Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 565 



worm. As this occurs the edges of the belt come together and a sealed capsule 

 or cocoon results. Within this the young worms develop and in about three 

 weeks, at least one or two of them emerge and make their way into the soil 

 without going through a swimming stage such as the trochophore of the marine 

 annelids. The cocoons of young earthworms, about the size of apple seeds, are 

 numerous in moist compost heaps in spring and summer. 



Regeneration. Adult earthworms can regenerate segments removed from 

 the ends of the body, accidentally or by experiment. According to recent in- 

 vestigations of G. B. Moment (1953), the complete number of segments is 

 present in the earthworm when it hatches and that number is not exceeded 

 either by its usual growth or by regeneration. No more than five new segments 

 will regenerate at the anterior end and no head will regenerate if 1 5 or more 

 segments have been cut off. Various combinations can be made by grafting 

 pieces together, fastening them by threads until they become united. It is 

 doubtful whether regeneration contributes to their survival as it does to 

 planarians and starfishes. In Tubifex, a common fresh-water oligochaete, the 

 posterior end of the worm regenerated 31 new segments in 32 days. This is 

 largely due to the totipotent cells (neoblasts) which migrate to the cut sur- 

 face, multiply and differentiate into one or another kind of cells during the 

 regeneration. Totipotent cells are those that have kept their embryonic charac- 

 ter and have the power to multiply with great rapidity somewhat as cancer 

 cells do. It is worth notice that such cells are killed by x-rays. 



Other Oligochaetes 



Most of the 2400 species of oligochaetes are smaller than Lumbricus 

 terrestris. The 10-foot giant earthworms of Australia are impressive excep- 

 tions. Two species of small earthworms are common all over North America; 

 one of them (AUoIobophoro caliginosa) lives in the soil; the other (Eisenia 

 [old name Allolobophora] foetida) lives in compost. 



The majority of oligochaetes are aquatic. The young ones called naiads are 

 transparent little bristle worms familiar to anybody who examines pond- 

 sweepings under the microscope. Slender red worms, Tubifex tubifex, about 

 an inch and a half long, live in tubes with their "tails" waving above the sur- 

 face of mud, usually odorous from decaying organisms. Milk-white enchy- 

 traeids {Enchytraeus albidus) about half an inch long are sold at pet shops for 

 turtle and fish foods. 



Class Polychaeta 



The Clamworm 



Ecology. Several species of clamworms live on sandy shores and clam flats 

 on both Atlantic and Pacific coasts of North America. The large clamworm. 

 Nereis virens, often a foot long, is one of the commonest annelids on the New 



