"Starting from the Protostraca," say Korschelt and Heider, "according to the present condition of our knowledge, we may, as has been already remarked, assume three great series of development of the Arthropodan stock, by the side of which a number of smaller independent branches have been retained. One of these series leads through the hypothetical primitive Phyllopod to the Crustacea; the second through the Palæostraca (Trilobita, Gigantostraca, Xiphosura) to the Arachnida; the third through forms resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and the Tardigrada must probably be regarded as smaller independent branches of the Arthropodan stock." To these "three great series of development of the Arthropodan stock" the evidence of Ammocoetes shows that a fourth must be added, which, starting also from the Protostraca, and closely connected with the second, palæostracan branch, leads through the Cephalaspidæ to the great kingdom of the Vertebrata. Such a direct linking of the earliest vertebrates with the Annelida through the Protostraca is of the utmost importance, as will be shown later in the explanation of the origin of the vertebrate coelom and urinary apparatus. {117}SUMMARY. The most important discovery of recent years which gives a direct clue to the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland is all that remains of a pair of median eyes which must have been functional in the immediate ancestor of the vertebrate, seeing how perfect one of them still is in Ammocoetes. The vertebrate ancestor, then, possessed two pairs of eyes, one pair situated laterally, the other median. In striking confirmation of the origin of the vertebrate from Palæostracans it is universally admitted that all the Eurypterids and such-like forms resembled Limulus in the possession of a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient mailed fishes the Ostracodermata, which are the earliest fishes known, are all said to show the presence of a pair of median eyes as well as of a pair of lateral eyes. This evidence directly suggests that the structure of both the median and lateral vertebrate eyes ought to be very similar to that of the median and lateral arthropod eyes. Such is, indeed, found to be the case. The retina of the simplest form of eye is formed from a group of the superficial epidermal cells, and the rods or rhabdites are formed from the cuticular covering of these cells; the optic nerve passes from these cells to the deeper-lying brain. This kind of retina may be called a simple retina, and characterizes the eyes, both median and lateral, of the scorpion group.