The Evolution of Man, V.2 by Ernst Haeckel (leveled readers .txt) 📖

sex-cells, are conveyed out behind by two pairs of short canals; the male opening (m) lies just behind the female (f). Most of the Platodaria have not the muscular pharynx, which is very advanced in the Turbellaria and Trematoda. On the other hand, they have, as a rule, before or behind the mouth, a bulbous sense-organ (auditory vesicle or organ of equilibrium, g), and many of them have also a couple of simple optic spots. The cell-pit of the ectoderm that lies underneath is rather thick, and represents the first rudiment of a neural ganglion (vertical brain or acroganglion).

The Turbellaria, with which the similar Platodaria were formerly classed, differ materially from them in the more advanced structure of their organs, and especially in having a central nervous system (vertical brain) and excretory renal canals (nephridia); both originate from the ectoderm. But between the two germinal layers a mesoderm is developed, a soft mass of connective tissue, in which the organs are embedded. The Turbellaria are still represented by a number of different forms, in both fresh and sea-water. The oldest of these are the very rudimentary and tiny forms that are known as Rhabdocoela on account of the simple construction of their gut; they are, as a rule, less than a quarter of an inch long and of a simple oval or lancet shape (Figure 2.240). The surface is covered with ciliated epithelium, a stratum of ectodermic cells. The digestive gut is still the simple primitive gut of the gastraea (d), with a single aperture that is both mouth and anus (m). There is, however, an invagination of the ectoderm at the mouth, which has given rise to a muscular pharynx (sd). It is noteworthy that the mouth of the Turbellaria (like the primitive mouth of the Gastraea) may, in this class, change its position considerably in the middle line of the ventral surface; sometimes it lies behind (Opisthostomum), sometimes in the middle (Mesostomum), sometimes in front (Prosostomum). This displacement of the mouth from front to rear is very interesting, because it corresponds to a phylogenetic displacement of the mouth. This probably occurred in the Platode ancestors of most (or all?) of the Coelomaria; in these the permanent mouth (metastoma) lies at the fore end (oral pole), whereas the primitive mouth (prostoma) lay at the hind end of the bilateral body.

In most of the Turbellaria there is a narrow cavity, containing a number of secondary organs, between the two primary germinal layers, the outer or animal layer of which forms the epidermis and the inner vegetal layer the visceral epithelium. The earliest of these organs are the sexual organs; they are very variously constructed in the Platode-class; in the simplest case there are merely two pairs of gonads or sexual glands--a pair of testicles (Figure 2.241 h) and a pair of ovaries (e). They open externally, sometimes by a common aperture (Monogonopora), sometimes by separate ones, the female behind the male (Digonopora, Figure 2.241). The sexual glands develop originally from the two promesoblasts or primitive mesodermic cells (Figure 1.83 p). As these earliest mesodermic structures extended, and became spacious sexual pouches in the later descendants of the Platodes, probably the two coelom-pouches were formed from them, the first trace of the real body-cavity of the higher Metazoa (Enterocoela).

The gonads are among the oldest organs, the few other organs that we find in the Platodes between the gut-wall and body-wall being later evolutionary products. One of the oldest and most important of these are the kidneys or nephridia, which remove unusable matter from the body (Figure 2.240 nc). These urinary or excretory organs were originally enlarged skin-glands--a couple of canals that run the length of the body, and have a separate or common external aperture (nm). They often have a number of branches. These special excretory organs are not found in the other Coelenteria (Gastraeads, Sponges, Cnidaria) or the Cryptocoela. They are first met in the Turbellaria, and have been transmitted direct from these to the Vermalia, and from these to the higher stems.

Finally, there is a very important new organ in the Turbellaria, which we do not find in the Cryptocoela (Figure 2.239) and their gastraead ancestors--the rudimentary nervous system. It consists of a couple of simple cerebral ganglia (Figure 2.241 g) and fine nervous fibres that radiate from them; these are partly voluntary nerves (or motor fibres) that go to the thin muscular layer developing under the skin; and partly sensory nerves that proceed to the sense-cells of the ciliated epiderm (f). Many of the Turbellaria have also special sense-organs; a couple of ciliated smell pits (na), rudimentary eyes (au), and, less frequently, auditory vesicles.

On these principles I assume that the oldest and simplest Turbellaria arose from Platodaria, and these directly from bilateral Gastraeads. The chief advances were the formation of gonads and nephridia, and of the rudimentary brain. On this hypothesis, which I advanced in 1872 in the first sketch of the gastraea-theory (Monograph on the Sponges), there is no direct affinity between the Platodes and the Cnidaria.

(FIGURE 2.240. A simple turbellarian (Rhabdocoelum). m mouth, sd gullet epithelium, sm gullet muscles, d gastric gut, nc renal canals, nm renal aperture, au eye, na olfactory pit. (Diagram.)

FIGURE 2.241. The same, showing the other organs. g brain, au eye, na olfactory pit, n nerves, h testicles, male symbol male aperture, female symbol female aperture, e ovary, f ciliated epiderm. (Diagram.)

(FIGURES 242 AND 243. Chaetonotus, a rudimentary vermalian, of the group of Gastrotricha. m mouth, s gullet, d gut, a anus, g brain, n nerves, ss sensory hairs, au eye, ms muscular cells, h skin, f ciliated bands of the ventral surface, nc nephridia, nm their aperture, e ovaries.))

