212 TEXTBOOK OF ZOOLOGY 



Regeneration 



Earthworms have been used extensively in regeneration experi- 

 ments because they possess the ability to regenerate lost parts. It 

 has been demonstrated that when the anterior end is cut off, in 

 front of the eighteenth segment, the segments from one to five will 

 be regenerated. If the cut is made posterior to segment eighteen 

 a new anterior end will not regenerate on the tail half, but instead 

 another tail will develop from the cut surface. This produces an 

 animal with two tails and no head, and death from starvation re- 

 sults. When any part of the tail region is cut off, the lost parts 

 readily regenerate. Numerous grafting experiments have also been 

 performed on earthworms. Almost any part of an individual 

 grafted to the cut surface (if properly located) of another will fuse 

 to it and grow. In this way numerous unusual forms of earth- 

 worms have been produced. 



Class Archiannelida (ar ki a nel'i da, first Annelida). — This class 

 includes numerous small marine forms which resemble Chaetopoda 

 in a number of ways. It is now believed that they have been de- 

 rived from that group by changes usually involving the reduction 

 or loss of certain structures. They are very small and lack both 

 setae and parapodia. Internally they are very similar to the earth- 

 worm. The best known example of this group is Polygordius, 

 which has a long cylindrical segmented body with a pair of ten- 

 tacles on the prostomium. Two ciliated pits are present as a reten- 

 tion of juvenile characters. The troehophore larva is common to 

 the entire group. 



Class Hirudinea (hlr u din'e a, leech). — These animals are com- 

 monly known as the leeches. They are usually flattened dorsoven- 

 trally, possess both an anterior sucker and a posterior sucker, have 

 characteristically thirty-two segments and possess no external ap- 

 pendages. The anterior sucker is formed from the prostomium and 

 first two segments, and the posterior one comes from the last seven. 

 Each segment shows externally a variable number of annuli or rings, 

 making the animal appear to possess more segments than are really 

 present. Leeches are commonly parasitic and live by sucking blood 

 from other animals. 



In a typical leech, of which Hirudo medicinalis is a good example, 

 there is a muscular pharynx, a short esophagus, midgut or crop, 



