PHYLUM ANNULATA 



501 



branches, with the capillaries of the skin. There is thus an 

 indirect connection, by means of capillaries, between the blood- 

 vessels and the sinuses, but no direct communication exists. The 

 sinuses in which the ciliated funnels are lodged open into the 

 ventral sinus. As we shall see more particularly in the general 

 account of the class, the vessels and sinuses represent a greatly 

 reduced coelome. 



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

 a small brain (Figs. 414 and 415, br.) situated above the anterior 

 end of the pharynx immediately behind the median dorsal jaw. 

 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), situated in the third or middle 

 ring of each segment, united by delicate double connectives 



its 



FIG. 417. Diagram of principal blood-channels of Leech, d. s. dorsal sinus ; I. v. lateral vessel ; 

 v. s. ventral sinus containing nerve-cord. 



and a slender median strand. The ganglion-cells are regularly 

 arranged in groups or packets. The first or sub-oesophageal 

 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 (Figs. 411 and 418) 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 seg- 

 mental sense-organs, with which they are obviously homologous. 



