48 THE DEPTHS OF THE SEA. [ CHAP. I. 
of their bodies of matter in solution, developing 
but little heat, and incurring a very small amount 
of waste by any manifestation of vital activity. 
According to this view, it seems probable that at 
all periods of the earth’s history some form of the 
Protozoa-—rhizopods, sponges, or both—predominated 
greatly over all other forms of animal life in the 
depths of the warmer regions of the sea. The rhizo- 
pods, like the corals of a shallower zone, form huge 
accumulations of carbonate of lime, and it is pro- 
bably to their agency that we must refer most of 
those great bands of limestone which have resisted 
time and change, and come in here and there with 
their rich imbedded lettering to mark hke milestones 
the progress of the passing ages. 
TINDHOLM. 
