25 



one another at a level about one-quarter ol" the distance 

 from the upper pole of the hydra (Fi^, 11). They are knob- 

 like v/hen they appear, but grow rapidly to a considerable 

 length, the few rjndodermal cells which form the core of 

 each increasing in number. Figure 11 shows a vertical sec- 

 tion of a two-tentacled polyp of the fifth week. The man- 

 ner of origin of the tentacles will be described in the 

 section on "Origin of Tentacles," under "The Medusa". 



The second pair of tentacles (Fig, 13) appear soon . 

 after the first, and by their rapid growth soon become as 

 large as the first pair from which they are then no longer 

 distinguishable. Irregularities are common in the appear- 

 ance of the tentacles of the hydra, as in the adult. It 

 frequently happens that only one of the second pair ever 

 makes its appearance (Fig. 24); or one may be slow in 

 arising, and always remain smaller than the other. On the 

 other hand, an abnormally large number of tentacles is fre- 

 quently developed; individuals with five or six tentacles 

 are not rare (Fig. 14). 



B. Fonn of Coelentoron . - 



The appearance of the tentacles is accompanied by al- 

 terations in the form of rho coel;nteric cavity. The rapid 



