52 Invertebrate Zoology. 



The Circulatory System is with difficulty followed out. 

 There is a dorsal and a ventral vessel, much as in Nereis. 



The Muscular System. What is the arrangement of 

 the muscle fibres of the integument ? How many longitu- 

 dinal bands are there? Examine again the pharyngeal 

 retractors. 



Draw. 



The Reproductive System. The sexes are separate. 

 The sexual glands are situated near the base of the ventral 

 pharyngeal retractors. The eggs or spermatozoa reach the 

 exterior through the single pair of nephridia. Fertilization 

 occurs after extrusion. 



The Excretory System. The nephridia are a pair of 

 large brown tubes which lie, somewhat freely, in the neigh- 

 borhood of the retractors. Their openings to the exterior 

 lie a little in front of the origin of the dorsal retractors. 



Draw. 



The Nervous System. The ventral cord lies along the 

 mid-ventral line. Examine with a lens and see if it is pro- 

 vided with ganglia. Do nerves leave the ventral cord? 

 Find the ccsophageal commissures and trace them to the 

 brain or supra-cesophageal ganglion. Are there pigmented 

 eye-spots ? 



Draw. 



