ANALYSIS OF FORM REGULATION. 1 63 



death-rate and in temperatures below 15° C. the regulatory- 

 processes are considerably retarded and certain complicating 

 features which result directly from low temperature appear. 

 With fairly constant temperature and regular renewal of the 

 fluid it has been possible to keep various individuals and pieces 

 alive in 1.5 per cent, alcohol for several months, and death, 

 when it occurred, was apparently the result of starvation. As 

 will appear below, however, the length of life of the pieces and 

 individuals varies greatly according to conditions. 



II. Certain Features of the Organization of Planaria. 

 In order to make clear certain of the results presented below 

 it is necessary to call attention to the fact that in Planaria doro- 

 tocephala (and in P. inaculata) the posterior region of the body in 

 all animals above a certain small size is specified to a greater or 

 less extent as a second individual or zooid, which, when it attains 

 a certain degree of physiological independence, separates from 

 the other part by an independent motor reaction and becomes a 

 complete animal. As I pointed out some years ago,^ the presence 

 of this second zooid is indicated by the regional differences in the 

 regulatory power of short pieces taken in sequence. When we 

 compare pieces not more than one tenth the length of the whole 

 animal and as nearly as possible of equal length, we find that 

 the ability to form a head decreases in each successive piece from 

 the original head-region posteriorly, until at about the region of 

 the old pharynx the power to form a head disappears entirely and 

 the pieces remain headless. But at about the middle of the post- 

 pharyngeal region, i. e., at the level where fission occurs, the 

 power of head-formation reappears rather suddenly. In species 

 like Planaria simplicissima, which do not undergo fission, there 

 is no increase in the power of head-formation in the postpharyngeal 

 region. Other regional differences which indicate the presence 

 of a second zooid in the posterior region of the body are described 

 in the paper referred to above. As further evidence along the 

 same line, I may add that I have been able to induce fission 

 experimentally without feeding under conditions where it does 



^ Child, "The Relation between Regulation and Fission in Planaria,''^ Biol. 

 Bull., XL, 3, 1906. 