Next to the ancient stem-group of the Turbellaria come a number of more recent chordonia ancestors, which we class with the Vermalia or Helminthes, the unarticulated worms. These true worms (Vermes, lately also called Scolecida) are the difficulty or the lumber-room of the zoological classifier, because the various classes have very complicated relations to the lower Platodes on the one hand and the more advanced animals on the other. But if we exclude the Platodes and the Annelids from this stem, we find a fairly satisfactory unity of organisation in the remaining classes. Among these worms we find some important forms that show considerable advance in organisation from the platode to the chordonia stage. Three of these phenomena are particularly instructive: (1) The formation of a true (secondary) body-cavity (coeloma); (2) the formation of a second aperture of the gut, the anus; and (3) the formation of a vascular system. The great majority of the Vermalia have these three features, and they are all wanting in the Platodes; in the rest of the worms at least one or two of them are developed.

Next and very close to the Platodes we have the Ichthydina (Gastrotricha), little marine and fresh-water worms, about 1/250 to 1/1000 inch long. Zoologists differ as to their position in classification. In my opinion, they approach very close to the Rhabdocoela (Figures 2.240 and 2.241), and differ from them chiefly in the possession of an anus at the posterior end (Figure 2.242 a). Further, the cilia that cover the whole surface of the Turbellaria are confined in the Gastrotricha to two ciliated bands (f) on the ventral surface of the oval body, the dorsal surface having bristles. Otherwise the organisation of the two classes is the same. In both the gut consists of a muscular gullet (s) and a glandular primitive gut (d). Over the gullet is a double brain (acroganglion, g). At the side of the gut are two serpentine prorenal canals (water-vessels or pronephridia, nc), which open on the ventral side (nm). Behind are a pair of simple sexual glands or gonads (Figure 2.243 e).

While the Ichthydina are thus closely related to the Platodes, we have to go farther away for the two classes of Vermalia which we unite in the group of the "snout-worms" (Frontonia). These are the Nemertina and the Enteropneusta. Both classes have a complete ciliary coat on the epidermis, a heritage from the Turbellaria and the Gastraeads; also, both have two openings of the gut, the mouth and anus, like the Gastrotricha. But we find also an important organ that is wanting in the preceding forms--the vascular system. In their more advanced mesoderm we find a few contractile longitudinal canals which force the blood through the body by their contractions; these are the first blood-vessels.

(FIGURE 2.244. A simple Nemertine. m mouth, d gut, a anus, g brain, n nerves, h ciliary coat, ss sensory pits (head-clefts), au eyes, r dorsal vessel, l lateral vessels. (Diagram.)

FIGURE 2.245. A young Enteropneust (Balanaglossus). (From Alexander Agassiz.) r acorn-shaped snout, h neck, k gill-clefts and gill-arches of the fore-gut, in long rows on each side, d digestive hind-gut, filling the greater part of the body-cavity, v intestinal vein or ventral vessel, lying between the parallel folds of the skin, a anus.

Figure 2.246. Transverse section of the branchial gut. A of Balanoglossus, B of Ascidia. r branchial gut, n pharyngeal groove, asterisk ventral folds between the two. Diagrammatic illustration from Gegenbaur, to show the relation of the dorsal branchial-gut cavity (r) to the pharyngeal or hypobranchial groove (n).)

The Nemertina were formerly classed with the much less advanced Turbellaria. But they differ essentially from them in having an anus and blood-vessels, and several other marks of higher organisation. They have generally long and narrow bodies, like a more or less flattened cord; there are, besides several small species, giant-forms with a width of 1/5 to 2/5 inch and a length of several yards (even ten to fifteen). Most of them live in the sea, but some in fresh water and moist earth. In their internal structure they approach the Turbellaria on the one hand and the higher Vermalia (especially the Enteropneusta) on the other. They have a good deal of interest as the lowest and oldest of all animals with blood. In them we find blood-vessels for the first time, distributing real blood through the body. The blood is red, and the red colouring-matter is haemoglobin, connected with elliptic discoid blood-cells, as in the Vertebrates. Most of them have two or three parallel blood-canals, which run the whole length of the body, and are connected in front and behind by loops, and often by a number of ring-shaped pieces. The chief of these primitive blood-vessels is the one that lies above the gut in the middle line of the back (Figure 2.244 r); it may be compared to either the dorsal vessel of the Articulates or the aorta of the Vertebrates. To the right and left are the two serpentine lateral vessels (Figure 2.244 l).

After the Nemertina, I take (as distant relatives) the Enteropneusta; they may be classed together with them as Frontonia or Rhyncocoela (snout-worms). There is now only one genus of this class, with several species (Balanoglossus); but it is very remarkable, and may be regarded as the last survivor of an ancient and long-extinct class of Vermalia. They are related, on the one hand, to the Nemertina and their immediate ancestors, the Platodes, and to the lowest and oldest forms of the Chordonia on the other.

The Enteropneusta (Figure 2.245) live in the sea sand, and are long worms of very simple shape, like the Nemertina. From the latter they have inherited: (1) The bilateral type, with incomplete segmentation; (2) the ciliary coat of the soft epidermis; (3) the double rows of gastric pouches, alternating with a single or double row of gonads; (4) separation of the sexes (the Platode ancestors were hermaphroditic); (5) the ventral mouth, underneath a protruding snout; (6) the anus terminating the simple gut-tube; and (7) several parallel blood-canals, running the length of the body, a dorsal and a ventral principal stem.

On the other hand, the Enteropneusta differ from their Nemertine ancestors in several features, some of which are important, that we may attribute to adaptation. The chief of these is the branchial gut (Figure