v] OF THE SIMPLER FORAMINIFEHA 255 



Among the Foraminifera we have an immense variety of forms, 

 Avhich, in the hght of surface tension and of the principle of 

 minimal area, are capable of explanation and of reduction to a 

 small number of characteristic types. Many of the Foraminifera 

 are composite structures, formed by the successive imposition of 

 cell upon cell, and these we shall deal with later on ; let us glance 

 here at the simpler conformations exhibited by the single cham- 

 bered or " monothalamic " genera, and perhaps one or two of the 

 simplest composites. 



We begin with forms, like Astrorhiza (Fig. 219, p. 464), which 

 are in a high degree irregular, and end with others which manifest a 

 perfect and mathematical regularity. The broad difference between 

 these two types is that the former are characterised, like Amoeba, 

 by a variable surface tension, and consequently by unstable equi- 

 librium ; but the strong contrast between these and the regular forms 

 is bridged over by various transition-stages, or differences of degree. 

 Indeed, as in all other Rhizopods, the very fact of the emission of 

 pseudopodia, which reach their highest development in this group 

 of animals, is a sign of unstable surface-equilibrium ; and we must 

 therefore consider that those forms which indicate symmetry and 

 equilibrium in their shells have secreted these during periods when 

 rest and uniformity of surface conditions alternated with the 

 phases of pseudopodial activity. The irregular forms are in 

 almost all cases arenaceous, that is to say they have no sohd shells 

 formed by steady adsorptive secretion, but only a looser covering 

 of sand grains with which the protoplasmic body has come in 

 contact and cohered. Sometimes, as in Ramulina, we have a 

 calcareous shell combined with irregularity of form ; but here we 

 can easily see a partial and as it were a broken regularity, the 

 regular forms of sphere and cylinder being repeated in various 

 parts of the ramified mass. When Ave look more closely at the 

 arenaceous forms, we find that the same thing is true of them ; 

 they represent, either in whole or part, approximations to the form 

 of surfaces of equilibrium, spheres, cylinders and so forth. In 

 Aschemonella w^e have a precise replica of the calcareous Ramulina ; 

 and in Astrorhiza itself, in the forms distinguished by naturalists 

 as A. crassatina, what is described as the " subsegmented interior* " 

 * Brady, Challenger Monograph, pi. xx, p. 233. 



