92 ANNELIDA 



side of the prostomium is a thick palp. Determine which 

 parts of the worm are most sensitive by gently stimulating 

 with a needle. 



7. Also in the head find the peristomium, the segment 

 which surrounds the mouth and bears four pairs of peris- 

 tomial cirri. Stretch the mouth with forceps. 



Make an enlarged drawing of the head. 



8. Hold it down against the bottom of the dish or place in 

 fresh water for a few minutes to induce it to protrude the 

 proboscis, the protrusible anterior portion of the alimentary 

 canal. This is lined with chitin and armed with numerous 

 denticles and a pair of lateral jaws. 



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 setae. 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? Examine a few of the small setae 

 microscopically. What is their structure? Why is it desir- 

 able to have so many of the small setae? Can you give any 

 reason for Nereis having more setae than the earthworm? 



Observe that each parapodium has a small dorsal and a 

 small ventral cirrus, and that the main portion of both noto- 

 podium and neuropodium has the form of a flattened blade, 

 somewhat divided into lobes. The largest lobe of the noto- 

 podium 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 neuro- 

 podial cirri. 



Internal Structure. — For dissection use a specimen that 

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



