Chordata

“ The vertebral axis may be either cartilaginous as in some fishes, or bony as in the greater number of animals belonging to this sub-kingdom.
Chordata.—
LINKS BETWEEN THE INVERTEBRATES AND FISHES.—The curious little ascidians or “sea-squirts,” belonging to the group Tunicata, are held by some authorities to be the degenerate descendants of a free-swimming animal having a complete notochord and nerve-tube, structures which are now only seen in the tails of their tadpole-like larvae. ”

“ Fishes, constituting the class Pisces, may be defined as Craniate Vertebrata, or Chordata, in which the anterior portion of the central nervous system is expanded into a brain surrounded by an unsegmented portion of the axial skeleton; which are provided with a heart, breathing through gills; and in which the limbs, if present, are in the form of fins, as opposed to the pentadactyle, structure common to the other Vertebrata. ”

“ The axis of the backbone of all vertebrates is formed by an elastic rod known as the 'notochord,' which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the phylum Chordata. ”

“ Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery (figs. 405 C and 410, po) . They appear to be derived from the epithelial cells amongst which they lie; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as the germinal epithelium. ”

“ Although the animals which are considered in this chapter are not admitted by all zoologists to be related to the Vertebrates, there can be no question that their respiratory organs closely resemble typical gill-slits. Since, moreover, they possess structures which can be regarded, with a fair amount of probability, as agreeing in essential respects with the notochord and the tubular dorsal nervous system of Vertebrates, it appears justifiable to include them in the Chordata, which are then subdivided into (1) Hemichordata, in which a "notochord" occurs in the anterior end of the body only ”

“ The system of binomial nomenclature has led to the development of the science of taxonomy, the systematic classification and naming of plants and animals according to their relationships.
THE UNITS OF CLASSIFICATION
The world of organic life has been divided into the plant and animal kingdoms. These kingdoms have been further divided into larger divisions called phyla (from the Greek word phylon, a race) . Each phylum is composed of organisms with certain characteristics in common. For example, all animals with a spinal cord (or notochord) are assigned to the phylum Chordata. ”

“ The Tunicata or Urochordata are hermaphrodite marine Chordate animals, which show in their development the essential Vertebrate characters, but in which the notochord is restricted to the posterior part of the body, and is in most cases present only during the free-swimming larval stages. The adult animals are usually sessile and degenerate, and may be either solitary or colonial, fixed or free. The nervous system is, in the larva, of the elongated, tubular, dorsal, Vertebrate type, but in most cases it degenerates in the adult to form a small ganglion placed above the pharynx. ”

“ Naturalists believe that all animals having a notochord at any stage in their existence are related to each other through being descended from a common ancestor, the first or oldest chordate or back-boned animal. In fact it is because all these chordate animals—the lancelets, lampreys, fishes, batrachians, the reptiles, the birds, and the mammals—have descended from a common ancestor that they all develop a notochord, and those most highly organized replace this by a complete back-bone. ”

“ It shows that every vertebrate, like every other animal, develops from a single cell, but that the course of its embryonic development is peculiar, and characterised by embryonic forms that are not found in the invertebrates. We find in them especially the chordula, or chorda-larva, a very simple worm-shaped embryonic form, without limbs, head, or higher sense-organs; the body consists merely of six very simple primitive organs. ”

“ But we may just as confidently affirm that the main features of their organisation have been preserved by heredity.
We must grant, however, that in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty—namely, that there was a long series of very different and totally extinct forms between the Gastraea and the Chordaea. Even in this modified chordaea-theory the six fundamental organs of the chordula would retain their great value. ”

“ General Structure.—The general plan of organisation of the body (see Fig. 71) is that a longitudinal skeletal axis, the notochord (nch) , separates a dorsal nervous system (sp.cd) from a ventral reduced coelom (coel) , in which lie the alimentary canal (int) , the gonads (gon) , and other organs. Thus a transverse section of the body (see Fig. 72) shows the typical Chordate arrangement of parts, and is comparable with a transverse section of a tadpole, a young fish, or a larval Ascidian. ”

“ The Protochordates.—Modern researches have shown that besides the ordinary back-boned animals certain other creatures easily to be mistaken for mollusks or worms and being chordate in structure must be regarded as offshoots from the vertebrate branch. These are degenerate allies, as is shown by the fact that their vertebrate traits are shown in their early or larval development and scarcely at all in their adult condition. ”

“ The reproduction takes place as far as is known at present, only by division. This, in many salt water forms, may take place in the swarming condition, and, in that case, is always parallel to the longitudinal axis. The daughter-individuals, each of which retains half of the original shell, sometimes do not separate at once from each other, and thus chains (e.g. in Ceratium) of several connected individuals may be formed. In others, the division occurs after the cilia have been thrown off and the cell-contents rounded. The daughter-cells then adopt entirely new cell-walls. ”

“ In the earlier stages of development these special characters of the placentalia are not to be detected. The significant embryonic or larval form of the chordula, which succeeds the gastrula, has substantially the [Pg 65] same structure in all vertebrates; a simple straight rod, the dorsal cord, lies lengthways along the main axis of the shield-shaped body—the “embryonic shield”; above the cord the spinal marrow develops out of the outer germinal layer, while the gut makes its appearance underneath. ”

