Fig. 45.—Ideal primitive vertebrate, seen from the left side. (After Häckel.) na, nose; au, eye; g, ear; md, mouth; ks, gill-openings; x, notochord; mr, spinal tube; kg, gill-vessels; k, gill-intestine; hz, heart; ms, muscles; ma, stomach; v, intestinal vein; c, body-cavity; a, aorta; l, liver; d, small intestine; e, ovary; h, testes; n, kidney canal; af, anus; lh, true or leather-skin; oh, outer-skin (epidermis); f, skin-fold, acting as a fin.
In order to trace these principles in the case of the Vertebrata, it is desirable first of all to obtain an idea of the anatomical features which most essentially distinguish the sub-kingdom as a whole.
Fig. 46.—The same in transverse section through the ovaries; lettering as in the preceding Fig. The following, then, is what may be termed the ideal plan of vertebrate organization, as given by Prof. Häckel. First, occupying the major axis of body we perceive the primitive vertebral column. The parts lying above this axis are those which have been developed from the ectoderm and mesoderm—viz. voluntary muscles, central nervous system, and organs of special sense. The parts lying below this axis are for the most part those which have been developed from the endoderm—namely, the digestive tract with its glandular appendages, the circulating system and the respiratory system. In transverse section, therefore, the ideal vertebrate consists of a solid axis, with a small tube occupied by the nervous system above, and a large tube, or body-cavity, below. This body-cavity contains the viscera, breathing organs, and heart, with its prolongations into the main blood-vessels of the organism. Lastly, on either side of the central axis are to be found large masses of muscle—two on the dorsal and two on the ventral. As yet, however, there are no limbs, nor even any bony skeleton, for the primitive vertebral column is hitherto unossified cartilage. This ideal animal, therefore, is to all appearance as much like a worm as a fish, and swims by means of a lateral undulation of its whole body, assisted, perhaps, by a dorsal fin formed out of skin.
Fig. 47.—Amphioxus lanceolatus. (After Häckel.) a, anus; au, eye; b, ventral muscles; c, body-cavity; ch, notochord; d, intestine; do and du, dorsal and ventral walls of intestine; f, fin-seam; h, skin; k, gills; ka, gill-artery; lb, liver; lv, liver-vein; m 1, brain-bladder; m 2, spinal marrow; mg, stomach; o, mouth; p, ventral pore; r, dorsal muscle; s, tail-fin; t, aorta; v, intestinal vein; x, boundary between gill-intestine and stomach-intestine; y, hypobranchial groove.
Now I should not have presented this ideal representation of a primitive vertebrate—for I have very little faith in the “scientific use of the imagination” where it aspires to discharge the functions of a Creator in the manufacture of archetypal forms—I say I should not have presented this ideal representative of a primitive vertebrate, were it not that the ideal is actually realized in a still existing animal. For there still survives what must be an immensely archaic form of vertebrate, whose anatomy is almost identical with that of the imaginary type which has just been described. I allude, of course, to Amphioxus, which is by far the most primitive or generalized type of vertebrated animal hitherto discovered. Indeed, we may say that this remarkable creature is almost as nearly allied to a worm as it is to a fish. For it has no specialized head, and therefore no skull, brain, or jaws: it is destitute alike of limbs, of a centralized heart, of developed liver, kidneys, and, in short, of most of the organs which belong to the other Vertebrata. It presents, however, a rudimentary backbone, in the form of what is called a notochord. Now a primitive dorsal axis of this kind occurs at a very early period of embryonic life in all vertebrated animals; but, with the exception of Amphioxus, in all other existing Vertebrata this structure is not itself destined to become the permanent or bony vertebral column. On the contrary, it gives way to, or is replaced by, this permanent bony structure at a later stage of development. Consequently, it is very suggestive that so distinctively embryonic a structure as this temporary cartilaginous axis of all the other known Vertebrata should be found actually persisting to the present day as the permanent axis of Amphioxus. In many other respects, likewise, the early embryonic history of other Vertebrata refers us to the permanent condition of Amphioxus. In particular, we must notice that the wall of the neck is always perforated by what in Amphioxus are the gill-openings, and that the blood-vessels as they proceed from the heart are always distributed in the form of what are called gill-arches, adapted to convey the blood round or through the gills for the purpose of aeration. In all existing fish and other gill-breathing Vertebrata, this arrangement is permanent. It is likewise met with in a peculiar kind of worm, called Balanoglossus—a creature so peculiar, indeed, that it has been constituted by Gegenbaur a class all by itself. We can see by the wood-cuts that it presents a series of gill-slits, like the homologous parts of the fishes with which it is compared—i. e. fishes of a comparatively low type of organization, which dates from a time before the development of external gills. (Figs. 48, 49, 50.) Now, as I have already said, these gill-slits are supported internally by the gill-arches, or the blood-vessels which convey the blood to be oxygenized in the branchial apparatus (see below, Figs. 51, 52, 53); and the whole arrangement is developed from the anterior part of the intestine—as is likewise the respiratory mechanism of all the gill-breathing Vertebrata. That so close a parallel to this peculiar mechanism should be met with in a worm, is a strong additional piece of evidence pointing to the derivation of the Vertebrata from the Vermes.