(FIGURES 242 AND 243. Chaetonotus, a rudimentary vermalian, of the group of Gastrotricha. m mouth, s gullet, d gut, a a.n.u.s, 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 cla.s.s 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 cla.s.sifier, because the various cla.s.ses 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 cla.s.ses. 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 a.n.u.s; 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 cla.s.sification. 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 a.n.u.s 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 cla.s.ses 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 ca.n.a.ls (water-vessels or p.r.o.nephridia, nc), which open on the ventral side (nm). Behind are a pair of simple s.e.xual 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 cla.s.ses of Vermalia which we unite in the group of the "snout-worms" (Frontonia). These are the Nemertina and the Enteropneusta. Both cla.s.ses 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 a.n.u.s, 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 ca.n.a.ls 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 a.n.u.s, 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 Aga.s.siz.) 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 a.n.u.s.

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 ill.u.s.tration from Gegenbaur, to show the relation of the dorsal branchial-gut cavity (r) to the pharyngeal or hypobranchial groove (n).)

The Nemertina were formerly cla.s.sed with the much less advanced Turbellaria. But they differ essentially from them in having an a.n.u.s 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-ca.n.a.ls, 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 cla.s.sed together with them as Frontonia or Rhyncocoela (snout-worms). There is now only one genus of this cla.s.s, with several species (Balanoglossus); but it is very remarkable, and may be regarded as the last survivor of an ancient and long-extinct cla.s.s 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 s.e.xes (the Platode ancestors were hermaphroditic); (5) the ventral mouth, underneath a protruding snout; (6) the a.n.u.s terminating the simple gut-tube; and (7) several parallel blood-ca.n.a.ls, running the length of the body, a dorsal and a ventral princ.i.p.al 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 2.245 k). The anterior section of the gut is converted into a respiratory organ, and pierced by two rows of gill-clefts; between these there is a branchial (gill) skeleton, formed of rods and plates of chitine. The water that enters at the mouth makes its exit by these clefts. They lie in the dorsal half of the fore-gut, and this is completely separated from the ventral half by two longitudinal folds (Figure 2.246 A*). This ventral half, the glandular walls of which are clothed with ciliary epithelium and secrete mucus, corresponds to the pharyngeal or hypo-branchial groove of the Chordonia (Bn), the important organ from which the later thyroid gland is developed in the Craniota (cf. Chapter 2.16). The agreement in the structure of the branchial gut of the Enteropneusts, Tunicates, and Vertebrates was first recognised by Gegenbaur (1878); it is the more significant as at first we find only a couple of gill-clefts in the young animals of all three groups; the number gradually increases. We can infer from this the common descent of the three groups with all the more confidence when we find the Balanoglossus approaching the Chordonia in other respects. Thus, for instance, the chief part of the central nervous system is a long dorsal neural string that runs above the gut and corresponds to the medullary tube of the Chordonia. Bateson believes he has detected a rudimentary chorda between the two.

Of all extant invertebrate animals the Enteropneusts come nearest to the Chordonia in virtue of these peculiar characters; hence we may regard them as the survivors of the ancient gut-breathing Vermalia from which the Chordonia also have descended. Again, of all the chorda-animals the Copelata (Figure 2.225) and the tailed larvae of the ascidia approach nearest to the young Balanoglossus. Both are, on the other hand, very closely related to the Amphioxus, the Primitive Vertebrate of which we have considered the importance (Chapters 2.16 and 2.17). As we saw there, the unarticulated Tunicates and the articulated Vertebrates must be regarded as two independent stems, that have developed in divergent directions. But the common root of the two stems, the extinct group of the Prochordonia, must be sought in the vermalia stem; and of all the living Vermalia those we have considered give us the safest clue to their origin. It is true that the actual representatives of the important groups of the Copelata, Balanoglossi, Nemertina, Icthydina, etc., have more or less departed from the primitive model owing to adaptation to special environment.

