Volume II Part 7 (2/2)
The second main cla.s.s or branch in the tribe of Zoophytes is formed by the Sea-nettles (Acalephae, or Cnidae). This interesting group of animals, so rich in forms, is composed of three different cla.s.ses, namely, the Hood-jellies (Hydromedusae), the Comb-jellies (Ctenophora), and the Corals (Coralla). The hypothetical, extinct Archydra must be looked upon as the common primary form of the whole group; it has left two near relations in the still living fresh-water polyps (Hydra and Cordylophora). The Archydra was very closely related to the simplest forms of Spongiae (Archispongia and Olynthus), and probably differed from them only by possessing nettle organs, and by the absence of cutaneous pores. Out of the Archydra there first developed the different Hydroid polyps, some of which became the primary forms of Corals, others the primary forms of Hydromedusae. The Ctenophora developed later out of a branch of the latter.
The Sea-nettles differ from the Spongiae (with which they agree in the characteristic formation of the system of the alimentary ca.n.a.l) princ.i.p.ally by the constant possession of nettle organs. These are small bladders filled with poison, large numbers-generally millions-of which are dispersed over the skin of the sea nettles, and which burst and empty their contents when touched. Small animals are killed by this; in larger animals this nettle poison causes a slight inflammation of the skin, just as does the poison of our common nettles. Any one who has often bathed in the sea, will probably have at times come in contact with large Hood-jellies (Jelly-fish), and become acquainted with the unpleasant burning feeling which their nettle organs can produce. The poison in the splendid blue Jelly-fish, Physalia, or Portuguese Man-of-war, acts so powerfully that it may lead to the death of a human being.
The cla.s.s of Corals (Coralla) lives exclusively in the sea, and is more especially represented in the warm seas by an abundance of beautiful and highly-coloured forms like flowers. Hence they are also called _Flower-animals_ (Anthozoa). Most of them are attached to the bottom of the sea, and contain an internal calcareous skeleton. Many of them by continued growth produce such immense stocks that their calcareous skeletons have formed the foundation of whole islands, as is the case with the celebrated coral reefs and atolls of the South Seas, the remarkable forms of which were first explained by Darwin.(13) In corals the counterparts, or antimera-that is, the corresponding divisions of the body which radiate from and surround the central main axis of the body-exist sometimes to the number of four, sometimes to the number of six or eight. According to this we distinguish three legions, the Fourfold (Tetracoralla), Sixfold (Hexacoralla), and Eightfold corals (Octocoralla). The fourfold corals form the common primary group of the cla.s.s, out of which the sixfold and eightfold have developed as two diverging branches.
SYSTEMATIC SURVEY
_Of the 4 Cla.s.ses and 30 Orders of the Animal Plants, or Zoophytes._
----------------+--------------------+--------------------+--------------- _Cla.s.s of the_ | _Legions of the_ | _Orders of the_ | _A Genus Name_ _Zoophytes._ | _Zoophytes._ | _Zoophytes._ | _as example._ ----------------+--------------------+--------------------+--------------- I. { I. Myxospongiae { 1. Archispongina | Archispongia { _Mucous Sponges_ { 2. Halisarcina | Halisarca =Sponges= { | { II. Fibrospongiae { 3. Chalynthina | Spongilla +Spongiae+ {_Fibrous Sponges_ { 4. Geodina | Ancorina or { { 5. Hexactinella | Euplectella +Porifera+ { | { III. Calcispongiae { 6. Ascones | Olynthus {_Calcareous Sponges_ { 7. Leucones | Dyssycus { { 8. Sycones | Sycurus | | II. { IV. Tetracoralla { 9. Rugosa | Cyathophyllum {_Fourfold Corals_ { 10. Paranemeta | Cereanthus =Corals= { | { V. Hexacoralla { 11. Cauliculata | Antipathes +Coralla+ {_Sixfold Corals_ { 12. Madreporaria | Astraea or { { 13. Halirhoda | Actinia +Anthozoa+ { | { VI. Octocoralla { 14. Alcyonida | Lobularia {_Eightfold Corals_ { 15. Gorgonida | Isis { { 16. Pennatulida | Veretillum | | III. { VII. Archydrae } 17. Hydraria | Hydra {_Primaeval Polyps_ } | =Jelly-polyps= { | { VIII. Leptomedusae { 18. Vesiculata | Sertularia +Hydromedusae+ {_Soft Jelly-fish_ { 19. Ocellata | Tubularia { { 20. Siphonophora | Physophora or { | { IX. Trachymedusae { 21. Marsiporchida| Trachynema =Hood-jellies= { _Hard Jelly-fish_ { 22. Phyllorchida | Geryonia { { 23. Elasmorchida | Charybdae +Medusa+ { | { X. Calycozoa } 24. Podactinaria | Lucernaria { _Stalked Jellies_ }| { | { XI. Discomedusae { 25. Semaeostomeae | Aurelia { _Disc-jellies_ { 26. Rhizostomeae | Crambessa | | IV. { XII. Eurystoma } 27. Beroida | Beroe { _Wide-mouthed_ } | =Comb-jellies= { | { XIII. Stenostoma { 28. Saccata | Cydippe +Ctenophora+ { _Narrow-mouthed_ { 29. Lobata | Eucharis { { 30. Taeniata | Cestum
Ctenophora Hydromedusae Taeniata Lobata Rhizostomeae | | | | | | ---v----/ Semaeostomeae Saccata DISCOMEDUSae STENOSTOMA | | Trachymedusae | Siphonophora | | | | | | Lucernaria | | EURYSTOMA> | Calycozoa | | | | | | | | | | | | -------------------------v----------------/ LEPTOMEDUSae Coralla | Octocoralla | Hexacoralla | | | | | | | | ----v----/ | Tetracoralla | Spongiae | | Fibrospongiae Calcispongiae | | Chalynthina Leucones Sycones| | | | | | | | | | | | | | | | | Hexactinella|Geodina Dyssycus Sycurus | Hydroida | | | | | | | Cordylophora | | | | | | | | Hydra ------v---/ -----v--/ | | | | | Ascones | | | | | Myxospongia | | -----v-------/ | Halisarcina | | HYDROIDA | | | Procorallum | | | | | | CHALYNTHUS | OLYNTHUS | | | | | -----v--------/ ------v----------/ Hydroida Archispongiae | | | | Archydra | | --------------v--------/ Protascus | Gastraea
The second cla.s.s of Sea-nettles is formed by the _Hood-jellies_ (Medusae) or _Polyp-jellies_ (Hydromedusae). While most corals form stocks like plants, and are attached to the bottom of the sea, the Hood-jellies generally swim about freely in the form of gelatinous bells. There are, however, numbers of them, especially the lower forms, which adhere to the bottom of the sea, and resemble pretty little trees. The lowest and simplest members of this cla.s.s are the little fresh-water polyps (Hydra and Cordylophora). We may look upon them as but little changed descendants of those _Primaeval polyps_ (Archydrae), from which, during the primordial period, the whole division of the Sea-nettles originated.
