C. {V. Bloodless worms {10. Planary worms 10. Platyhelminthes =Worms= { _Aclomi_ { {11. Round worms 11. Nemathelminthes +Vermes+ { VI. Blood-bearing {12. Moss-polyps 12. Bryozoa { worms {13. Sac-worms 13. Tunicata { _Clomati_ {14. Proboscideans 14. Rhynchocla { {15. Star-worms 15. Gephyrea { {16. Wheel animalcules 16. Rotatoria { {17. Ring-worms 17. Annelida

D. {VII. Headless sh.e.l.lfish {18. Lamp-sh.e.l.ls 18. Spirobranchia =Molluscs= { _Acephala_ {19. Mussels 19. Lamellibranchia { +Mollusca+ { VIII. Head-bearing {20. Snails 20. Cochlides { _Eucephala_ {21. Cuttles 21. Cephalopoda

E. { IX. Ringed-arms {22. Sea-stars 22. Asterida =Star-fishes= { _Colobrachia_ {23. Lily-stars 23. Crinoida { +Echinoderma+ { X. Armless {24. Sea-urchins 24. Echinida { _Lipobrachia_ {25. Sea-cuc.u.mbers 25. Holothuriae

F. { XI. Gill-breathers {26. Crab-fish 26. Crustacea =Articulated= { _Carides_ { =Animals= { { XII. Tube-breathers {27. Spiders 27. Arachnida +Arthropoda+ { _Tracheata_ {28. Centipedes 28. Myriopoda { {29. Flies 29. Insecta

{ XIII. Skull-less {30. Lancelets 30. Leptocardia { _Acrania_ { G. { =Vertebrate= { XIV. Single-nostriled {31. Lampreys 31. Cyclostoma =Animals= { _Monorrhina_ { { +Vertebrata+ { XV. Amnion-less {32. Fishes 32. Pisces { _Anamnia_ {33. Mud-fish 33. Dipneusta { {34. Sea dragons 34. Halisauria { {35. Amphibians 35. Amphibia { { XVI. Amnion-bearing {36. Reptiles 36. Reptilia { _Amniota_ {37. Birds 37. Aves { {38. Mammals 38. Mammalia

+Vertebrata+ (_Vertebrated animals_) Craniota | +Arthropoda+ | +Mollusca+ (_Articulated Animals_) | (_Molluscs_) +Echinoderma+ Tracheata | Eucephala (_Star-fishes_) | | | | | | Lipobrachia Crustacea Acrania | | Annelida | | Tunicata Acephala | | | | | Bryozoa | Colobrachia | | | | | | | Gephyrea| | Rotatoria --v---/ --v---/ | | | | | | | | | | | | | | ---------v-------------------/ ------v-----/ | | | +Vermes+ | | (_Worms_) | ------------------v-------------------/ | | CLOMATI (_Worms with a body-cavity_) | Platyhelminthes | | | | -----v------/ +Zoophyta+ | (_Animal-Plants_) +ACLOMI+ Spongiae Acalephae (_Worms without body-cavity_) | | | | | | ------v-------/ | | | Protascus Prothelmis | | | | -----------v---------------/ | | +Protozoa+ | (_Primaeval animals_) | |/-------------^------------- +GASTRaeA+ | | Infusoria | | +PLANaeA+ | Gregarinae | | | | | | +SYNAMBae+ ----v---/ | | | | ---------v--------/ +AMBae+ | +MONERA+

The first province of the Protozoa consists of the _Egg animals_ (Ovularia); we include among them all _single-celled animals_, all animals whose body, in the fully developed state, possesses the form-value of a _simple plastid_ (of a cytod or a cell), also those simple animal forms whose body consists of an aggregation of several cells perfectly similar one to another.

The _Archaic animals_ (Archezoa) form the first cla.s.s in the series of Egg animals. It contains only the most simple and most ancient primary forms of the animal kingdom, whose former existence we have proved by means of the fundamental law of biogenesis; they are, (1) Animal Monera; (2) Animal Ambae; (3) Animal Synambae. We may, if we choose, include among them a portion of the still living Monera and Ambae, but another portion (according to the discussion in Chapter XVI.) must on account of their neutral nature be considered as Protista, and a third portion, on account of their vegetable nature, must be considered as plants.

A second cla.s.s of the egg animals consists of the _Gregarines_ (Gregarinae), which live as parasites in the intestines and body-cavities of many animals. Some of these Gregarines are perfectly simple cells like the Ambae; some form chains of two or three identical cells, one lying behind the other. They differ from the naked Ambae by possessing a thick, simple membrane, which surrounds their cell-body; they can be considered as animal Ambae which have adopted a parasitical mode of life, and in consequence have surrounded themselves with a secreted covering.