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. The medullary tube would be originally a chemical sensory organ, a dorsal olfactory tube, taking in respiratory-water and food by the neuroporus in front and conveying them by the neurenteric ca.n.a.l into the primitive gut. This olfactory tube would afterwards become the nervous centre, while the expanding gonads (lying to right and left of the primitive mouth) would form the coeloma. The chorda may have been originally a digestive glandular groove in the dorsal middle line of the primitive gut. The two secondary gut-openings, mouth and a.n.u.s, may have arisen in various ways by change of functions. In any case, we should ascribe the same high value to the chordula as we did before to the gastrula.

In order to explain more fully the chief stages in the advance of our race, I add the hypothetical sketch of man's ancestry that I published in my Last Link [a translation by Dr. Gadow of the paper read at the International Zoological Congress at Cambridge in 1898]:--

A. MAN'S GENEALOGICAL TREE, FIRST HALF: EARLIER SERIES OF ANCESTORS, WITHOUT FOSSIL EVIDENCE.

COLUMN 1 : CHIEF STAGES.

COLUMN 2 : ANCESTRAL STEM-GROUPS.

COLUMN 3 : LIVING RELATIVES OF ANCESTORS.

STAGES 1 TO 5. PROTIST ANCESTORS. UNICELLULAR ORGANISMS.

1 to 2. Protophytes. : 1. Monera. Without nucleus. : Chromacea.

(Chroococcus.) Phycochromacea.

1 to 2. Protophytes. : 2. Algaria. Unicellular algae. : 2. Paulotomea.

Palmellacea. Eremosphaera.

3 to 5. Protozoa. : 3. Lobosa. Unicellular (amoebina) rhizopods. : 3.

Amoebina. Amoeba Leucocyta.

3 to 5. Protozoa. : 4. Infusoria. Unicellular. : 4. Flagellata.

Euflagellata. Zoomonades.

3 to 5. Protozoa. : 5. Blastaeades. Multicellular hollow spheres. : 5.

Catallacta. Magosphaera, Volvocina, Blastula.

STAGES 6 TO 11. INVERTEBRATE METAZOA ANCESTORS.

6 to 8. Coelenteria, without a.n.u.s and body-cavity. : 6. Gastraeades.

With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria.

6 to 8. Coelenteria, without a.n.u.s and body-cavity. : 7. Platodes I.

Platodaria (without nephridia). : 7. Cryptocoela. Convoluta, Proporus.

6 to 8. Coelenteria, without a.n.u.s and body-cavity. : 8. Platodes II.

Platodinia (with nephridia). : 8. Rhabdocoela. Vortex, Monotus.

9 to 11. Vermalia, with a.n.u.s and body-cavity. : 9. Provermalia.

(Primitive Worms.) Rotatoria. : 9. Gastrotricha. Trochozoa, Trochophora.

9 to 11. Vermalia, with a.n.u.s and body-cavity. : 10. Frontonia.

(Rhynchelminthes.) Snout-worms. : 10. Enteropneusta. Balanoglossus, Cephalodiscus.

9 to 11. Vermalia, with a.n.u.s and body-cavity. : 11. Prochordonia.

Chorda-worms. : 11. Copelata. Appendicaria. Chordula-larvae.

STAGES 12 TO 15. MONORHINA ANCESTORS.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 12.

Acrania I. (Prospondylia.) : 12. Amphioxus larva.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 13.

Acrania II. More recent. : 13. Leptocardia. Amphioxus.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 14.

Cyclostoma I. (Archicrania.) : 14. Petromyzonta larvae.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 15.

Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta.

B. MAN'S GENEALOGICAL TREE, SECOND HALF: LATER ANCESTORS, WITH FOSSIL EVIDENCE.

COLUMN 1 : GEOLOGICAL PERIODS.

COLUMN 2 : ANCESTRAL STEM-GROUPS.

COLUMN 3 : LIVING RELATIVES OF ANCESTORS.

Silurian. : 16. Selachii. Primitive fishes. Proselachii. : 16.