The extensive beds of nummulitic limestones found in various parts of South Europe and North Africa are also composed largely of foraminifer shells, the most conspicuous of which are those already referred to as nummulites--disc-shaped shells of a spiral form, in which the older chambers overlap and hide those that enclose the earlier portion of the colony. Before concluding our brief account of these interesting marine protozoons, it may be well to point out that, although the foraminifera belong to the lowest class of the lowest sub-kingdom of animals, yet there are some rhizopods--the _Monera_, which are even simpler in structure. These are mere specks of undifferentiated protoplasm, not protected by any shell, and not even possessing a nucleus, and are the simplest of all animal beings. The second division of the Protozoa--the class _Protoplasta_--has already received a small share of attention, inasmuch as the amœba, which was briefly described as a type of the whole sub-kingdom, belongs to it. The study of the amœba is usually pursued by means of specimens obtained from fresh-water pools, and reference has been made to it in a former work dealing particularly with the life of ponds and streams; but it should be observed that the amœba inhabits salt water also, and will be frequently met with by those who search for the microscopic life of the sea, especially when the water examined has been taken from those sheltered nooks of a rocky coast that are protected from the direct action of the waves, or from the little pools that are so far from the reach of the tides as to be only occasionally disturbed. Here the amœba may be seen creeping slowly over the slender green threads of the confervæ that surround the margin of the pool. The third class--_Radiolaria_--is of great interest to the student of marine life, on account of the great beauty of the shells; but, as with the other members of this sub-kingdom, a compound microscope is necessary for the study of them. The animals of this group resemble the foraminifers in that they throw out fine thread-like pseudopods, but they are distinguished from them by the possession of a membranous capsule in the centre of the body, surrounding the nucleus, and perforated in order to preserve the continuity of the deeper with the surrounding protoplasm. They have often a central contractile cavity, and further show their claim to a higher position in the animal scale than the preceding classes by the possession of little masses of cells and a certain amount of fatty and colouring matter. [Illustration: FIG. 62.--A GROUP OF RADIOLARIAN SHELLS, MAGNIFIED]