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 hj^drothecae, 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." ^ 



This correlation and proportionality among repetitive 

 parts is frequently observed within the bounds of particular 



S^ 



-V— V 



a. 



figure 57 — tentacle of halocynthia johnsoni (schematic; after 

 ritter). 

 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." ^ For the rest, the figure 

 (Figure 57) tells its own story. 



