NEREIS VIRENS. 85 



Order 1. Rhynchobdellida. 



Anterior end of body forming a proboscis or in- 

 trovert. No jaws. (Glossiphonia, Macrob- 

 della, Clepsine.) 

 Order 2. Gnathobdellida. 



No proboscis or introvert. Mouth usually with 

 three teeth. (Hirudo.) 



Hatschek: Studien liber Entwicklungsgeschichte der Anneliden. Arb. 



Zool. Inst. Wien, 1, 1878. 

 Norman: Durfen wir aus den Reactionen neiderer Thiere auf des Vorhan- 



densein von Schmerzempfindungen Schliessen? Arch. ges. Physiol., 67, 



1897. 



CH-ETOPODA. 



NEREIS VIRENS. (Clam-Worm.) 



These animals may be found inhabiting mud-flats from which 

 ;he 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 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. 



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? 



