PEDIGREE OF WORMS
| Chætopoda | ||||||||||
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| Phracthelminthes | │ | |||||||||
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| Echiurida | │ | │ | Hirudinea | |||||||
| Sipunculida | │ | │ | │ | │ | ||||||
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| │ | │ | │ | Onychophora | ||||||
| Gephyrea | │ | │ | │ | │ | Arctisca | |||||
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| Chætognatha | │ |
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| Nematoda | │ | │ | │ |
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| │ │ | Acantho-cephala | │ │ | Stelmopoda | Annelida | ||||||
| │ | │ | │ | │ | │ | Enteropneusta | |||||
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| Nemathelminthes | │ │ | Lophopoda Bryozoa | │ │ | Ascidia | │ │ | |||||
| │ | │ | │ | │ | Thalicea | │ | │ | Nemertina | |||
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| │ | │ | │ | Rotifera | │ | │ | │ |
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| │ | │ | │ | │ | │ | │ | │ | Rhynchocœla | |||
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| │ | │ | │ | │ | Tunicata | │ | |||||
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| Cœlomati (_worms with body-cavity_) | ||||||||||
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| Cestoda | │ | |||||||||
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| Trematoda | │ | |||||||||
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| Turbellaria | │ | |||||||||
| Platyhelminthes | │ | |||||||||
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| Acœlomi (_worms without body-cavity_) | ||||||||||
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| Archelminthes | ||||||||||
| Prothelmis | ||||||||||
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| Gastræa | ||||||||||
The Round Worms (Nemathelminthes) which we mention as the first class of the Cœlomati, and which are characterized by their cylindrical form, consist principally of parasitical Worms which live in the interior of other animals. Of human parasites, the celebrated Trichinæ, the Maw-worms, Whip-worms, etc., for example, belong to them. The Star-worms (Gephyrea) which live exclusively in the sea are allied to round worms, and the comprehensive class of Ring-worms (Annelida) are allied to the former. To the Ring-worms, whose long body is composed of a number of segments, all alike in structure, belong the Leeches (Hirudinea), Earth-worms (Lumbricina), and all the marine bristle-footed Worms (Chætopoda). Nearly akin to them are the Snout-worms (Rhynchocœla), and the small microscopic Wheel-worms (Rotifera). The unknown, extinct, primary forms of the tribe of Sea-stars (Echinoderma), and of the tribe of the articulated animals (Arthropoda), were nearest akin to the Ring-worms. On the other hand, we must probably look for the primary forms of the great tribe of Molluscs in extinct Worms, which were very closely related to the Moss-polyps (Bryozoa) of the present day; and for the primary forms of the Vertebrata in the unknown Cœlomati, whose nearest kin of the present day are the Sea-sacs, especially the Ascidia.
The class of Sea-sacs (Tunicata) is one of the most remarkable among Worms. They all live in the ocean, where some of the Ascidiæ adhere to the bottom, while others (the sea-barrels, or Thaliacea) swim about freely. In all of them the non-jointed body has the form of a simple barrel-shaped sack, which is surrounded by a thick cartilaginous mantle. This mantle consists of the same non-nitrogenous combination of carbon, which, under the name of cellulose, plays an important part in the Vegetable Kingdom, and forms the largest portion of vegetable cellular membranes, and consequently also the greater part of wood. The barrel-shaped body generally possesses no external appendages. No one would recognise in them a trace of relationship to the highly differentiated vertebrate animals. And yet this can no longer be doubted, since Kowalewsky’s investigations, which in the year 1867 suddenly threw an exceedingly surprising and unmistakable light upon them. From these investigations it has become clear that the individual development of the adherent simple Ascidian Phallusia agrees in most points with that of the lowest vertebrate animal, namely, the Lancelet (Amphioxus lanceolatus). The early stages of the Ascidia possess the beginnings of the spinal marrow and the spinal column (chorda dorsalis) lying beneath it, which are the two most essential and most characteristic organs of the vertebrate animal. Accordingly, of all invertebrate animals known to us, the Tunicates are without doubt the nearest blood relations of the Vertebrates, and must be considered as the nearest relations of those Worms out of which the vertebrate tribe has developed. (Compare Plates [XII]. and [XIII].)
While thus different branches of the Cœlomatous group of the Worms furnish us with several genealogical links leading to the four higher tribes of animals, and give us important phylogenetic indications of their origin, the lower group of Acœlomi, on the other hand, show close relationships to the Zoophytes, and to the Primæval animals. The great phylogenetic interest of the Worm tribe rests upon this peculiar intermediate position.