[CHAPTER XVIII]

GEOLOGICAL HISTORY OF ANIMALS (EXCLUDING VERTEBRATES)

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A STUDY of the animals of the past is not only of great interest in itself, but also it furnishes a mainstay of the great doctrine of organic evolution. At the very outset of our discussion the reader should have already in mind at least the main subdivisions of the animal kingdom in order to reasonably well understand where the important animal types of the different geological ages fit in, and how those types bear upon the doctrine of evolution. The accompanying, very brief, general classification includes the usually recognized subkingdoms with special reference to representatives of those which are of most geological and evolutionary significance. Reading downward in this classification, the degree of complexity of organization steadily increases from single-celled animals to man himself.

I.Protozoans,e.g.foraminifers (with lime carbonate shells)
II.Cœlenterates,e.g.Sponges
So-called “jellyfishes,” graptolites
Corals
III.Echinoderms,e.g.So-called “sea lilies”
So-called “starfishes”
So-called “sea urchins”
IV.Worms,
V.Molluscoids,e.g.So-called “sea mosses”
Brachiopods
VI.Mollusks,e.g.Clams, oysters
Snails{Pearly nautilus, ammonites
Cephalopods, e.g{So-called “cuttle fishes”
VII.Arthropods,e.g.Trilobites
Crabs, lobsters
So-called “sea scorpions”
Insects
VIII.Vertebrates,e.g.Ostracoderms
Fishes
Amphibians
Reptiles
Birds
Mammals (including man)

Before entering into a brief but rather systematic discussion of some of the most important types of animals which lived during geological time, it may be well for the reader to have in mind some of the most important conclusions which have been reached as a result of the study of the fossil animal records. These conclusions may be summarized as follows:

1. Animal life existed many millions of years ago.

2. Not only the animals of to-day, but also those of any given geological period, directly descended from those of preceding geological periods.