NEREIS VIRENS. gg 



6. In a freshly killed or preserved worm, count the segments or 

 metameres and compare it with your neighbor's to ascertain 

 whether the number is constant. What segments, if any, are 

 devoid of parapodia? Why? 



7. In the head distinguish the prostomium, which bears the 

 four eyes and a pair of short terminal tentacles. At each side of the 

 prostomium is a thick palp. Determine which parts of the 

 worm are most sensitive by gently stimulating with a needle. 



8. Also in the head find the peristomium, the segment which 

 surrounds the mouth and bears four pairs of lateral tentacles or 

 cirri. Stretch the mouth with forceps. 



Make an enlarged drawing of the head. 



9. Find the small terminal anus and a pair of caudal cirri on 

 the last segment. 



10. With scissors cut off a parapodium close to the body and 

 observe that it has a dorsal blade and a ventral blade (notopodium 

 and neuropodium). Each of these contains a bundle of bristles 

 or set(B. What use can you ascribe to the setae ? In each bundle 

 is one very thick seta, the aciculum, which extends into the body 

 and is attached to muscles. Of what use is the aciculum? Ex- . 

 amine a few of the small setse microscopically. What is their 

 structure? Why is it desirable to have so many of the small 

 setse? Why does this animal need more than an earthworm 

 needs ? 



Observe that each parapodium has a small dorsal and a small 

 ventral cirrus, and that the main portion of both notopodium 

 and neuropodium has the form of a flattened blade, somewhat 

 divided into lobes. The largest lobe of the notopodium is very 

 thin and vascular. What function can you ascribe to it? 



Draw a parapodium. 



11. Look for the nephridiopores, minute apertures which are 

 segmentally placed on the ventral surface near the neuropodial 

 cirri. 



Internal Structure. — For dissection use a specimen that has 

 been killed and fasten it down by a pin through the head and one 

 through the posterior part. With scissors cut through the body- 

 wall, longitudinally, near the mid-dorsal line. 



