THE EARTHWORM AND ANNELIDS IN GENERAL 189 



PHYSIOLOGICAL STATE. From the foregoing account it might 

 be inferred that only external stimuli are factors in the behavior of 

 the earthworm. This, however, is not the case, since the physio- 

 logical condition, which depends largely upon previous stimula- 

 tion, determines the character of the response. Different physio- 

 logical states may be recognized, ranging from a state of rest in 

 which slight stimuli are not effective, to a state of great excite- 

 ment caused by long-continued and intense stimulation, in which 

 condition slight stimuli cause violent responses (145). 



Regeneration. A general account of this phenomenon has 

 already been given on pages 138-139, and this should be read in 

 order that the following paragraphs may be perfectly clear. If 

 the anterior portion of an earthworm is cut off at any point be- 

 tween the end of the prostomium and the fifteenth to the eight- 

 eenth segment, a new anterior end will grow out from the cut 

 end of the body. The piece regenerated will consist of one seg- 

 ment, if only one segment is removed; of two segments, if two 

 segments are removed; of three, four, or five segments, if three, 

 four, or five segments are removed; but never more than seg- 

 ments I to V are regenerated, regardless of the number removed 

 (Fig. 96, A), and no new reproductive organs appear if the origi- 

 nal ones were contained in the severed piece. If the cut is made 

 behind segment XVIII, a tail will grow out from the cut surface 

 of the posterior piece, producing a worm consisting of two tails 

 joined at the center (Fig. 96, B). Such a creature cannot take 

 in food, and must slowly starve to death. When the regenerated 

 part is different from the part removed, as in the case just cited, 

 the term heteromorphosis is given to the phenomenon. 



If the posterior portion of an earthworm is cut off at any point 

 between the anal segment and the twelfth to the fifteenth seg- 

 ment, a new tail will grow out from the cut surface of the part 

 remaining (Fig. 96, C). Regeneration of a tail differs from that 

 of a head, since more than five segments are replaced. The anal 

 segment develops first, and then a number of new segments 

 are introduced between it and the old tissue. 



