128 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



soon as that has happened, and a process of growth at the 

 end will serve to drive the new head out of the so-called 

 perisarc or horny skeleton, which surrounds the stem. By 

 comparing the two figures, 1 2 e, and g, you easily find out 



a 



<j y u ^ 



..Qf. 

 a.* 



f 



Ill 



II 



FIG. 12. TUBULARIA. 



n. Diagram of the " Hydranth," with its short and long tentacles. 



b. Restitution of a new hydranth inside the perisai'C (p). 



c. The same later stage ; the tentacles are complete ; the whole hydranth will be driven 



out of the perisarc by a process of growth that occurs at the locality marked A. 



d. A stem of Tubular ia cut either at ai 5j or at a% bo, or at i c. 



e. Position of tentacles in the piece cut at aj bj. 



/. ,, ,, ,, ao b 2 , which is equal in length to aj 61. 



g. ,, ,, ,, aj c, which is half as long as a^ bj. 



that the absolute lengths of the two tentacle rings are very 

 different, and that both are in proportion 1 to the actual size 

 of the stem (Fig. 12). 



1 This statement is not strictly correct for Tubularia. I found (Archiv / 

 Entivickelungsmechanik, ix. 1899), that a reduction of the length of the 

 stem is always followed by a reduction of the size of the hydranth-primor- 

 dium, but there is no real proportionality between them. It is only for 

 theoretical simplification that a strict proportionality is assumed here, both 

 in the text and the diagram. But there is an almost strict proportionality 

 in all cases of " closed forms. " 



