7. The next higher stage is represented by such low animals as the Gastrotricha—e.g., Chætonotus among the Rotatoria, which differ from the rhabdocœlous Turbellarians chiefly by the formation of a vent and the beginnings of a cœlom, or cavity, between gut and body wall. The addition of a primitive vascular system and a pair of nephridia, or excretory organs, is first met with in the Nemertines.
8. These, together with the Enteropneusta (Balanoglossus), are comprised under the name of Frontonia, or Rhynchelminthes, and form the highest group of the Vermalia.
The Enteropneusta especially fix our attention, because they alone, although essentially 'worms,' exhibit certain characteristics which make it possible to bridge over the gulf which still separates the Invertebrata from the vertebrate phylum. The anterior portion of the gut is transformed into a breathing apparatus—hence Gegenbaur's term of Enteropneusta, or Gut-breathers. Moreover, Balanoglossus and Cephalodiscus possess another modification of the gut—namely, a peculiar diverticulum, which, in the present state of our knowledge, may be looked upon as the forerunner of the chorda dorsalis.
9. Stage of Prochordonia, as indicated by the larval form, called Chordula, which is common to the Tunicata and all the Vertebrata. These two groups possess three most important features: (a) A chorda dorsalis, a stiff rod lying in the long axis of the body, dorsally from the gut and below the central nervous system. This latter, for the first time in the animal kingdom, appears in the shape of a spinal cord. (b) The use of the anterior portion of the gut for respiratory purposes. (c) The larval development of the Tunicata is essentially the same as that of the Vertebrata in its early stages. Only the free-swimming Copelata or Appendicularia among the Tunicates retain most of these features. The others, which become sessile—namely, the Ascidiæ, or sea-squirts—degenerate and specialize away from the main line.
[ANCESTRAL TREE OF THE VERTEBRATA]
Abridged from 'Systemat. Phylogenie,' § 15.
Names underlined refer to hypothetical groups.
| Aves | Mammalia | |
| | | Reptilia | | |
| '——— | ———| | | |
| |——— | ———' | |
| | | ||
| Proreptilia | ||
| Pisces | | | Amphibia |
| | | |——— | ———' |
| | | | | |
| | | Stegocephali | Dipnoi |
| | | |——— | ———' |
| '——— | ———| | |
| Proselachii | Cyclostomata | |
| | | | | |
| |——— | ———' | |
| | | ||
| Tunicata | Archicrania | Acrania |
| | | |——— | ———' |
| | | Prospondylia | |
| | | | | |
| '——— | ———| | |
| Prochordonia |
10. Stage of the Acrania, represented by Amphioxus. The early development of this little marine creature agrees closely with that of the Tunicates; but one important feature is added to its organization—namely, metamerism, segmentally arranged mesoderm. Amphioxus still possesses neither skull nor vertebræ, neither ribs nor jaws, and no limbs. But it is a member of the Vertebrata if we define these as follows: Bilateral symmetrical animals with segmentally arranged mesoderm, with a chorda dorsalis between the tubular nervous system and the gut, and with respiratory organs which arise from the anterior portion of the gut. We do not assume that Amphioxus stands in the direct ancestral line; it is probably much specialized, partly degenerated, and represents a side-branch; but it is, nevertheless, the only creature, hitherto known, which satisfactorily connects the Vertebrata with their invertebrate ancestors. Many other efforts have been made to solve the mystery of the origin of the Vertebrata—all less satisfactory than the present suggestion, or even absolutely futile. This remark applies especially to the attempts to derive them from either Articulata or Echinoderms. The other great and highly developed phylum, the Mollusca, is quite out of the question. We have to go back to a level at which all these principal phyla meet, and there we find the Vermalia, the lower of which alone permit connection in an upward direction with the higher phyla.
11. Stage of Cyclostomata. This now small group of Lampreys and Hagfishes represents the lowest Craniota; and although much specialized as a side-branch of the main-stem from which the other Craniota have sprung, they give us an idea of what the direct ancestors of the latter must have been like:—still without visceral arches, without jaws and without paired limbs; with a persistent pronephros; the ear with one semicircular canal only; mouth suctorial; cranium very primitive; and the metamerism of the vertebral column indicated only by little blocks of cartilage in the perichordal sheath. Such creatures must have existed at least as early as the Lower Silurian epoch; but until 1890 fossil Cyclostomes were unknown. Their life in the mud, or as endoparasites of fishes, coupled with their soft structure, makes them very unfit for preservation. This gives all the greater importance to Traquair's discovery, in 1890, of many little creatures, called by him Palæospondylus gunni, in the Old Red Sandstone of Caithness, which seem to be very closely allied to Cyclostomata.
12. The Elasmobranchi (sharks and skates), with their immediate forerunners, the Acanthodi of the Devonian and Carboniferous age, are the first typical fishes. That they existed as far back as the Silurian age is proved by many enamelled spines of the dermal armour, chiefly from the dorsal fins. This higher stage is characterized by the possession of typical jaws, by visceral or gill-bearing arches, and by two pairs of limbs. None of the Elasmobranchs, fossil or recent, stands in the direct ancestral line; but they are the lowest Gnathostomata, jaw-and-limb-possessing creatures, known.