Volume II Part 7 (1/2)

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 relations.h.i.+p 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 was.h.i.+ng. 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.”