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In their internal organs arthropods agree in their general relations with the higher worms, hence most zoölogists agree that they have directly originated from the annelid worms.

The position and general shape of the digestive canal, of the nervous and circulatory systems, are the same in Arthropoda as in annelid (oligochete) worms, so much so that it is generally thought that the Arthropoda are the direct descendants of the worms. It is becoming evident, however, that there was no common ancestor of the Arthropoda as a whole, and that the group is a polyphyletic one. Hence, though a convenient group, it is a somewhat artificial one, and may eventually be dismembered into at least three or four phyla or branches.

The following diagram may serve to show in a tentative way the relations of the classes of Arthropoda to each other, and also may be regarded as a provisional genealogical tree of the branch.

We will now rapidly review the leading features of the classes of Arthropoda.

The Crustacea.