The Origin of Vertebrates

by Walter Holbrook Gaskell

At the same time one discrepancy still exists which requires consideration—viz. in no arthropod eye possessing a compound retina is the retina inverted. All the known cases of inversion among arthropods occur in eyes, the retina of which is simple, and are all natural consequences of the process of invagination by which the retina is formed. On the other hand, eyes with an inverted compound retina are not entirely unknown among invertebrates, for the eyes of Pecten and of Spondylus possess a retina which is inverted after the vertebrate fashion and still may be spoken of as compound rather than simple. It is clear that an invagination, the effect of which is an inversion of the retinal layer, would lead to the same result, whether the retinal optic nerves were short or long, whether, in fact, a retinal ganglion existed or not. Undoubtedly the presence of the retinal ganglion tends greatly to obscure any process of invagination, so that, as already mentioned, many observers, with Parker, consider the retina of the crustacean lateral eye to be formed by a thickening only, without any invagination, while Reichenbach says an obscure invagination does take place at a very early stage. So in the vertebrate eye most observers speak only of a thickening to form the retina, but Götte's observation points to an invagination of the optic plate at an early stage. So also in the eye of Pecten, Korschelt and Heider consider that the thickening, by which the retina is formed according to Patten, in reality hides an invagination process by means of which, as Bütschli suggests, an optic vesicle is formed in the usual manner. The retina is formed from the anterior wall of this vesicle, and is therefore inverted.

The origin of the inverted retina of the vertebrate eye does not seem to me to present any great difficulty, especially when one takes into consideration the fact that the retina is inverted in the arachnid group, only in the lateral eyes. The inversion is usually regarded as associated with the tubular formation of the vertebrate retina, and it is possible to suppose that the retina became inverted in consequence of the involvement of the eye with the gut-diverticulum. I do not myself think any such explanation is at all probable, because I cannot conceive such a process taking place without a temporary derangement—to say the least of it—of the power of vision, and as I do not believe that evolution was brought about by sudden, startling changes, but by gradual, orderly adaptations, and as I also believe in the paramount importance of the organs of vision for the evolution of all the higher types of the animal kingdom, I must believe that in the evolution from the Arthropod to the Cephalaspid, the lateral eyes remained throughout functional. I therefore, for my own part, would say that the inversion of the retina took place before the complete amalgamation with the gut-diverticulum, that, in fact, among the proto-crustacean, proto-arachnid forms there were some sufficiently arachnid to have an inverted retina, and at the same time sufficiently crustacean to possess a compound retina, and therefore a compound inverted retina after the vertebrate fashion existed in combination with the anterior gut-diverticula. Thus, when the eye and optic nerve sank into and amalgamated with the gut-diverticulum, neither the dioptric apparatus nor the nervous arrangements would suffer any alteration, and the animal throughout the whole process would possess organs of vision as good as before or after the period of transition.

Further, not only the retina but also the dioptric apparatus of the vertebrate eye point to its origin from a type that combined the peculiarities of the arachnids and the crustaceans. In the former it is difficult to speak of a true lens, the function of a lens being undertaken by the cuticular surface of the cells of the corneagen (Mark's 'lentigen'), while in the latter, in addition to the corneal covering, a true lens exists in the shape of the crystalline cones. Further, these crustacean lenses are true lenses in the vertebrate sense, in that they are formed by modified hypodermal cells, and not bulgings of the cuticle, as in the arachnid. We see, in fact, that in the compound crustacean eye an extra layer of hypodermal cells has become inserted between the cornea and the retina to form a lens. So also in the vertebrate eye the lens is formed by an extra layer of the epidermal cells between the cornea and the retina. The fact that the vertebrate eye possesses a single lens, though its retina is composed of a number of ommatidia, while the crustacean eye possesses a lens to each ommatidium, may well be a consequence of the inversion of the vertebrate retina. It is most probable, as Korschelt and Heider have pointed out, that the retina of the arachnid eyes is composed of a number of ommatidia, just as in the crustacean eyes and in the inverted eyes it is probable that the image is focussed on to the pigmented tapetal layer, and thence reflected on to the percipient visual rods. In such a method of vision a single lens is a necessity, and so it must also be if, as I suppose, eyes existed with an inverted compound retina. Owing to the crustacean affinities of such eyes, a lens would be formed and the retina would be compound: owing to the arachnid affinities, the retina would be inverted and the hypodermal cells which formed the lens would be massed together to form a single lens, instead of being collected in groups of four to form a series of crystalline cones.

To sum up: The study of the vertebrate eyes, both median and lateral, leads to most important conclusions as to the origin of the vertebrates, for it shows clearly that whereas, as pointed out in this and subsequent chapters, their ancestors possessed distinct arachnid characteristics, yet that they cannot have been specialized arachnids, such as our present-day forms, but rather they were of a primitive arachnid type, with distinct crustacean characteristics: animals that were both crustacean and arachnid, but not yet specialized in either direction: animals, in fact, of precisely the kind which swarmed in the seas at the time when the vertebrates first made their appearance. In the opinion of the present day, the ancestral forms of the Crustacea, which were directly derived from the Annelida, may be classed as an hypothetical group the Protostraca, the nearest approach to which is a primitive Phyllopod.

"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 Ammocœtes 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 cœlom and urinary apparatus.

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 Ammocœtes. 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.

In other cases a portion of the optic ganglion remains at the surface, when the brain sinks inwards, in close contiguity to the epidermal sense-cells which form the retina; a tract of fibres connects this optic ganglion with the underlying brain, and is known as the optic nerve. Such a retina may be called a compound retina and characterizes the lateral eyes of both crustaceans and vertebrates. Also, owing to the method of formation of the retina by invagination, the cuticular surface of the retinal sense-cells, from which the rods are formed, may be directed towards the source of light or away from it. In the first case the retina may be called upright, in the second inverted.