254 HYDROID POLYPES LESS, xxm 



bearing round its edge a number of close-set tentacles, and 

 on its under side a central tubular organ (hy) with a ter- 

 minal mouth, like the manubrium of a medusa, surrounded 

 by a great number of structures like hollow tentacles (hy')> 

 The discoid body is supported by a sort of shell having the 

 consistency of cartilage and divided into chambers which 

 contain air (B, sh). 



Accurate examination shows that the manubrium-like 

 body (hy) on the under surface is a hydranth, that the short, 

 hollow, tentacle-like bodies (hy'} surrounding it are mouthless 

 hydranths, and that the disc represents the common stem of 

 Diphyes or Bougainvillea. So that Porpita is not what it 

 appears at first sight, a single individual, like a Medusa or a 

 Hydra, but a colony in which the constituent zooids have 

 become so modified in accordance with an extreme division 

 of physiological labour, that the entire colony has the char- 

 acter of a single physiological individual. 



It was pointed out in the previous lesson (p. 233) that 

 Hydra, while morphologically the equivalent of an indefinite 

 number of unicellular organisms, was yet physiologically a 

 single individual, its constituent cells being so differentiated 

 and combined as to form one whole. A further stage in this 

 same process of individuation is seen in Porpita, in which not 

 cells but zooids, each the morphological equivalent of an 

 entire Hydra, are combined and differentiated so as to form 

 a colony which, from the physiological point of view, has 

 the characters of a single individual. 



