LESSON XXIII 



HYDROID POLYPES : BOUGAINVILLEA, DIPHYES, AND PORPITA 



IT was stated in the previous lesson (p. 234) that in a 

 budding Hydra the buds do not always become detached 

 at once, but may themselves bud while still in connection 

 with the parent, temporary colonies being thus produced. 



Suppose this state of things to continue indefinitely : the 

 result would be a tree-like colony or compound organism 

 consisting of a stem with numerous branchlets each ending 

 in a Hydra-like zooid. Such a colony would bear much the 

 same relation to Hydra as Zoothamnium bears to Vorticella 

 (see p. 134). 



As a matter of fact this is precisely what happens in a 

 great number of animals allied to Hydra and known by the 

 name of Hydroid polypes. 



Every one is familiar with the common Sertularians of the 

 sea-coast, often mistaken for sea-weeds : they are delicate, 

 much-branched, semi-transparent structures of a horny con- 

 sistency, the branches beset with little cups, from each of 

 which, during life, a Hydra-like body is protruded. 



A very convenient genus for our purpose is Bougainvillea, 

 a hydroid polype found as little tufts a few centimetres long 

 attached to rocks and other submarine objects. Fig. 54, A 



