84 HISTORY OF BRITISH ZOOPHYTHES. 
upwards, forming additional cells, till the polypidom has 
attained its usual size. 
The polypes, with the exception of the Zubularina, can 
retreat within their cells, hiding themselves from danger. 
Their body is very contractile, and can change rapidly from 
a cylindrical to a globular, or from a globular to a cylindri- 
cal form. The tentacula, which 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 by 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. When 
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, which 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 adhering young its nascent form expands ; 
