NEREIS VIRENS. 71 



Order 1. Rhynchobdellida. 



Anterior end of body forming a proboscis or in- 

 trovert. No jaws. (Glossiphonia, Pontob- 

 della, Clepsine.) 



Order 2. Gnathobdellida. 



No proboscis or introvert. Mouth usually with 

 three teeth. (Hirudo.) 



CHJETOPODA. 



NEREIS VIRENS. (Clam-Worm.) 



These animals may be found inhabiting mud-flats from which 

 the water flows at low tide. Occasionally they may be seen with 

 their head ends protruding from their burrows, but generally 

 specimens will have to be dug. Notice the conditions under 

 which the animals live and the forms with which they are asso- 

 ciated. It should also be understood that many of their worst 

 enemies are present only when the water covers their burrows. 



External Structure. 1. Examine a living worm in a dish 

 of sea-water, noting the motions of the body and of the parapodia 

 or swimming feet. 



Make a drawing of the animal. 



2. Hold it down against the bottom of the dish to induce it 

 to protrude the proboscis, the protrusible anterior portion of the 

 alimentary canal. This is lined with chitin and armed with nu- 

 merous denticles and a pair of lateral jaws. 



3. Is the general surface clean or slimy? Compare with the 

 earthworm in this respect and explain the basis for the difference. 



4. Determine the direction of the peristaltic waves in the 

 dorsal blood-vessel. 



5. Is the median ventral nerve cord visible through the body- 

 wall? 



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 