“ Thus in the salt-water Crustacean, Artemia salina, which possesses 168 minute granular chromosomes, each of these chromosomes must be regarded as an id, for each can in certain circumstances be thrown out from the ovum by the reducing division, or it can be brought into the most various combinations with other chromosomes by fertilization. Each of them must therefore consist of perfect germ-plasm in the sense that all the parts of an individual are virtually contained in it; each is a biological unity, an id. ”

“ THE PROCHORDONIA. Man's ancestors have substantially the organisation of a simple unarticulated Chordonium (Copelata and Ascidia-larvae) . The unsegmented chorda develops between the dorsal medullary tube and the ventral gut-tube. The simple coelom-pouches divide by a frontal septum into two on each side; the dorsal pouch (episomite) forms a muscle-plate ”

“ The analogy indeed in structure and development between chorda and cartilage cells and the cells of plants seemed to him complete. The substance of the notochord consisted of polyhedral cells having attached to their wall an oval disc similar in all respects to the nucleus of the plant-cell, and like it containing one or more nucleoli. Inside the mother-cell were to be found young developing cells of spherical shape, lacking however a nucleus. Cartilage was even more like plant tissue. It was composed of cells, each with its cell membrane. ”

“ A parthenogenetic ovum gives off one polar body—though without reduction in the number of chromosomes—and then proceeds by asexual multiplication or ordinary cell division to build up a body. The spore of a fern or a moss has only half the number of chromosomes that the cells of its producer have, yet it proceeds by asexual multiplication or ordinary cell-division to build up the gametophyte or sexual generation. ”

“ If a protozoon, the weaker progeny arising by division of it will originate an inferior stirp, unable to compete successfully with that arising from the sister-cell endowed with a larger portion of chromatin. By continual elimination of the varieties which produce unequal halving, necessarily at a disadvantage if a moiety of their members tend continually to disappear, there will be established a variety in which the halving is exact: the character of this variety being such that all its members aid the permanent multiplication of the species. ”

“ A study of the phenomena of polymorphism in hydroids and insects will beautifully and most interestingly illustrate latent hereditary units.
Much that is speculative and fanciful is included under the subject of atavism, and the safest plan for pathologists and biologists, in considering any abnormality, is to remember a golden rule of Gegenbaur, that only those structures are reversional which are taxonomically not far distant or phylogenetically not very old. Embryology is also a very important check in considering such subjects. ”

“ It is indeed usually assumed that the mitotic mechanism is not merely for the distribution, but for the equal distribution, of the sister granules to the two daughter nuclei. The conspicuous part the chromatin is seen to play in the whole mechanism of heredity—in maturation, fertilization and development—indicating as it does that the chromatin is the chief, if not the only, bearer of the specific qualities of the organism, sufficiently clearly emphasizes the importance of the equal distribution of this substance between the daughter cells at successive cell-divisions. ”

“ The North American and Kamtschatkan species—the Laminaria longicrucis—has a frond as large as a table-cloth, and a stem of proportionate length. The English species attain very frequently to six or eight feet, although in their native habitats they may be gathered of every size, and in every stage of growth; and to reduce such giants to the scale of a few inches would give no idea of their grandeur or beauty.
Of those immensely long and slender sea-weeds, placed by algologists in a distinct genus, with the expressive name of Chorda, little use, I think, can be made in the way of design. ”

“ Graber classifies the chordotonal organs into truncal and membral. In Coleoptera and Trichoptera they may occur on several joints of the leg; others are more localized,—thus he distinguishes femoral (Pediculidæ) , tibial (Orthoptera, Perlidæ, Formicidæ) , and tarsal organs (Coleoptera) .
A type of chordotonal organ, observed in the body-segments of the larvæ of several insects by Leydig, Weismann, Graber, Grobben, and Bolles Lee, is to be seen in the transparent larva of Corethra (Fig. 296) , where the auditory organ extends to the skin. ”

“ According to Nägeli, the stages of phylogeny are based upon true qualitative differences in the idioplasm: the germ-plasm of a worm is qualitatively different from that of Amphioxus, a frog, or a mammal. But if such phyletic stages occur crowded together in the ontogeny of a single species, they are said to be based upon different ‘conditions of tension and movement’ of one and the same idioplasm! ”

“ Section of the egg of the beetle, Calligrapha, showing the pigment at one end where the germ cells will later develop as shown in the other two figures. (After Hegner.) At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance. ”

“ Grafts are, however, parts of the soma of a previously existent tree, and we are not therefore concerned, in this method of propagation, with a succession of generations, but with the successive distribution of one and the same individual over many wild stocks. But no one will doubt that one and the same individual can be gradually changed during the course of its life, by the direct action of external influences. The really doubtful point is whether such changes can be transmitted by means of the germ-cells. ”

“ But how can a course of development, which seems to be impossible in phylogeny, occur as the regular method of ontogeny? And quite apart from the question of possibility, we have to ask for proofs of the actual occurrence of cyclical development. Such a proof would be afforded if it could be shown that the nucleoplasm of those somatic cells which (e.g. in Hydroids) are transformed into germ-cells passes backwards through many stages of development into the nucleoplasm of the germ-cell. ”