92 INDIVIDUALITY IN ORGANISMS 



The primary form of asexual reproduction in Tubu- 

 laria is represented in Fig. 43. When the stem and 

 stolon together attain a certain length, which varies with 

 the metabolic condition of the animal but under favor- 

 able conditions may be five to eight centimeters, the 

 stolon turns away from the substratum and gives rise to a 

 hydranth; then a stem forms and elongates below this 

 hydranth, and a new stolon arises from the base of this 

 stem. This process of reproduction itself suggests that 

 the tip of the stolon is subordinate to the original 

 hydranth until it attains a certain distance from it and 

 then is able to produce a new hydranth, and experi- 

 ments show that this is true. If the original stem 

 elongates still further new hydranths may arise along 

 the stolon and at the base of the stem, as these regions 

 become physiologically isolated. 



In Corymorpha, a form related to Tuhularia, the 

 hydranth is much larger, the stem naked except near 

 the base and reaching a length of ten to twelve centi- 

 meters, and instead of a stolon the basal end is imbed- 

 ded in sand and bears delicate root-like outgrowths as 

 holdfasts (see Figs. 74, 78, pp. 143, 145). 



Planaria dorotocephala (Fig. 30, p. 82), a flatworm 

 and one of a number of species much used in experiment, 

 is a much more highly differentiated, bilaterally sym- 

 metrical form, with distinct head and "brain" and two 

 ventraJ nerve cords, and with definite, though rather 

 diffuse, alimentary and excretory organs. Sexual organs 

 appear in this form only under certain conditions. This, 

 as well as various other species of the group, undergoes 

 fission after it attains a certain variable size, the separa- 

 tion usually occurring at about the level f in Fig. 44 



