The Origin of Vertebrates

by Walter Holbrook Gaskell

This theory implies that the vertebrate alimentary canal is a new formation necessitated by the urgency of the case, and, indeed, there was cause for urgency, for the general plan of the evolution of the invertebrate from the cœlenterate involved the piercing of the anterior portion of the central nervous system by the œsophagus, while, at the same time, upward progress meant brain-development; brain-development meant concentration of nervous matter at the anterior end of the animal, with the result that in the highest scorpion and spider-like animals the brain-mass has so grown round and compressed the food-tube that nothing but fluid pabulum can pass through into the stomach; the whole group have become blood-suckers. These kinds of animals—the sea-scorpions—were the dominant race when the vertebrates first appeared: here in the natural competition among members of the dominant race the difficulty must have become acute. Further upward evolution demanded a larger and larger brain with the ensuing consequence of a greater and greater difficulty of food-supply. Nature's mistake was rectified and further evolution secured, not by degeneration in the brain-region, for that means degradation not upward progress, but by the formation of a new food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus the great and mighty kingdom of the Vertebrata was evolved with its culminating organism—man—whose massive brain with all its possibilities could never have been evolved if he had still been compelled to pass the whole of his food through the narrow œsophageal tube, still existent in him as the infundibular tube. This, then, is the working hypothesis upon which this book is written. If this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it follows that we have various fixed points of comparison in the central nervous systems of the two groups of animals from which to search for further clues. It further follows that from such starting-point every organ of importance in the body of the arthropod ought to be visible in the corresponding position in the vertebrate, either as a functional or rudimentary organ. The subsequent chapters will deal with this detailed comparison of organs in the arthropod and vertebrate respectively.

CHAPTER II

THE EVIDENCE OF THE ORGANS OF VISION

Different kinds of eye.—Simple and compound retinas.—Upright and inverted retinas.—Median eyes.—Median or pineal eyes of Ammocœtes and their optic ganglia.—Comparison with other median eyes.—Lateral eyes of vertebrates compared with lateral eyes of crustaceans.—Peculiarities of the lateral eye of the lamprey.—Meaning of the optic diverticula.—Evolution of vertebrate eyes.—Summary.

The Different Kinds of Eye.

In all animals the eyes are composed of two parts. 1. A set of special sensory cells called the retina. 2. A dioptric apparatus for the purpose of forming an image on the sensory cells. The simplest eye is formed from a modified patch of the surface-epithelium; certain of the hypodermal cells, as they are called, elongate, and their cuticular surface becomes bulged to form a simple lens. These elongated cells form the retinal cells, and are connected with the central nervous system by nerve-fibres which constitute an optic nerve; the cells themselves may contain pigment.

The more complicated eyes are modifications of this type for the purpose of making both the retina and the dioptric apparatus more perfect. According to a very prevalent view, these modifications have been brought about by invaginations of the surface-epithelium. Thus if ABCD (Fig. [28]) represents a portion of the surface-epithelium, the chitinous cuticle being represented by the dark line, with the hypodermal cells beneath, and if the part C is modified to form an optic sense-plate, then an invagination occurring between A and B will throw the retinal sense-cells with the optic nerve further from the surface, and the layers B and A between the retina and the source of light will be available for the formation of the dioptric apparatus. The lens is now formed from the cuticular surface of A, and the hypodermal cells of A elongate to form the layer known by the name of corneagen, or vitreogen, the cells of B remaining small and forming the pre-retinal layer of cells. The large optic nerve end-cells of the retinal layer, C, take up the position shown in the figure, and their cuticular surface becomes modified to form rods of varying shape called rhabdites, which are attached to the retinal cells. Frequently the rhabdites of neighbouring cells form definite groups, each group being called a rhabdome. Whatever shape they take it is invariably found that these little rods (bacilli), or rhabdites, are modifications of the cuticular surface of the cells which form the retinal layer. Also, as must necessarily be the case from the method of formation, the optic nerve arises from the nuclear end of the retinal cells, never from the bacillary end. As in the case first mentioned, so in this case, the light strikes direct upon the bacillary end of the retinal cells; such eyes, therefore, are eyes with an upright retina.

Fig. 28.—Diagram of Formation of an Upright Simple Retina.

It may happen that the part invaginated is the optic sense-plate itself, as would be the case if in the former figure, instead of C, the part B was modified to form a sense-plate. This will give rise to an eye of a character different from the former (Fig. [29]). The optic nerve-fibres now lie between the source of light and the retinal end-cells, the layer A as before forms the cuticular lens, and its hypodermal cells elongate to form the corneagen; there is no pre-retinal layer, but, on the contrary, a post-retinal layer, C, called the tapetum, and, as is seen, the light passes through the retinal layer to the tapetum. The cuticular surface of the retinal cells forming the rods or bacilli is directed towards the tapetal layer away from the source of light, and the nuclei of the retinal cells are pre-bacillary in position, in contradistinction to the upright eye, where they are post-bacillary. The retinal end-cells are devoid of pigment, the pigment being in the tapetal layer.