72 THE EVOLUTION OF CONTINUITY 



and there in the ectoderm of the tentacles, the function 

 doubtless being the paralysing of any prey seized ; and 

 there is also a very primitive communicating " nervous 

 system " represented by small stellate cells whose branching 

 processes communicate, and keep one part of the body 

 in touch with the others. These nerve-cells lie between 

 the ectoderm and endoderm. 



The Hydra zooid is only a part of the Hydra Individual, 

 and during the Summer produces other zooids similar to 

 itself by a process of budding or " gemmation." The 



Fig. 16.— 1. General plan of Hydra zooid, showing the cell- 

 layers, and the production of a daughter-zooid by gemma- 

 tion. The testis and ovary are also represented, though when 

 these were present there would be no gemmation. 

 ec, ectoderm ; en, endoderm ; t, tentacles ; c, body-cavity ; 

 d, disc of attachment ; g, developing budded zooid ; ts, testis ; 

 ov, ovary. (Partly after Shipley and MacBride.) 



2. Hydra vulgaris, with several budded zooids about to 

 be detached. (Nicholson.) 



budded zooid originates as a little protrusion of the body-wall 

 enclosing a small diverticulum of the producing zooid's 

 body-cavity. It gradually elongates, a mouth opens at its 

 distal end, tentacles arise around the mouth, and finally 

 the new zooid becomes detached and is free to lead an 

 in dependentexistence and to bud like its producer. But at 

 the commencement of Winter this discontinuous gemmation 

 ceases, and the zooids form sexual elements. These are 

 male and female, the former being produced in numbers 



