PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 115 



tions, acclimate more rapidly and more completely than the large ani- 

 mals. They become active earlier and show less differential death than 

 the older individuals. After a week or two disintegration usually begins 

 to appear in the posterior region of the anterior zooid in some of the larger 

 animals, that is, just anterior to the level of fission, and progresses ante- 

 riorly from this region (Fig. 34, A). The body gradually becomes sepa- 

 rated into two independent pieces, the posterior zooid region and the an- 

 terior part of the anterior zooid (Fig. 34, B). After progressing a greater 

 or less distance anteriorly in the anterior zooid, the disintegration may 

 cease, and healing of the wound and regenera- 

 tion of a new posterior end may occur slowly 

 in the same concentration that brought about 

 the disintegration (Fig. 34, C). The posterior 

 zooid may also remain active and slowly re- 

 generate a new head (Fig. 34, C). In a num- 

 ber of individuals of the same length in the 

 same container, differences in the time of disin- 

 tegration and the rate and amount of its prog- 

 ress anteriorly occur. Some individuals may 

 disintegrate completely (the head region and 

 the posterior zooid last of all) , while in others 

 disintegration is limited to the posterior part 

 of the anterior zooid, and both anterior piece 

 and posterior zooid or zooids regenerate and re- 

 ,main alive indefinitely. Small, physiologically 

 young animals usually remain alive and intact 



in concentrations in which partial death of the large, old individuals oc- 

 curs; in slightly higher concentrations they may show a similar partial 

 death. The range of concentrations giving these results differs with tem- 

 perature, nutritive condition, and size of animals and must be determined 

 experimentally for a particular stock. Susceptibility to toxic and lethal 

 effects of these low concentrations increases with decrease and decreases 

 with increase of temperature, the reverse of susceptibiHty to concentra- 

 tions above the limit of tolerance. 



Occasionally heads only, or heads and a short anterior portion of a few 

 of the larger animals, may disintegrate early, and a new, more or less in- 

 hibited head develops slowly. In somewhat higher concentrations this 

 may sometimes occur in smaller animals. These cases represent the begin- 

 ning of the death gradient characteristic of higher concentrations. The 



Fig. 34, A~C. — Dugesia doro- 

 tocephala, differential tolerance 

 and differential acclimation. 



