3. Radiolaria:—Possessing a central membranous capsule. Usually supported by a flinty skeleton.
4. Infusoria:—Outer protoplasm firmer and denser; therefore of more definite shape.
Possess permanent threadlike extensions of protoplasm instead of pseudopods.
We shall now observe the principal marine members of the protozoa, commencing with the lowest forms, and dealing with each in its proper zoological order as expressed in the above table.
Marine Rhizopods
When we stand on a beach of fine sand on a very calm day watching the progress of the ripples over the sand as the tide recedes we frequently observe whitish lines marking the limits reached by the successive ripples as they advance toward the shore. If, now, we scrape up a little of the surface sand, following the exact course of one of these whitish streaks, and examine the material obtained by the aid of a good lens, we shall in all probability discover a number of minute shells among the grains of sand.
These shells are of various shapes—little spheres, discs, rods, spirals, &c.; but all resemble each other in that they are perforated with a number of minute holes or foramina. They are the skeletons of protozoons, belonging to the class Rhizopoda, and they exist in enormous quantities on the beds of certain seas.
Fig. 54.—A group of Foraminifers, magnified
We will first examine the shells, and then study the nature of the little animals that inhabit them.