ix PHYLUM ANNULATA 209 



position on the ventral side, and enclosing the ventral 

 nerve-cord. 



The nervous system is of the same general type as that of 

 Nereis. There is a small brain (Fig. 112, br.) situated above 

 the anterior end of the pharynx, immediately behind the 

 median dorsal jaw. It is connected by a very short pair of 

 cesophageal 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 embryonic 

 ganglia : the last ganglion is also of unusual size, and results 

 from the fusion of six distinct ganglia in the embryo. The 

 ventral nerve-cord is contained in the ventral sinus. 



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

 five pairs 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 

 a series of papillae, the lateral sense organs, with which they 

 are obviously homologous. The margin of the anterior 

 sucker also bears a large number of goblet-shaped organs, 

 which are very probably organs of taste. The minute 

 structure both of these and of the lateral sense organs is very 

 similar to that of the eyes. The function of the lateral 

 sense-organs is unknown. 



The Leech is monoecious. There are nine pairs of testes 

 (Fig. 112, ts.\ in the form of small spherical sacs situated in 

 segments 12-20. Each gives off from its outer surface a 

 narrow efferent duct, which opens into a common vas deferens 

 (v,d.\ In the tenth segment the vas deferens increases in 

 width and forms a complex coil, the vesicula seminalis (v. sem.), 

 from which is continued anteriorly a somewhat dilated 

 muscular tube, the ductiis ejaculatorius (d. ej.). From each 

 Man. Zoo], p 



