PLANT-ANIMALS, OR ZOOPHYTES. 157 



tentacles are arranged round the mouth, in one or 

 two series ; they are thread-like appendages, very 

 flexible, and extensile, and are rough on the surface 

 with a vast number of thread-cells, containing the 

 only organs of offence and defence which these 

 animals possess. These cells are imbedded in the 

 flesh, and each contains a long delicate thread, or 

 dart, often covered with barbs, and when at rest, lies 

 spirally coiled up in the thread-cell. Undoubtedly 

 these darts are deadly instruments when brought 

 into operation, as they bury themselves in any soft 

 substance with which they come in contact, and pro- 

 bably convey some poisonous fluid into the wound. 



When the food is collected at the mouth, by means 

 of the tentacles, and conveyed into the stomach, it is 

 digested, and prepared for the support of the whole 

 structure, and then passes down by the posterior 

 opening into the canal, which traverses the whole 

 colony. The circulation through this canal is very 

 simple, the walls of the canal are lined with vibratile 

 cilia, which, by their motion, keep up a continuous 

 stream, from the stomachs of the polyps towards the 

 trunk, and back again to the remotest branchlet, and 

 thus every part of the structure is in communication 

 with the myriad mouths, and is constantly supplied 

 with all that is necessary to maintain the life, health, 

 and vigour of the entire community. 



The individual polypes often die, or become ab- 



