THE NERVOUS SYSTEM. 83 



cells (i.e. to coagulate the protoplasm of the 

 cells). 



3. Warm very gently over a flame until the prepara- 



tion begins to dry. Then cover with a drop 

 of dahlia solution (-25 per cent.) and allow to 

 stain for three minutes. 



4. By means of a pipette wash with water and mount 



in a small drop of glycerin on a slide. 



5. Examine under the microscope and draw the 



several stages shown. 



The vesiculse contain spermatozoa in all 

 stages of development : (1) the massed sper- 

 mocytes as they leave the testis ; (2) a stage in 

 which the tubercles, or spermatids, are more 

 distinct and are produced outwards into fila- 

 ments ; (3) later stages in which the filaments 

 are elongated to form the tails of the sperma- 

 tozoa, while the bodies of the spermatids form 

 their rod-like heads. 



Stages in the life-history of the Protozoon 

 parasite, Monocystis (p. 15), usually occur in the 

 vesiculae. 



F. The Nervous System. 



This consists of a ventral cord, swollen slightly in each 

 segment ; and a pair of dorsal ganglia in front, connected with 

 the ventral cord by a pair of connectives running round the 

 sides of the alimentary canal. 



1. The nerve-collar is a small ring of nervous substance 

 surrounding the buccal sac just in front of the 

 pharynx, in the third segment. It consists of a pair 

 of pyriform supra-cesophageal ganglia, united by 

 their broader ends in the median plane, and sending 

 large nerves to the prostomium ; a pair of lateral 

 connectives running round the sides of the buccal 

 sac ; and a pair of closely apposed ventral ganglia, 

 the first pair of the ventral series. 



