CHAPTER XII 



THE SPIRAL SHELLS OF THE FORAMINIFERA 



We have already dealt in a few simple cases with the shells of 

 the Foraminif era * ; and we have seen that wherever the shell is 

 but a single unit or single chamber, its form may be explained 

 in general by the laws of surface tension: the assumption being 

 that the httle mass of protoplasm which makes the simple shell 

 behaves as a fluid drop, the form of which is perpetuated when 

 the protoplasm acquires its solid covering. Thus the spherical 

 Orbuhnae and the flask-shaped Lagenae represent drops in 

 equihbrium, under various conditions of freedom or constraint; 

 while the irregular, amoeboid body of Astrorhiza is a manifestation 

 not of equihbrium, but of a varying and fluctuating distribution 

 of surface energy. When the foraminiferal shell becomes multi- 

 locular, the same general principles continue to hold ; the growing 

 protoplasm increases drop by drop, and each successive drop has 

 its particular phenomena of surface energy, manifested at its fluid 

 surface, and tending to confer upon it a certain place in the system 

 and a certain shape of its own. 



It is characteristic and even diagnostic of this particular 

 group of Protozoa (1) that development proceeds by a well-marked 

 alternation of rest and of activity — of activity during which the 

 protoplasm increases, and of rest during which the shell is formed ; 

 (2) that the shell is formed at the outer surface of the protoplasmic 

 organism, and tends to constitute a continuous or all but continuous 

 covering ; and it follows (3) from these two factors taken together 

 that each successive increment is added on outside of and distinct 

 from its predecessors, that the successive parts or chambers of 



* Cf. pp. 255, 463, etc. 



