OBOAN-riPE CORAL. 175 



fctandiug the tubular form assumed by the hardened 

 envelopes. The polypi in the tubipora are, in fact, 

 completely independent of one another. When 

 fresh inhabitants are added to the colony, they grow 

 by placing themselves parallel to tlieir predecessors, 

 to whom they are attached by transverse partitions. 



Fig. 31. — Organ-pipe Conil. 



Each polypus lives and grows in its own tube, and 

 holds no relations with the others, exce})t that of good 

 neighbourhood, when it leaves its abode to spread its 

 snares for the little marine animals. It is not so 

 with the true hydras. In them the hardened enve- 

 lope resembles a continuous canal, uniting all the 

 polypi one with another. At whatever point of the 

 common trunk a bud is produced, the calcareous 

 matter envelopes it, and establishes its immediate 

 relaliuuship with the rest of the colony. 



