472 



ZOOLOGY 



SECT. 



ventral side, and enclosing the ventral nerve-cord. The two sinuses 

 are in connection with one another posteriorly, and are also in com- 

 munication, by means of their branches, with the capillaries of the 

 skin. There is thus an indirect connection, by means of capil- 

 laries, between the blood-vessels and the sinuses, but no direct 

 communication exists. The sinuses in which the nephrostomes 

 are lodged open into the ventral sinus. As we shall see more par- 

 ticularly in the general account of the class, the sinuses represent 

 a greatly reduced ccelome. 



The nervous system is of the usual annulate type. There is 

 a small brain (Figs. 373 and 374, &r.) situated above the anterior 

 end of the pharynx immediately behind the median dorsal jaw. 



FIG. 376. Diagram of principal blood-channels of Leech, d. s. dorsal sinus ; 1. v. lateral vessel ; 

 v. s. ventral sinus containing nerve-cord. 



It is connected by a very short pair of oesophageal connectives 

 with the ventral nerve-cord, which consists of twenty-three well- 

 marked rounded ganglia (gn. 1-23) united by delicate double 

 connectives. The first, or sub-cesophageal ganglion is larger 

 than the others, and is shown by development to be made up of 

 five united pairs of embryonic ganglia : the last ganglion is also of 

 unusual size, and results from the fusion of six pairs of ganglia 

 distinct in the embryo. The whole ventral nerve-cord is contained 

 in the ventral sinus. Nerves are given off from the ganglia, but 

 not, as in the Earthworm, from the connectives, in which also, 

 nerve-cells are wholly absent. 



The principal sense-organs are the eyes, of which there are five 

 pairs (Fig. 377) situated round the margin of the anterior sucker, 

 on the dorsal side, one pair in each of the first five segments. 

 They occupy positions taken in the succeeding segments by lateral 



