i8o 



CORALS 



There are still some gaps to be filled up in our knowledge 

 of the life-history of Polytrema, but it is known that before 

 the young Polytrema becomes fixed to its support it lives 

 a free life like the majority of the Foraminifera and possesses 

 a shell of three or four chambers which has a close re- 

 semblance to the shells of the genus Rotalia. This stage is 

 known as the " rotaliform young." At a subsequent stage, 

 when successive chambers have been formed around the 

 primary ones, it assumes a roughly globular form like a 

 raspberry, and if this stage continues and it becomes more 

 irregular it assumes a form like that of some species of 





. « • - ' 



:;•:: 





A. 



B. 



Fig. 



-Surface views of A, Polytrema ; B, Homotrema ; C, Sporadotrema. 

 X about :;o diams. 



Gypsina. If the Gypsina-like form finds a suitable object 

 it becomes attached to it and constructs an irregular thin 

 plate of chambers, connecting it with its host, which sub- 

 sequently increases in thickness and submerges the primary 

 chambers. At a later stage the beginning of the stem is 

 seen arising as a dome in the centre of the upper surface.^ 



The account that has been given of the general form of 

 the full-grown Polytrema applies to specimens which have 

 been able to develop freely in comparatively quiet waters 

 or sheltered places. But the coral is so brittle that the 

 stem and branches are very liable to be broken off in their 

 natural habitat in the sea, or more particularly in the process 

 of collecting and the subsequent handling of the specimens. 

 It thus comes about that the most familiar form of Poly- 

 trema is not the branching form but that of little pink 



^ For a full account of this development see Heron- Allen and Karland, 

 Zoology of the " Terra Nova " Expedition, xo\. vi. No. i, 1922, p. zzi. 