As a third cla.s.s of egg animals, we adopt the real _Infusoria_ (Infusoria), embracing those forms to which modern zoology almost universally limits this cla.s.s of animals. The princ.i.p.al portion of them consists of the small _ciliated Infusoria_ (Ciliata), which inhabit all the fresh and salt waters of the earth in great numbers, and which swim about by means of a delicate garb of vibratile fringes. A second and smaller division consists of the adherent _sucking Infusoria_ (Acinetae), which take their food by means of fine sucking-tubes. Although during the last thirty years numerous and very careful investigations have been made on these small animalcules,-which are mostly invisible to the naked eye,-still we are even now not very sure about their development and form-value. We do not even yet know whether the Infusoria are single or many-celled; but as no investigator has as yet proved their body to be a combination of cells, we are, in the mean time, justified in considering them as single-celled, like the Gregarines and the Ambae.

The second main cla.s.s of primaeval animals consists of the _Germ animals_ (Blastularia). This name we give to those extinct Protozoa which correspond to the two ontogenetic embryonic forms of the six higher animal tribes, namely, the Planula and the Gastrula. The body of these Blastularia, in a perfectly developed state, was composed of many cells, and these cells moreover differentiated-in two ways at least-into an external (animal or dermal) and an internal (vegetative or gastral) ma.s.s. Whether there still exist representatives of this group is uncertain. Their former existence is undoubtedly proved by the two exceedingly important ontogenetic animal forms which we have already described as Planula and Gastrula, and which still occur as a transient stage of development in the ontogeny of the most different tribes of animals. Corresponding to these, we may, according to the biogenetic principle, a.s.sume the former existence of two distinct cla.s.ses of Blastularia, namely, the _Planaeada_ and _Gastraeada_. The type of the _Planaeada_ is the _Planaea_-long since extinct-but whose historical portrait is still presented to us at the present day in the widely distributed _ciliated larva_ (Planula). (Frontispiece, Fig. 4.) The type of the _Gastraeada_ is the _Gastraea_, of whose original nature the mouth-and-stomach larva (Gastrula), which recurs in the most different animal tribes, still gives a faithful representation. (Frontispiece Fig.

5, 6.) Out of the Gastraea, as we have previously mentioned, there were at one time developed two different primary forms, the Protascus and Prothelmis; the former must be looked upon as the primary form of the Zoophytes, the latter as the primary form of Worms. (Compare the enunciation of this hypothesis in my Monograph of the Calcareous Sponges, vol i. p. 464.)

The _Animal-plants_ (Zoophyta, or Clenterata) which const.i.tute the second tribe of the animal kingdom, rise considerably above the primitive animals in the characters of their whole organisation, while they remain far below most of the higher animals. For in the latter (with the exception only of the lowest forms) the four distinct functions of nutrition-namely, digestion, circulation of the blood, respiration, and excretion-are universally accomplished by four perfectly different systems of organs: by the intestines, the vascular system, the organs of respiration, and the urinary apparatus. In Zoophytes, however, these functions and their organs are not yet separate, and are all performed by a single system of alimentary ca.n.a.ls, by the so-called gastro-vascular system, or the clenteric apparatus of the intestinal cavity. The mouth, which is also the a.n.u.s, leads into a stomach, into which the other cavities of the body also open. In Zoophytes the body-cavity, or "cloma," possessed by the four higher tribes of animals is still completely wanting, likewise the vascular system and blood, as also the organs of respiration, etc.

All Zoophytes live in water; most of them in the sea, only a very few in fresh water, such as fresh-water sponges (Spongilla) and some primaeval polyps (Hydra, Cordylophora). A specimen of the pretty flower-like forms which are met with in great variety among Zoophytes is given on Plate VII. (Compare its explanation in the Appendix.)

The tribe of animal-plants, or Zoophytes, is divided into two distinct provinces, the _Sponges_, or _Spongiae_, and the _Sea-nettles_, or _Acalephae_ (p. 144). The latter are much richer in forms and more highly organized than the former. In all Sponges the entire body, as well as the individual organs, are differentiated and perfected to a much less extent than in Sea-nettles. All Sponges lack the characteristic _nettle-organs_ which all Sea-nettles possess.

The common primary form of all Zoophytes must be looked for in the _Protascus_, an animal form long since extinct, but whose existence is proved according to the biogenetic principle by the Ascula. This Ascula is an ontogenetical development form which, in Sponges as well as in Sea-nettles, proceeds from the Gastrula. (Compare the Ascula of the calcareous sponge on the Frontispiece, Fig. 7, 8.) For after the Gastrula of zoophytes has for a time swum about in the water it sinks to the bottom, and there adheres by that pole of its axis which is opposite to the opening of the mouth. The external cells of the ectoderm draw in their vibrating, ciliary hairs, whereas, on the contrary, the inner cells of the entoderm begin to form them. Thus the Ascula, as we call this changed form of larva, is a simple sack, its cavity (the cavity of the stomach or intestine) opening by a mouth externally, at the upper pole of the longitudinal axis (opposite the basal point of fixture). The entire body is here in a certain sense a mere stomach or intestinal ca.n.a.l, as in the case of the Gastrula. The wall of the sack, which is both body wall and intestinal wall, consists of two layers or coats of cells, a fringed _entoderm_, or gastral layer (corresponding with the inner or vegetative germ-layer of the higher animals), and an unfringed exoderm or dermal layer (corresponding with the external or animal germ-layer of the higher animals). The original _Protascus_, a true likeness of which is still furnished by the Ascula, probably formed egg-cells and sperm-cells out of its gastral layer.

