220 DESCRIPTIONS OF PREPARATIONS. 



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 em- 

 bryonic 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 ven- 

 tral 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 dis- 

 coverer. Each of these commissures has its own sheath, and all three have in ad- 

 dition a common sheath, the outer surface of which is pigmented. The three com- 

 missures, 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 con- 

 nection 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. Anatomic, Tubingen, 1864, Taf. i. figs. 4, 6, 7 ; Taf. ii. figs, i, 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, &c., 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; Hoff- 

 mann, Untersuchungen iiber den Bau, &c. der Hirudeen, Verh. Ak. Amsterdam, 

 1880; Kohler, Systeme nerveux de Nephelis, Nancy, 1883, have not been acces- 

 sible to me. 



