The Evolution of Man, vol 2 - Ernst Haeckel (sight word books .TXT) 📗

of the Catallacta. In September, 1869, I studied the development of one of these graceful animals on the island of Gis-Oe, off the coast of Norway (Magosphaera planula), Figures 2.231 and 2.232). The fully-formed body is a gelatinous ball, with its wall composed of thirty-two to sixty-four ciliated cells; it swims about freely in the sea. After reaching maturity the community is dissolved. Each cell then lives independently for some time, grows, and changes into a creeping amoeba. This afterwards contracts, and clothes itself with a structureless membrane. The cell then looks just like an ordinary animal ovum. When it has been in this condition for some time the cell divides into two, four, eight, sixteen, thirty-two, and sixty-four cells. These arrange themselves in a round vesicle, thrust out vibratory lashes, burst the capsule, and swim about in the same magosphaera-form with which we started. This completes the life-circle of the remarkable and instructive animal.

If we compare these permanent blastulae with the free-swimming ciliated larvae or blastulae, with similar construction, of many of the lower animals, we can confidently deduce from them that there was a very early and long-extinct common stem-form of substantially the same structure as the blastula. We may call it the Blastaea. Its body consisted, when fully formed, of a simple hollow ball, filled with fluid or structureless jelly, with a wall composed of a single stratum of ciliated cells. There were probably many genera and species of these blastaeads in the Laurentian period, forming a special class of marine protists.

It is an interesting fact that in the plant kingdom also the simple hollow sphere is found to be an elementary form of the multicellular organism. At the surface and below the surface (down to a depth of 2000 yards) of the sea there are green globules swimming about, with a wall composed of a single layer of chlorophyll-bearing cells. The botanist Schmitz gave them the name of Halosphaera viridis in 1879.

The next stage to the Blastaea, and the sixth in our genealogical tree, is the Gastraea that is developed from it. As we have already seen, this ancestral form is particularly important. That it once existed is proved with certainty by the gastrula, which we find temporarily in the ontogenesis of all the Metazoa (Figure 1.29 J, K). As we saw, the original, palingenetic form of the gastrula is a round or oval uni-axial body, the simple cavity of which (the primitive gut) has an aperture at one pole of its axis (the primitive mouth). The wall of the gut consists of two strata of cells, and these are the primary germinal layers, the animal skin-layer (ectoderm) and vegetal gut-layer (entoderm).

The actual ontogenetic development of the gastrula from the blastula furnishes sound evidence as to the phylogenetic origin of the Gastraea from the Blastaea. A pit-shaped depression appears at one side of the spherical blastula (Figure 1.29 H). In the end this invagination goes so far that the outer or invaginated part of the blastoderm lies close on the inner or non-invaginated part (Figure 1.29 J). In explaining the phylogenetic origin of the gastraea in the light of this ontogenetic process, we may assume that the one-layered cell-community of the blastaea began to take in food more largely at one particular part of its surface. Natural selection would gradually lead to the formation of a depression or pit at this alimentary spot on the surface of the ball. The depression would grow deeper and deeper. In time the vegetal function of taking in and digesting food would be confined to the cells that lined this hole; the other cells would see to the animal functions of locomotion, sensation, and protection. This was the first division of labour among the originally homogeneous cells of the blastaea.

(FIGURE 2.231. The Norwegian Magosphaera planula, swimming about by means of the lashes or cilia at its surface.

FIGURE 2.232. Section of Magosphaera planula, showing how the pear-shaped cells in the centre of the gelatinous ball are connected by a fibrous process. Each cell has a contractile vacuole as well as a nucleus.)

The effect, then, of this earliest histological differentiation was to produce two different kinds of cells—nutritive cells in the depression and locomotive cells on the surface outside. But this involved the severance of the two primary germinal layers—a most important process. When we remember that even man’s body, with all its various parts, and the body of all the other higher animals, are built up originally out of these two simple layers, we cannot lay too much stress on the phylogenetic significance of this gastrulation. In the simple primitive gut or gastric cavity of the gastrula and its rudimentary mouth we have the first real organ of the animal frame in the morphological sense; all the other organs were developed afterwards from these. In reality, the whole body of the gastrula is merely a “primitive gut.” I have shown already (Chapters 1.8 and 1.9) that the two-layered embryos of all the Metazoa can be reduced to this typical gastrula. This important fact justifies us in concluding, in accordance with the biogenetic law, that their ancestors also were phylogenetically developed from a similar stem-form. This ancient stem-form is the gastraea.

The gastraea probably lived in the sea during the Laurentian period, swimming about in the water by means of its ciliary coat much as free ciliated gastrulae do to-day. Probably it differed from the existing gastrula only in one essential point, though extinct millions of years ago. We have reason, from comparative anatomy and ontogeny, to believe that it multiplied by sexual generation, not merely asexually (by cleavage, gemmation, and spores), as was no doubt the case with the earlier ancestors. Some of the cells of the primary germ-layers probably became ova and others fertilising sperm. We base these hypotheses on the fact that we do to-day find the simplest form of sexual reproduction in some of the living gastraeads and other lower animals, especially the sponges.

