NEREIS 91 



Order 1. Rhynchobdellida. 



Anterior end of body forming a proboscis or 



introvert. No jaws. (Glossiphonia, Macrob- 



della, Clepsine.) 

 Order 2. Gnathobdellida. 



No proboscis or introvert. Mouth usually 



with three teeth. (Hirudo.) 



Hatschek: Studien iiber Entwicklungsgeschichte der Anneliden. Arb. 



Zool. Inst. Wien, 1, 1878. 

 Norman: Diirfen wir aus den Reactionen niederer Thiere auf des Vor- 



handensein von Schmerzempfindungen schliessen? Arch. ges. Physiol., 



67, 1897. 



CHAETOPODA 



NEREIS VTRENS (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 associated. 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 para- 

 podia or swimming feet. 



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

 the earthworm in this respect and explain the basis for the 

 difference. 



3. Determine the direction of the peristaltic waves in the 

 dorsal blood vessel. 



4. Is the median ventral nerve cord visible through the 

 body wall? 



5. In an anesthetized or dead 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? Explain. 



6. In the head distinguish the prostomium, which bears 

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



