Fig. 246—Transverse section of the branchial gut. A of Balanoglossus, B of Ascidia. r branchial gut, n pharyngeal groove, * 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).

After the Nemertina, I take (as distant relatives) the Enteropneusta ; they may be classed together with them as Frontonia or Rhyncocœla (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 (Fig. 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 (Fig. 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 (Fig. 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. p. 184). 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 (Fig. 225) and the tailed larvæ 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 XVI and XVII). 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 Gastræads 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 Gastræa and the Chordæa. Even in this modified chordæa-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 canal 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 cœloma. 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 anus, 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.—Man’s Genealogical Tree, First Half:
EARLIER SERIES OF ANCESTORS,
WITHOUT FOSSIL EVIDENCE.

Chief Stages Ancestral Stem-groups Living Relatives of
Ancestors
Stages 1–5:
Protist
ancestors
Unicellular
organisms.
1–2:
Prototypes
3–5:
Protozoa 		1.Monera
Without nucleus

2. Algaria
Unicellular algæ
 1. Chromacea
(Chroococcus)
Phycochromacea
2. Paulotomea
Palmellacea
Eremosphæra
3. Lobosa
Unicellular (amœbina)
rhizopods
4. Infusoria
Unicellular

5.Blastæades
Multicellular hollow spheres
3. Amœbina
Amœba Leucocyta

4. Flagellata
Euflagellata
Zoomonades
5.Catallacta
Magosphæra, Volvocina,
Blastula
Stages 6–11:
Invertebrate
metazoa
ancestors
6–8:
Cœlenteria
without anus and
body-cavity
9–11:
Vermalia, with
anus and
body-cavity 		6.Gastræades
With two germ-layers

7 PlatodesI
Platodaria
(without nephridia)
8. PlatodesII
Platodinia (with nephridia) 		6.Gastrula
Hydra, Olynthus,
Gastremaria
7.Cryptocœla
Convoluta, Porporus

8.Rhabdocœla
Vortex, Monolus
9. Provermalia
(Primitive worms)
Rotatoria
10. Frontonia
(Rhynchelminthes)
Snout-worms
11. Prochordonia
Chorda-worms
9. Gastrotricha
Trochozoa,Trochophora

10. Enteropneusta
Balanglossus
Cephalodiscus
11. Copelata
Appendicaria
Chordula-larvæ
Stages 12–15:
Monorhina
ancestors
Oldest vertebrates
without jaws or
pairsof limbs,
single nose 		12. Acrania I
(Prospondylia)
13. Acrania II
More recent
14.Cyclostoma I
(Archicrania)
15. Cyclostoma II
More recent 		12. Amphioxus larvæ

13.Leptocardia
Amphioxus
14. Petromyzonta larvæ

15. Marsipobranchia
Petromyzonta

B.—Man’s Genealogical Tree, Second Half:
LATER ANCESTORS, WITH FOSSIL EVIDENCE.