The Protascads-as we will call the most ancient group of vegetable animals, represented by the Protascus-type-divided into two lines or branches, the Spongiae and the Sea-nettles, or Acalephae. I have shown in my Monograph of the Calcareous Sponges (vol. i. p. 485) how closely these two main cla.s.ses of Zoophytes are related, and how they must both be derived, as two diverging forms, from the Protascus-form. The primary form of Spongiae, which I have there called Archispongia, arose out of the Protascus by the formation of pores through its body-wall; the primary form of Sea-nettles, which I there called Archydra, developed out of the Protascus by the formation of nettle-organs, as also by the formation of feelers or tentacles.

The main-cla.s.s or branch of the _Sponges_, _Spongiae_, or _Porifera_, lives in the sea, with the single exception of the green fresh-water Sponge (Spongilla). These animals were long considered as plants, later as Protista; in most Manuals they are still cla.s.sed among the primaeval animals, or Protozoa. But since I have demonstrated their development out of the Gastrula, and the construction of their bodies of two cellular germ-layers (as in all higher animals), their close relationship to Sea-nettles, and especially to the Hydrapolyps, seems finally to be established. The _Olynthus_ especially, which I consider as the common primary form of calcareous sponges, has thrown a complete and unmistakable light upon this point.

The numerous forms comprised in the cla.s.s of Spongiae have as yet been but little examined; they may be divided into three legions and eight orders. The first legion consists of the soft, gelatinous _Mucous Sponges_ (Myxospongiae), which are characterized by the absence of any hard skeleton. Among them are, on the one hand, the long-since-extinct primary forms of the whole cla.s.s, the type of which I consider to be the Archispongia; on the other hand there are the still living, gelatinous sponges, of which the _Halisarca_ is best known. We can obtain a notion of the Archispongia, the most ancient primaeval sponge, if we imagine the Olynthus (see Frontispiece), to be deprived of its radiating calcareous spiculae.

The second legion of Spongiae contains the _Fibrous Sponges_ (Fibrospongiae), the soft body of which is supported by a firm, fibrous skeleton. This fibrous skeleton often consists merely of so-called "h.o.r.n.y fibres," formed of a very elastic, not readily destructible, organic substance. This is the case for instance in our common bathing Sponge (Euspongia officinalis), the purified skeleton of which we use every morning when washing. Blended with the h.o.r.n.y, fibrous skeleton of many of these Sponges, there are numerous flinty spicula; this is the case for example with the fresh-water Sponge (Spongilla). In others the whole skeleton consists of only calcareous or silicious spicula which are frequently interwoven into an extremely beautiful lattice-work, as in the celebrated Venus' Flower Basket (Euplectella). Three orders of fibrous sponges may be distinguished according to the different formation of the spicula, namely, Chalynthina, Geodina, and Hexactinella. The natural history of the fibrous sponges is of especial interest to the Theory of Descent, as was first shown by Oscar Schmidt, the greatest authority on this group of animals. In no other group, perhaps, can the unlimited pliability of the specific form, and its relation to Adaptation and Inheritance, be so clearly followed step by step; perhaps in no other group is the species so difficult to limit and define.

[Ill.u.s.tration: Pl. VII.

E. Haeckel del. Lagesse sc.]

This proposition, which applies to the great legion of the Fibrous Sponges, applies in a still higher degree to the smaller but exceedingly interesting legion of the calcareous sponges (Calcispongiae), on which in 1872, after five years' careful examination, I published a comprehensive Monograph. The sixty plates of figures accompanying this Monograph explain the extreme pliability of these small sponges "good species" of which, in fact, cannot be spoken of in the usual systematic sense. We find among them only varying series of forms, which do not even completely transmit their specific form to their nearest descendants, but by adaptation to subordinate, external conditions of existence, perpetually change. It frequently occurs here, that there arise out of one and the same stock different form-species, which according to the usual system would belong to several quite distinct genera; this is the case, for instance, with the remarkable Ascometra (Frontispiece, Fig.

10.) The entire external bodily form is much more pliable and protean in Calcareous Sponges than in the silicious sponges, which are characterized by possessing silicious spicula, forming a beautiful skeleton. Through the study of the comparative anatomy and ontogeny of calcareous sponges, we can recognise, with the greatest certainty, the common primary form of the whole group, namely, the sack-shaped _Olynthus_, whose development is represented in the Frontispiece (compare its explanation in the Appendix). Out of the Olynthus (Fig. 9 on the Frontispiece), the order of the Ascones was the first to develop, out of which, at a later period, the two other orders of Calcareous Sponges, the _Leucones_ and _Sycones_, arose as diverging branches.

Within these orders, the descent of the individual forms can again be followed step by step. Thus the Calcareous Sponges in every respect confirm the proposition which I have elsewhere maintained: that "the natural history of sponges forms a connected and striking argument in favour of Darwin."

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 astonishingly 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 relationships 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.)