104 ANNELIDA. 



5. Between the ninth and tenth and the tenth and eleventh 

 somites, on the ventral side, are two pairs of white, rounded 

 pouches, the seminal receptacles, that open externally but not 

 internally. Understand their function. Make a drawing of 

 the reproductive system. 



Nervous System. — 1. On the dorsal surface of the pharynx, 

 near its anterior end, are the two cerebral ganglia. They lie on 

 either side of the median line and are connected by a stout com- 

 missure. In what somite do they lie ? 



2. The remainder of the ganglia lie ventral to the alimentary 

 canal. The first ventral ganglia are connected with the cerebral 

 ganglia by connectives that pass around the sides of the pharynx. 

 Adjacent ganglia of the ventral chain are united by connec- 

 tives. The ganglia of each somite, and the cords that connect 

 those of adjacent somites, are fused so that the original paired 

 condition is not very apparent. How far does the ventral chain 

 of ganglia extend ? Where do nerves leave it ? 



Draw the nervous system into the figure that shows the repro- 

 ductive system. 



Notice the sacs that inclose the setae and indicate them in 

 the above figure. 



Examine prepared serial microscopic sections.^ 



1. The cuticle will probably be absent in most sections, in 

 which case the outer covering will be the cellular hypodermis or 

 skin. How many cells thick is this layer? Look for the gland 

 cells that keep the living worm moist. Do you know how the 

 cuticle is formed? 



2. Beneath the hypodermis is the circular muscle layer, which 

 is followed by the longitudinal muscle layer. The fibers of the 



* Small worms should be kept in a dish and fed on clean moistened 

 filter-paper, which they will eat readily, until the alimentary canal is free 

 from grit, before they are preserved for sectioning. It is well to narcotize 

 them by placing them in a small quantity of water and adding a little 

 alcohol from time to time (never enough to make the worms squirm vio- 

 lently) until they cease to move. They may then be killed with subli- 

 mate acetic or other killing agent and treated in the usual manner. 



