98 



The Unity of the Organism 



only do the hydrocladia possess fewer and fewer hydroth- 

 ecae, but the dimensions of the latter through the mesial 

 nematophore reaches its minimum more and more rapidly. 

 Since the hydrothecae, once formed, do not enlarge with age, 

 it is clear that for such colonies as this, there is a limit of 

 growth and a specific form." 1 



This correlation and proportionality among repetitive 

 parts is frequently observed within the bounds of particular 



a. 



FIGURE 57 TENTACLE OF HALOCYNTHIA JOHNSONI (SCHEMATIC; AFTER 

 RITTEH). 

 'a, axis, a', axis of branch, b', b", primary and secondary branches. 



organs, as for example, in the branching tentacles occurring 

 in various groups. A detailed study of one of these cases 

 was made by me some years ago on the tentacles of an as- 

 cidian. Figure (pi. 12, figure 13) of this study, sup- 

 plemented by the following statement, illustrates the 

 point. "Although this figure is diagrammatic in a way, it is 

 accurate as to numbers of branches. The positions, too, of 

 all the branches and length of the primaries were deter- 

 mined by micrometer measurements, and the secondaries were 

 drawn as accurately as possible." 2 For the rest, the figure 

 (Figure 57) tells its own story. 



