AN INTRODUCTION TO ZOOLOGY 



The rate of regenerative growth depends upon the amount of 

 old tissue removed. Thus, if only a few segments of the posterior 

 end are cut off, a new tail regenerates very slowly; if more is 

 removed, the new tissue is added more rapidly. In fact, the rate 

 of growth increases up to a certain point as the amount removed 

 increases. The factors regulating the rate of regeneration have 

 not yet been fully determined, although several possible explana- 

 tions have been suggested. 



Grafting. Pieces of earthworms may be grafted upon 

 other worms without much difficulty. Several results are 

 shown in Figure 97. Three pieces may be so united as to 

 produce a very long worm (A); the tail of one animal may 

 be grafted upon the side of another, producing a double- 

 tailed worm (B); or the anterior end of one individual may 

 be united with that of another (C). In all these experiments 

 the parts were held together by threads until they became 

 united. 



2. ANNELIDS IN GENERAL 



The two chief classes of Annelids are the Chaetopoda or Earth- 

 worms and marine Annelids, and the Hirudinea or Leeches. 

 Class Chaetopoda may be divided into two subclasses, the Oli- 

 gochaeta, to which the earthworm belongs, and the Polychaeta. 

 Table VI contrasts the characters of these two divisions, and 

 Figure 98 shows one of the most common Polychaets, the marine 

 annelid, Nereis. 



The members of the Class Hirudinea have neither setae nor 

 parapodia. They move by means of a ventral sucker near the 

 posterior end, and a sucker mouth on the ventral surface at the 

 anterior end. They are flattened dorso-ventrally. The grooves 

 on the outside of the body do not correspond in number or posi- 

 tion to the septa within, there being several external rings to each 

 internal segment. The ccelom is not as large as in the Chaetopods, 

 being reduced to the cavities in which the germ cells lie, and a 



