PLANT-ANIMALS, OR ZOOPHYTES. 153 



the least alarm the tentacles, and partly protruded 

 body, are with- 

 drawn into the 

 openings of the 

 polypary, and a 

 minute trap-door 

 closes over them, 

 to protect them 

 from injury. This 

 is a remarkable 

 contrivance, 

 which, although 

 not universal, is 

 nevertheless com- 

 mon, each open- 

 ing being fur- 

 nished with a tiny 

 operculum, or lid 

 (fig. 25), fixed by 

 a hinge on one 

 side, which is 

 pushed up when 

 the animal pro- 

 trudes itself, and 



f ,. . FIG. 24. SERTULARIA PUMILA (magnified}. 



falls back into its 



place when the retreat takes place. 1 It is no easy 



1 Further information for those desirous of pursuing the 



