FORAMINIFERAN AND OTHER CORALS 179 



lips of this aperture there may be seen in well-preserved 

 specimens a number of needle-like spicules. The presence 

 of these spicules carefully arranged in this position to act 

 as scaffolding poles for the support of the new chambers 

 as they are formed has given rise to some controversy. 

 They are not composed of the same chemical substance 

 (calcium carbonate) as the walls of the chambers and are 

 not solvible in weak acids, and it is generally supposed that 

 they are the siliceous spicules of some sponges which the 

 pseudopodia have collected from the surrounding medium 

 and placed in this 

 position. 



The habit of col- 

 lecting the spicules of 

 sponges, grains of 

 sand, and other foreign 

 bodies, and incor- 

 porating them in the 

 skeletal structures is 

 found in many other 

 genera of Foramini- 

 fera, so that in this 

 respect Polytrema is 

 not peculiar; b'ut 

 there are many inter- 

 esting questions that 

 arise about this habit which require further careful investiga- 

 tion. It is, for example, very difficult to understand how the 

 Polytrema can find the required spicules in some localities, 

 how they can select spicules of the proper length and kind, 

 and how they are dissolved at a later period when the 

 calcareous secretions have surrounded them in the con- 

 struction of the chambers. 



As a final word in this very brief account of the structure 

 of Polytrema it should be said that the calcareous skeleton 

 is extremely brittle. The stem and branches can be easily 

 crushed between the finger and thumb. This is in striking 

 contrast to the next two genera to be described in this 

 chapter, which are more solidly built. 



Fig. 87. — Polytrema miniaccum. \ branch- 

 ing specimen attached to a piece of RamuUna. 

 The fragile ends of the branches are broken off, 

 showing the chambers. x 3 diams. 