Scarcely distinguishable from the Hydra are the adherent Hydroid polyps (Campanularia, Tubularia), which produce freely swimming medusae by budding, and out of the eggs of these there again arise adherent polyps.
These freely swimming Hood-jellies are mostly of the form of a mushroom, or of an umbrella, from the rim of which many long and delicate tentacles hang. They are among the most beautiful and most interesting inhabitants of the sea. The remarkable history of their lives, and especially the complicated alternation of generation of polyps and medusae, are among the strongest proofs of the truth of the theory of descent. For just as Medusae still daily arise out of the Hydroids, did the freely swimming medusa-form originally proceed, phylogenetically, out of the adherent polyp-form. Equally important for the theory of descent is the remarkable _division of labour_ of the individuals, which among some of them is developed to an astonis.h.i.+ngly high degree, more especially in the splendid _Siphonophora_.(37) (Plate VII. Fig. 13.)
The third cla.s.s of Sea-nettles-the peculiar division of Comb-jellies (Ctenophora), probably developed out of a branch of the Hood-jellies.
The Ctenophora, which are also called Ribbed-jellies, possess a body of the form of a cuc.u.mber, which, like the body of most Hood-jellies, is as clear and transparent as crystal or cut gla.s.s. Comb or Ribbed-jellies are characterized by their peculiar organs of motion, namely, by eight rows of paddling, ciliated leaflets, which run in the form of eight ribs from one end of the longitudinal axis (from the mouth) to the opposite end. Those with narrow mouths (Stenostoma) probably developed later out of those with wide mouths (Eurystoma). (Compare Plate VII. Fig. 16.)
The third tribe of the animal kingdom, the phylum of _Worms_ or worm-like animals (Vermes, or Helminthes), contains a number of diverging branches. Some of these numerous branches have developed into well-marked and perfectly independent cla.s.ses of Worms, but others changed long since into the original, radical forms of the four higher tribes of animals. Each of these four higher tribes (and likewise the tribe of Zoophytes) we may picture to ourselves in the form of a lofty tree, whose branches represent the different cla.s.ses, orders, families, etc. The phylum of Worms, on the other hand, we have to conceive as a low bush or shrub, out of whose root a ma.s.s of independent branches shoot up in different directions. From this densely branched shrub, most of the branches of which are dead, there rise four high stems with many branches. These are the four lofty trees just mentioned as representing the higher phyla-the Echinoderma, Articulata, Mollusca, and Vertebrata.
These four stems are directly connected with one another at the root only, to wit, by the common primary group of the Worm tribe.
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 cla.s.ses 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 Aclomi and Clomati. For all the lower Worms which are comprised in the cla.s.s 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 clome); they are, therefore, called Aclomi. The true cavity, or clome, 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 cla.s.s them together as _Clomati_.
The main division of _Bloodless Worms_ (Aclomi) 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 Primaeval Worms (Archelminthes). The type of these _Primaeval Worms_, the ancient Prothelmis, may be directly derived from the Gastraea (p. 133). Even at present the Gastrula-form-the faithful historical portrait of the Gastraea-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 Primaeval 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 Aclomi arose the second main division of the Worm tribe, the Worms with blood and body-cavity (Clomati): of these there are seven different cla.s.ses.
The Pedigree on p. 151 shows how the obscure phylogeny of the seven cla.s.ses of Clomati may be supposed to stand. We shall, however, mention these cla.s.ses here quite briefly, as their relations.h.i.+ps and derivation are, at present, still very complicated and obscure. More numerous and more accurate investigations of the ontogeny of the different Clomati will at some future time throw light upon their phylogenesis.
The Round Worms (Nemathelminthes) which we mention as the first cla.s.s of the Clomati, and which are characterized by their cylindrical form, consist princ.i.p.ally of parasitical Worms which live in the interior of other animals. Of human parasites, the celebrated Trichinae, 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 cla.s.s 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 (Chaetopoda). Nearly akin to them are the Snout-worms (Rhynchocla), 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 Clomati, whose nearest kin of the present day are the Sea-sacs, especially the Ascidia.
SYSTEMATIC SURVEY
_Of the 8 Cla.s.ses and 22 Orders of the Worm Tribe._
(Compare Gen. Morph. ii. Plate V. pp. 75-77.)
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