84 HISTORY OF BRITISH ZOOPHYTES. 



upwards, forming . additional cells^ till the polypidom has 

 attained its usual size. 



The polypes, with the exception of the Tubularina, can 

 retreat within their cells, hiding themselves from danger. 

 Their body is very contractile, and can change rapidly from 

 a cyHndrical to a globular, or from a globular to a cylindri- 

 cal form. The tentacula, w'hich are irregular in number, 

 can be extended to a great length, or can instantly contract 

 into little knobs, shrinking within the cell. In the centre 

 of the circle formed bv the tentacula is the mouth of the 

 polype, pouting upwards, and ready to receive whatever prey 

 the prehensile tentacula from time to time bring to it. 



The hydroid zoophytes increase by buds or eggs. AYlien 

 the increase is by buds, it may be said to be a perpetuation 

 of the same individual animal, as a plant perpetuated by a 

 layer. When it is by eggs, new individual animals of the 

 same species are produced. Every species begins its exist- 

 ence by a single polype, wliich grows up to a polypidom, 

 containing, it may be, hundreds of polypes. Darwin, in 

 his ' Temple of Nature,' thus sings : — 



" New buds and bulbs the living fibre shoots 

 On lengthening branches and protruding roots ; 

 Or, on the father's side, from bursting glands 

 The adherin;; young its nascent form expauds ; 



