102 ANNELIDA 



more or less ventral and is overhung by a narrow prostomium 

 surrounded by a peristomial rim. 

 A drawing is desirable. 



Sayles: External Features of Regeneration in Clymenella torquata. 



Jour. Exp. Zool., 62, 1932. 

 : Regeneration in the Polychaete Clymenella torquata. Physiol. 



Zool., 7, 1934. 



ARENICOLA^CRISTATA (Lug-worm) 



This worm lives in sand flats in U-shaped burrows. Dur- 

 ing the breeding season the burrows of females may be lo- 

 cated by the large, elongated, gelatinous egg masses. 



External Structure. — 1. Examine a living worm, noting 

 its movements. Are they associated with locomotion? Com- 

 pare with Nereis and Amphitrite. What is the effect of the 

 tube habit on locomotion? Compare the general shapes of 

 Arenicola, Amphitrite and Nereis. In what respects are the 

 shapes of the first two similar? How do you account for 

 this? Is the color of Arenicola a pigment or is it due to 

 iridescence? 



2. On the head locate the prostomium, a small, three- 

 lobed structure embedded in the dorsal surface of the first 

 segment or peristomium. Unless the specimen has been well 

 narcotized, the prostomium will be withdrawn into the nuchal 

 groove, which lies just posterior to it. Prostomial tentacles 

 and palps are absent. Compare with Amphitrite. Eyes are 

 present as minute, subepidermal structures not visible ex- 

 ternally. 



3. The body is divisible into three regions: an anterior 

 one bearing parapodia but no gills ; a middle one bearing both 

 parapodia and gills; a posterior one bearing neither. 



4. Examine a parapodium and find both notopodium and 

 neuropodium. Explain the position of the gills so near the 

 dorsal surface. Compare with Amphitrite. Examine setae 

 from both divisions of the parapodium. In the posterior re- 

 gion of the body note the small papillae which occur on the 

 fourth annulus and are probably vestigial parapodia. 



