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).)