits, and notice the three large white buccal muscles 

 which occupy the spaces between these grooves or channels, 

 of which the three slits are the external ends. In the 

 medicinal leech the teeth are placed upon the inner surfaces 

 of these muscles. 



