PHYLUM ANNULATA 



513 



The two principal sinuses are respectively dorsal (Figs. -405 and 

 410, d. s.) and ventral (v. s.), the former lying just above the 

 enteric canal in the middle dorsal line, the latter occupying a 

 similar position on the ventral side, and enclosing the ventral 

 nerve-cord. The two sinuses are in connection with one another 

 pcsteriorly, and are also in communication, by means of their 

 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 sinuses represent a greatly reduced coelome. 



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

 a small brain (Figs. 407 and 408, br.) situated above the anterior 



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Fig. 410.— Diagmra of principal blood-channels of lieech. d. s. dorsal sinus ; I. v. lateral vessel ; 

 V. s. ventral sinus containing nerve-cord. 



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 

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

 arranged in groups or packets. 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. 



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