Many of these "minims of nature" consist of only one chamber, and hence are called unilocular or monothalamous; but a vast proportion consist of several chambers, and hence are called multilocular or polythalamous. The latter, however numerous their chambers or seemingly complex their structure, always originate as a single shell. The primitive jelly-sphere, or first sarcode segment, secretes around itself its appropriate calcareous envelope. Having grown too large for its habitation, it protrudes a portion of itself without, and thus forms a second segment. If by a process of spontaneous fission this segment becomes quite detached from its parent, and repeats the life and method of reproduction of the latter, a series of monothalamous shells will be formed. But if by means of a sarcode band the primitive segment maintains its connection with its immediate offspring, and this, repeating the reproductive process, does the same, a compound shell will, of course, be the result. Among the microscopic denizens of the ocean, the Polycystina rival the Foraminifera both by their number and their wonderful elegance of form and structure. Their body consists of the same viscid homogeneous plastic mass, termed "sarcode" by the naturalists; like them they are capable of protruding it through the foramina with which their shell is pierced, and consequently they are ranked with them among the Rhizopods, or root-footed animalcules, that form the lowest order of the Protozoa, the lowest class of the animal world. [Illustration: Polycystina. _a. Podocyrtis Schomburgkii. b. Haliomma Humboldtii._] It is a peculiar feature of these beautiful little shells (whose delicate sculpture frequently reminds the observer of the finest specimens of the hollow ivory balls carved by the Chinese) that they are usually surmounted by a number of spine-like projections, very frequently having a radiate disposition. Some have an oblong shape (Podocyrtis), others a discoid form (Haliomma), from the circumference of which the silicious spines project at regular intervals, so as to give them a star-like aspect. They are generally of a smaller size than even the Foraminifera, appear to be almost as widely diffused, and have also largely contributed to the structure of the earth-rind. They were first discovered by Professor Ehrenberg at Cuxhaven, on the North Sea; they were afterwards found by him in collections made in the antarctic seas, and have been brought up by the sounding lead from the bottom of the Atlantic at depths of from 1,000 to 2,000 fathoms.