Page:Popular Science Monthly Volume 82.djvu/425

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As long ago as 1889, while working on the development of the eyes of arthropods, the author discovered that the forebrain of the embryo scorpion is gradually covered by an overgrowing fold of skin that converts the brain into a hollow vesicle. During this process, one or two pairs of eyes are transferred from the outer surface of the head to the blind end of a median tube that projects from the membranous roof of the brain.

The details of the whole process by which the eyes were transferred from the outer surface of the head to the inside of the brain were unique in the invertebrates, and so similar to what takes in the formation of the rudimentary pineal eye of vertebrates, that it clearly pointed to some intimate genetic relation between the two groups.

To test what at first sight appeared to be so improbable, a careful study of the anatomy and development of several types of arachnids was made, and, much to our astonishment, it was found that the brain of the arachnids resembled that of the vertebrates in its general shape, in its subdivision into several regions, in the general nature of the functions performed by these regions, and in the character of their appropriate nerves, ganglia and sense organs; that the arachnids possessed skeletal structures comparable, respectively, with the dermal bones, cranium, gill-bars and notochord of vertebrates; and finally it was seen that the development of the embryo and the formation of the germ layers in the arachnids, not only harmonized with, but illuminated the corresponding conditions in the vertebrates.

It was evident that in their fundamental structure the arachnids resembled the vertebrates more than did any other invertebrates; and they resembled them in so many different ways that it became more and more improbable that all these resemblances could be mere coincidences, or could be reasonably accounted for as duplications of structure due to similar functions, or to environment, or to any conceivable cause other than community of origin. Nevertheless, it was hardly possible that the vertebrates came from modern air-breathing scorpions, or spiders, for the lowest vertebrates undoubtedly came from marine animals.

But the modern land arachnids are descendants of a large group of very ancient marine arachnids, the trilobites and merostomata, or giant sea-scorpions, which flourished in the early Cambrian and Ordovician periods, long before any vertebrates were known to exist. They were also found, although in rapidly diminishing numbers, in the two