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A compilation of data on numbers of known species of living and extinct invertebrate animals indicates a total of approximately 1,100,000 of which 86 per cent are arthropods and 79 per cent insects. Mollusks include about 6 per cent. Discovered kinds of fossil invertebrates aggregate an estimated 80,000, and at least 78,000 of these are marine. It is evident that the paleontological record comprises a minor fraction (about 7 per cent) of the described species of invertebrate life on the globe. Even so, paleontological calibration of the fossil-bearing part of the geologic column depends more on invertebrate remains than on evidence of vertebrate animals or plants, which is explained chiefly by the enormous abundance of invertebrate skeletal parts in marine strata ranging from Lower Cambrian to Recent. Indeed, many marine sedimentary deposits are predominantly composed of such parts.

Characterization of major and minor divisions of the geologic time scale in terms of their contained invertebrate faunas depends on (1) definition of time-stratigraphic units derived mainly from early geologic studies in Europe, leading to what seems an arbitrary classification of deposits in many other areas; (2) empirical determination of significant steps in evolution as discernible in successive fossil faunas; (3) general conditions of sedimentary environment affecting the life of invertebrates and controlling preservation of their remains; and (4) completeness of discoveries of fossil invertebrates combined with adequacy of investigations and publications concerning them. A review of present knowledge of the fossil record for each invertebrate phylum or class emphasizes the disparity in importance of each such assemblage compared with others, taking the geologic time scale as a whole, and of different time-defined segments within an individual phylum or class when these are compared with each other. For example, in the phylum Mollusca, the class Cephalopoda is roughly equal in magnitude to the class Pelecypoda as regards occurrence of fossils, but cephalopods far outrank pelecypods as useful indicators of various parts of the time scale. Also, extremely varied sorts of nautiloid cephalopods in early Paleozoic periods stand in contrast to the relative paucity and lack of variety of this group in later parts of geologic time, whereas abundant kinds of ammonoid cephalopods are found to represent only late Paleozoic and Mesozoic periods and their subdivisions. These aspects of the subject are summarized and illustrated.

Acellular organisms, many of which exhibit intergrading plantlike and animal-like attributes, are designated as representatives of a separate Kingdom called Protista. Although many protistans (such as diatoms and other algae) are not classified as invertebrates, they are discussed in this paper.

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