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let also, the anterior end of the primitive intestine becomes the branchial cavity. But with reference to the vertebrate animals, the most important parts of the Ascidian larva are the following. It possesses a true spinal cord with a vesicularly expanded brain (ra). The distribution and position of this organ agrees accurately with the corresponding parts of the vertebrate animal, and Kupfer has even discerned the rudiments of nerves. (sss), which, if the observation is confirmed, will still more incontrovertibly establish the homology of the organ in question with the spinal cord of the Vertebrata and the nerves proceeding from it in pairs. But we know that it is not the spinal cord alone, but its combination with the vertebral column which constitutes the characteristic feature of the vertebrate animal. This vertebral column the Ascidian larva likewise possesses (c) in the form of the noto-chord, and, as in the vertebrate animal, this embryonic vertebral column lies between the intestine and the spinal cord. So far goes the accordance; henceforth, the development of this part, so important to the vertebrate animal, becomes retrogressive in the Ascidian. The rudder-like tail, with the spinal cord contained in it, and the noto-chord, are cast off when the animal becomes fixed; the larval brain which promised so well, shrinks into an insignificant nervous ganglion, and the complete animal gives no cause for suspecting its analogy with the Vertebrata.
These laborious observations prove that the Vertebrata are not the sole proprietors of the spinal cord and vertebral column, but received these organs as a heritage from lower grades of organization as their progenitors. It does not occur to the Darwinists to regard man as the
direct offspring of the present apes; neither do they infer from these observations on the Ascidian larva that vertebrate animals are descended from the Ascidians. Their accordance much rather forces us to assume an unknown primordial vertebrate family, springing from some branch of the heterogeneous division of the Annulosa. From these diverged on one side the Testacea, who might perhaps be called mischanced vertebrata, and on the other the true vertebrate animals.75
The Amphioxus which lives in the sand in shallow places on various coasts, and is daily caught by thousands at Messina for example, is five or six centimetres in length, and is compressed after the manner of a
fish, pointed at both ends, and semi- FIG. 24. Full-grown Ascidian. transparent whilst alive. It possesses no trace of limbs, at the posterior end only a pair of minute membranous margins, the indication of dorsal and caudal fins, and is so simple in its internal structure that it is usually, though inaccurately, termed a fish. Its skeleton is limited to the noto-chord, and some minute cartilaginous rods at the mouth and gills. It has no brain, and, except a small ciliated sac, perhaps to be interpreted as an olfactory organ, no sensory apparatus; the heart is tubular. And thus between the lancelet and other true fishes there exists so wide a difference that the possibility remains open that the fishes passed through some other course of development than phases like that of the Amphioxus.
Our knowledge of the genealogy of the fishes may be laid down in the following diagram:
The Marsipobranchii (Cycloştomi), it is true, exhibit important peculiarities, such as deficiency of limbs, entire absence of bony plates or scales on the integument; but the brain, heart, and vertebral column (which, although persistently cartilaginous, is far superior to that of the Amphioxus), show their direct coherence with the fishes. Fossil remains of these animals, universally known in the genus lamprey (Petromyzon), are not forthcoming, and, at the most, only their horny teeth could have been preserved.
After these manifest gaps in our knowledge, the succeeding orders of fishes present themselves in a connection all the more conspicuous. The starting-point is formed by the Elasmobranchii, to which belong the true chimeras, sharks, and rays. Brain and gills testify their kindred with the Marsipobranchii. In the construction. of the cranium, facial bones, pectoral and pelvic arches, and the anterior extremities, heart and intestine, they exhibit forms to which, as Gegenbaur has shown in his
well-known observations, the homologous parts of the Ganoids are related either as progressive developments or as reductions. Huxley has also prepared the way for a correct apprehension of these relations. To be fully convinced of this, detailed study is certainly requisite; for in its absence it is impossible to imagine, how in the Elasmobranchii the true branchial apparatus is wanting, and how the cartilaginous arch, which, in them, replaces the gills, is applied in the Ganoids, partly as the palate, and partly as the attachment for the true lower jaw, while the internal gills of the former, become the external gills of the latter; how in the skeleton of the anterior extremities, a gradual simplification may be exhibited, step by step, from the sharks and rays to the Ganoids, and especially the sturgeon, a process of which the two extremes are reached in the Teleostei on the one side and the higher Vertebrata on the other-in the latter in the multiform perfection of the arm and hand.
Of the Ganoids only scattered remnants survive, the sturgeon family and some few American and African genera, of which, as Rütimeyer says, a flight into fresh water has been the salvation. They just suffice to explain the relation of this once extraordinarily extensive group, to the Elasmobranchii as well as the Teleostei.
In the Teleostei, the metamorphosis of the organization of the Elasmobranchii initiated in the Ganoids, is carried yet further. It is only with great qualification that they can be termed "more highly developed," in the skeleton perhaps, to which older zoologists attributed too much importance. Brain, heart, the form of the extremities, and the reproductive system, are indeed distinct developments which, in combination with the external shape and integuments, have exhibited great
powers of adaptation, but have not proved capable of any further development. Comparative anatomy has vainly spent much labour in attempting to trace the condition of the higher animals from the special organization of the Teleostei, or to explain the peculiarities of the Teleostei from above downwards. It was labour lost, for the solution is to be reached only by the method indicated in the derivation of the Telcostei, through the Ganoids, from the shark-like fishes.
Hence, at the present period, a development is concluded with the Teleostei, and we must look to another grade for the transition from the fishes to the amphibians. We find one in the order of the mud-fishes (Dipnoi), scantily represented by only few species (Lepidosiren Protopterus). These fish-like animals, living in American and African rivers which dry up in the hot season of the year, are fish by right of their skeleton and scales, and some other characteristics; the skull, however, almost resembles that of an amphibian, and they also provisionally use their swim-bladders as lungs; and by thus breathing alternately water and air, they set before us the transition of the gill-breathing larvæ of the amphibians to the phase of air-breathing. Of the true fishes at the present time, they most nearly approach the family of the Crossopterygii, represented by the African Polypterus; and the discovery of a very remarkable Australian fish, the Ceratodus, confirms this affinity.
Through forms thus resembling the Dipnoi, the advance from the fishes to the amphibians was probably accomplished. But, as a scientific friend, profoundly versed in the history of development, has pointed out to me,-supporting his remark on the comparison of the