PHYLUM ANNULATA 



209 



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) united 

 by delicate double connectives. The first, or sub-cesopha- 

 geal 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. 



v$ rt.c ou 



Fig. 121. — Transverse section of Hirudo : diagrammatic; cr, crop; d. s, dorsal 

 sinus which encloses the dorsal vessel ; /. v, lateral vessel ; «. c, nerve cord; 

 nph. t nephridium; nst., nephrostome ; ov, ovary; is, testis ; v. s. ventral sinus. 

 (After Bourne.) 



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 five segments. 

 They occupy positions taken in the succeeding segments by 

 a series of papilla?, 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 

 v 



