analogy in the beautiful craters encircling a central jet (as the collar of Codosiga encircles the flagellum), which we see produced in the later stages of the splash of a pebble40. {255}
Among the Foraminifera we have an immense variety of forms, which, in the light of surface tension and of the principle of minimal area, are capable of explanation and of reduction to a small number of charôÙacôÙterôÙisôÙtic 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 chambered or ãmonothalamicã 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 mathôÙeôÙmatôÙiôÙcal 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 equiôÙlibôÙrium; 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-equiôÙlibôÙrium; and we must therefore consider that those forms which indicate symmetry and equiôÙlibôÙrium in their shells have secreted these during periods when rest and uniformity of surface conditions alternated with the phases of pseudopodial