132 Descriptions of Preparat io ns . 



larger than any which come after it; it gives off five pairs of 

 nerves, and is connected by very short commissural cords to the 

 supra-oesophageal ganglia anteriorly, as also to the second ventral 

 ganglion posteriorly. The commissures between the second and 

 third, and the third and fourth ventral ganglia increase in length, 

 though they are shorter than those conaecting the ganglia be- 

 longing to the middle region of the body; the ganglia at the 

 posterior extremity of the animal are again closely aggregated 

 together. The last ganglion of the ventral chain is much larger 

 than any of the series except the first, and gives off from seven 

 to nine branches to the posterior sucker ; the other ganglia give 

 off each two trunks, which distribute themselves to the muscles of 

 the body, with the exception of the penultimate ganglion, which 

 gives off only a single nerve on either side. The paired nerves 

 are given off very close to each other, and the one which ramifies 

 nearer to or in connection with the anterior wall of each disse- 

 piment, and takes a dorsal rather than a ventral direction, arises 

 above rather than behind the other. This latter nerve has, just 

 at the point of its bifurcation, a small ganglionic mass developed 

 upon it, which may represent the accessory ganglia seen upon the 

 pedal nerves in Nereis, and called ' ganglions de renforcement ' by 

 De Quatrefages. With a microscope, a small detached ganglion, 

 probably homologous with the lateral ganglia of the system of the 

 nervi transversi in insects, may be seen apposed to but not inclosed 

 v/ithin the capsule of each ventral ganglion, in the int-rval between 

 the points of origin of these paired nerves. A second element 

 homologous to a portion of the Arthropodous system of nervi 

 transversi is presented to us in the Leech by an '^intermediary' 

 nerve, which runs in the interval between the two fibrous strands 

 connecting the several pairs of ganglia, but which does not give 

 off any branches. The bilateral character of the chain of nerve 

 ganglia is as plainly seen in the Leech as in the worm ; but there 

 are no nerves given off from the inter-ganglionic commissural cords 

 in the Hirudineae. A third nerve of the sympathetic class exists 

 in the Leech, in the form of an azygos nerve trunk, with ganglion 

 cells appended to it throughout its course, which corresponds to 

 the ventral aspect of the digestive tract, and has two lateral arms 

 prolonged in relation witli the two posterior lateral coeca of the 

 'stomach' or 'oesophagus.' The removal of the digestive tract 



