272 



GENERAL ZOOLOGY 



the cells of the branches are probably supplied with food 

 by the polyps nearest at hand. 



Reproduction in Bougainvillia may be a mere increase in 

 the number of polyps by a process of budding similar to 

 that described for the hydra. Occasionally, however, a bud 



develops into a structure totally dif- 

 ferent from an ordinary polyp, and 

 after a series of stages, indicated in 

 Fig. 142, 2, 3, 4, finally becomes sepa- 

 rated from the colony and floats 

 away as a free-swimming individual 

 (Fig. 143). This is called the medusa 

 stage of the animal. The medusa is 

 shaped something like an umbrella. 

 The structure hanging within the 

 cavity of thebell is called ihemanubri- 

 um. The mouth is located at the end 

 of the manubrium (Fig. 143, 8). The 

 gastrovascular cavity sends out four 

 slender tubes (Fig. 143, 4) radially 

 to the margin of the body, where a 

 circular canal joins them. Tentacles 

 (Fig. 143, 2) stream downward from 

 the margin, and at their bases are 

 minute sense organs (Fig. 143, 5). 

 There are male and female medusa?. The ovaries and the 

 spermaries .of Bougainvillia are located on the manubrium. 

 When the eggs and spermatozoa are ripe, they pass out 

 through the mouth, and fertilization takes place in the 

 water outside. The embryo swims about for a time and 

 then settles to some fixed or floating object, attaches itself, 

 and soon another colony of the hydroid (hydra-like) stage 

 is developed by budding. The life history of Bougainvillia 

 illustrates alternation of generations. The fixed, colonial, 



Fig. 143. Medusa. (Much 

 enlarged) 



1, bell ; 2, tentacle ; 3, manu- 

 brium ; 4, radial tubes ; 5, 

 sense organs. (After Allman) 