The fact that there are still in existence various kinds of gastraeads, or lower Metazoa with an organisation little higher than that of the hypothetical gastraea, is a strong point in favour of our theory. There are not very many species of these living gastraeads; but their morphological and phylogenetic interest is so great, and their intermediate position between the Protozoa and Metazoa so instructive, that I proposed long ago (1876) to make a special class of them. I distinguished three orders in this class—the Gastremaria, Physemaria, and Cyemaria (or Dicyemida). But we might also regard these three orders as so many independent classes in a primitive gastraead stem.

The Gastremaria and Cyemaria, the chief of these living gastraeads, are small Metazoa that live parasitically inside other Metazoa, and are, as a rule, 1/50 to 1/25 of an inch long, often much less (Figure 2.233, 1 to 15). Their soft body, devoid of skeleton, consists of two simple strata of cells, the primary germinal layers; the outer of these is thickly clothed with long hair-like lashes, by which the parasites swim about in the various cavities of their host. The inner germinal layer furnishes the sexual products. The pure type of the original gastrula (or archigastrula, Figure 1.29 I) is seen in the Pemmatodiscus gastrulaceus, which Monticelli discovered in the umbrella of a large medusa (Pilema pulmo) in 1895; the convex surface of this gelatinous umbrella was covered with numbers of clear vesicles, of 1/25 to 1/8 inch in diameter, in the fluid contents of which the little parasites were swimming. The cup-shaped body of the Pemmatodiscus (Figure 2.233, 1) is sometimes rather flat, and shaped like a hat or cone, at other times almost curved into a semi-circle. The simple hollow of the cup, the primitive gut (g), has a narrow opening (o). The skin layer (e) consists of long slender cylindrical cells, which bear long vibratory hairs; it is separated by a thin structureless, gelatinous plate (f) from the visceral or gut layer (i), the prismatic cells of which are much smaller and have no cilia. Pemmatodiscus propagates asexually, by simple longitudinal cleavage; on this account it has recently been regarded as the representative of a special order of gastraeads (Mesogastria).

Probably a near relative of the Pemmatodiscus is the Kunstleria Gruveli (Figure 2.233, 2). It lives in the body-cavity of Vermalia (Sipunculida), and differs from the former in having no lashes either on the large ectodermic cells (e) or the small entodermic (i); the germinal layers are separated by a thick, cup-shaped, gelatinous mass, which has been called the “clear vesicle” (f). The primitive mouth is surrounded by a dark ring that bears very strong and long vibratory lashes, and effects the swimming movements.

Pemmatodiscus and Kunstleria may be included in the family of the Gastremaria. To these gastraeads with open gut are closely related the Orthonectida (Rhopalura, Figure 2.233, 3 to 5). They live parasitically in the body-cavity of echinoderms (Ophiura) and vermalia; they are distinguished by the fact that their primitive gut-cavity is not empty, but filled with entodermic cells, from which the sexual cells are developed. These gastraeads are of both sexes, the male (Figure 1.3) being smaller and of a somewhat different shape from the oval female (Figure 1.4).

The somewhat similar Dicyemida (Figure 1.6) are distinguished from the preceding by the fact that their primitive gut-cavity is occupied by a single large entodermic cell instead of a crowded group of sexual cells. This cell does not yield sexual products, but afterwards divides into a number of cells (spores), each of which, without being impregnated, grows into a small embryo. The Dicyemida live parasitically in the body-cavity, especially the renal cavities, of the cuttle-fishes. They fall in several genera, some of which are characterised by the possession of special polar cells; the body is sometimes roundish, oval, or club-shaped, at other times long and cylindrical. The genus Conocyema (Figures 1.7 to 1.15) differs from the ordinary Dicyema in having four polar pimples in the form of a cross, which may be incipient tentacles.

The classification of the Cyemaria is much disputed; sometimes they are held to be parasitic infusoria (like the Opalina), sometimes platodes or vermalia, related to the suctorial worms or rotifers, but having degenerated through parasitism. I adhere to the phylogenetically important theory that I advanced in 1876, that we have here real gastraeads, primitive survivors of the common stem-group of all the Metazoa. In the struggle for life they have found shelter in the body-cavity of other animals.

(FIGURE 2.233. Modern gastraeads. Figure 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal section. Figure 2. Kunstleria gruveli (Delage), in longitudinal section. (From Kunstler and Gruvel.) Figures 3 to 5. Rhopalura Giardi (Julin): Figure 3 male, Figure 4 female, Figure 5 planula. Figure 6. Dicyema macrocephala (Van Beneden). Figures 7 to 15. Conocyema polymorpha (Van Beneden): Figure 7 the mature gastraead, Figures 8 to 15 its gastrulation. d primitive gut, o primitive mouth, e ectoderm, i entoderm, f gelatinous plate between e and i (supporting plate, blastocoel).)

The small Coelenteria attached to the floor of the sea that I have called the Physemaria (Haliphysema and Gastrophysema) probably form a third order (or class) of the living gastraeads. The genus Haliphysema (Figures 2.234 and 2.235) is externally very similar to a large rhizopod (described by the same name in 1862) of the family of the Rhabdamminida, which was at first taken for a sponge. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophysema is a simple cylindrical or oval tube, with a two-layered wall. The hollow of the tube is the gastric cavity, and the upper opening of it the mouth (Figure 2.235 m). The two strata of cells that form the wall of the tube are the primary germinal layers. These rudimentary zoophytes differ from the swimming gastraeads chiefly in being attached at one end (the end opposite