The extraordinary difficulties which the systematic arrangement of Worms presents, for this reason merely, are still more increased by the fact that we do not possess any fossil remains of them. Most of the Worms had and still have such soft bodies that they could not leave any characteristic traces in the neptunic strata of the earth. Hence in this case again we are entirely confined to the records of creation furnished by ontogeny and comparative anatomy. In making then the exceedingly difficult attempt to throw a few hypothetical rays of light upon the obscurity of the pedigree of Worms, I must therefore expressly remark that this sketch, like all similar attempts possesses only a provisional value.
The numerous classes distinguished in the tribe of Worms, and which almost every zoologist groups and defines according to his own personal views, are, in the first place, divided into two essentially different groups or branches, which in my Monograph of the Calcareous Sponges I have termed Acœlomi and Cœlomati. For all the lower Worms which are comprised in the class of Flat-worms (Platyhelminthes), (the Gliding-worms, Sucker-worms, Tape-worms), differ very strikingly from other Worms, in the fact that they possess neither blood nor body-cavity (no cœlome); they are, therefore, called Acœlomi. The true cavity, or cœlome, is completely absent in them as in all the Zoophytes; in this important respect the two groups are directly allied. But all other Worms (like the four higher tribes of animals) possess a genuine body-cavity and a vascular system connected with it, which is filled with blood; hence we class them together as Cœlomati.
The main division of Bloodless Worms (Acœlomi) contains, according to our phylogenetic views, besides the still living Flat-worms, the unknown and extinct primary forms of the whole tribe of Worms, which we shall call the Primæval Worms (Archelminthes). The type of these Primæval Worms, the ancient Prothelmis, may be directly derived from the Gastræa (p. [133]). Even at present the Gastrula-form—the faithful historical portrait of the Gastræa—recurs in the ontogenesis of the most different kinds of worms as a transient larva-form. The ciliated Gliding-worms (Turbellaria), the primary group of the present Planary or Flat-worms (Platyhelminthes), are the nearest akin to the Primæval Worms. The parasitical Sucker-worms (Trematoda) arose out of the Gliding-worms, which live freely in water, by adaptation to a parasitical mode of life; and out of them later on—by an increasing parasitism—arose the Tape-worms (Cestoda).
Out of a branch of the Acœlomi arose the second main division of the Worm tribe, the Worms with blood and body-cavity (Cœlomati): of these there are seven different classes.
The Pedigree on p. [151] shows how the obscure phylogeny of the seven classes of Cœlomati may be supposed to stand. We shall, however, mention these classes here quite briefly, as their relationships and derivation are, at present, still very complicated and obscure. More numerous and more accurate investigations of the ontogeny of the different Cœlomati will at some future time throw light upon their phylogenesis.
| SYSTEMATIC VIEW | ||||||
| Of the 8 Classes and 22 Orders of the Worm Tribe. | ||||||
| (Compare Gen. Morph. II. Plate V. pp. 75-77.) | ||||||
| Classes of the Worm Tribe. | Orders of the Worm Tribe. | Systematic Name of the Orders of Worms. | Name of a Genus as example. | |||
| 1. Flat Worms Platyhelminthes |
| 1. | Primæval worms | 1. | Archelminthes | Prothelmis |
| 2. | Gliding-worms | 2. | Turbellaria | Planaria | ||
| 3. | Sucker-worms | 3. | Trematoda | Distoma | ||
| 4. | Tape-worms | 4. | Cestoda | Tænia | ||
| 2.Round Worms Nemathelminthes |
| 5. | Arrow-worms | 5. | Chætognatha | Sagitta |
| 6. | Thread-worms | 6. | Nematoda | Trichina | ||
| 7. | Hook-headed-worms | 7. | Acanthocephala | Echinorhynchus | ||
| 3. Moss Polyps Bryozoa |
| 8. | Horse-shoe-lipped | 8. | Lophopoda | Alcyonella |
| 9. | Circle-lipped | 9. | Stelmopoda | Retepora | ||
| 4. Sea-sacs Tunicata |
| 10. | Sea-squirts | 10. | Ascidia | Phallusia |
| 11. | Sea-barrels | 11. | Thaliacea | Salpa | ||
| 5. Proboscideans Rhynchocœla |
| 12. | Tongue-worms | 12. | Enteropneusta | Balanoglossus |
| 13. | Cord-worms | 13. | Nemertina | Borlasia | ||
| 6. Star-Worms Gephyrea |
| 14. | Star-worms without bristles | 14. | Sipunculida | Sipunculus |
| 15. | Star-worms with bristles | 15. | Echiurida | Echiurus | ||
| 7. Wheel Animalcule Rotifera |
| 16. | Wheel-worms | 16. | Rotatoria | Hydatina |
| 8. Ring Worms Annelida |
| 17. | Bear-worm | 17. | Arctisca | Macrobiotus |
| 18. | Worms with claws | 18. | Onychophora | Peripatus | ||
| 19. | Leeches | 19. | Hirudinea | Hirudo | ||
| 20. | Land-worms | 20. | Drilomorpha | Lumbricus | ||
| 21. | Mailed worms | 21. | Phracthelminthes | Crossopodia | ||
| 22. | Bristle-footed worms | 22. | Chætopoda | Aphrodite | ||
