CHAPTER XII 



THE SPIRAL SHELLS OF THE FORAMINIFERA 



We have already dealt in a few simple cases with the shells x)f the 

 Foraminifera * ; 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 argument (or assumption) being 

 that the little 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 Orbulinae 

 and the flask-shaped Lagenae represent drops in equilibrium, under 

 various conditions of freedom or constraint; while the irregular, 

 amoeboid body of Astrorhiza is a manifestation not of equilibrium, 

 but of a varying and fluctuating distribution of surface energy. 

 When the foraminiferal shell becomes multilocular, 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 alterna- 

 tion 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 covermg ; 

 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 the shell are 

 of different and successive ages, so that one part of the shell is always 

 relatively new, and the rest old in various grades of seniority. 



The forms which we set together in the sister-group of Radiolaria 

 are very differently characterised. Here the cells or vesicles of 

 which each little composite organism is made up are but httle 



* Cf. pp. 420, 702, etc. 



