Dissected so as to show its nervous system.
A PART of the pharynx with the jaws has been left in situ, and a black bristle also passed down it through the oesophageal or nerve-collar. The supra-oesophageal ganglion is seen above the pharynx, part of the glandular and muscular walls of which have been removed to show it in situ. It lies immediately behind the dorsally placed jaw, which is very visible in this preparation. It is connected by commissures forming an extremely narrow ring to the ventral chain of ganglia. This chain, counting the infra-oesophageal ganglion as the first of the series, numbers twenty-three ganglia in all. The infra-oesophageal ganglion and the second ganglion, which is closely apposed to it, are to be seen with difficulty here. The first ganglion, easily seen, is the third of the series. The longitudinal commissures between the third and fourth, the fourth and fifth ganglia increase in length, though they are shorter than those connecting the ganglia belonging to the middle region of the body. The sixth ganglion is concealed by the prostate gland at the base of the muscular penis. It is close to the seventh ganglion. The ganglia at the posterior extremity of the body, beginning with the nineteenth ganglion, are again closely aggregated together.
The last ganglion is much larger than any of the series except the first.
It can be readily seen with the naked eye that nerves are given off laterally from each ganglion. They are in reality paired, but one branch is dorsal, the other ventral. There are no nerves given off between the ganglia in the Leech, as there appear to be in the Earthworm. But in the Leech the ganglion cells are really aggregated in the ganglia, not scattered along the whole cord as in the last-named worm.
In front of the supra-oesophageal ganglion lie three minute ganglia closely connected to it, one median and two lateral. The nerves given off by these ganglia supply the three jaws. The nerves originating from the supra-oesophageal ganglia supply the eyes and the goblet-shaped organs of Leydig.
The infra-oesophageal ganglion, according to Leuckart, is composed of three ganglia in the embryo which fuse in the adult. As figured by Leydig, it is composed of two halves, right and left, connected by five transverse fibrous commissures.
Through the four interspaces between these commissures and in front of the first one of the series small bundles of muscle fibres pass vertically. According to Ley-dig each ventral ganglion is pierced by a central hole which transmits a similar muscular bundle. The terminal ganglion of the chain is composed of seven embryonic ganglia. According to Vignal the distinction between these ganglia can be traced in the adult. There appears to be a larger number of fused embryonic ganglia in some other Leeches.
The infra-oesophageal ganglion gives off five nerves on each side. The two nerves given off by the other ganglia of the chain on each side are dorsal and ventral respectively. The ventral nerve of the third ganglion in the chain and its successors has a minute ganglion at the spot where it first bifurcates.
The ganglion cells in the Leeches tend to accumulate in masses on the outer surface of the ganglia. This follicular appearance is much more marked in some other instances than it is in Hirudo, e. g. in Haemopis and Nephelis. At the point of origin between the two nerves of each ganglion on either side lies a large ganglion cell, the connections of which are not known. The fibrous commissures between the ganglia of the ventral chain are three in number, two large and lateral, one small, median and dorsal. The latter is the intermediary nerve of Faivre, its discoverer. Each of these commissures has its own sheath, and all three have in addition a common sheath, the outer surface of which is pigmented. The three commissures, according to Vignal, fuse centrally in each ganglion.
Brandt discovered in connection with the ventral surface of the crop a median nerve which bifurcates posteriorly in correspondence with the last pair of caeca. This nerve forms a rich plexus on the walls of the crop, and the filaments of the plexus are in connection with numerous ganglion cells. The exact mode of connection of this sympathetic system, which appears to be chiefly in relation with the muscles of the crop, to the ventral chain is not known. It takes place probably through branches of the nerves given off by the ganglia.
The ventral chain of ganglia is contained within a ventral blood sinus in all Hirudinea.
Nervous system. Leydig, Bau des Thierischen Korpers, Tubingen, 1864; Remy Saint-Loup, A. Sc. N. (6), xviii. 1884. For figures, see also Leydig, Tafeln zur Vergleich. Anatomie, Tubingen, 1864, Taf. i. figs. 4, 6, 7; Taf. ii. figs. 1, 3, and 5. Cp. figures of other genera on same plates and on Taf. iii. figs. 4 and 5.
Histology only. Vignal, A. Z. Expt. (2), i. 1883, p. 343.
Lateral ganglion cell, Leydig, Tafeln, etc, Taf. ii. fig. 3, 1.
Sympathetic nerve. Id. op. cit. Taf. i. fig. 4; Taf. ii. fig. 5.
Hermann, Centralnervensystem von H. medicinalis, Munchen, 1875; Hoffmann, Untersuchungen uber den Bau, etc. der Hirudeen, Verh. Ak. Amsterdam, 1880; Kohler, Systeme nerveux de Nephelis, Nancy, 1883, have not been accessible to me.