_Of the 4 Cla.s.ses, 9 Sub-cla.s.ses, and 20 Orders of Star-fishes._

(Compare Gen. Morph. II. Plate IV. pp. 62-67.)

----------------+-------------------------+---------------------------+------------------ _Cla.s.ses of the | _Sub-cla.s.ses of the | _Orders of the | _Systematic Name Star-fishes._ | Star-fishes._ | Star-fishes._ | of the Orders._ ----------------+-------------------------+---------------------------+------------------ | { I. { 1. Primary Stars | 1. Tecastra { Sea Stars with radiated { 2. Articulated Stars | 2. Colastra I. { stomach { 3. Brisinga Stars | 3. Brisingastra =Sea Stars= { _Actinogastra_ | +Asterida+ { { II. { 4. Serpent Stars | 4. Ophiastra { Sea Stars with disc- { 5. Tree Stars | 5. Phytastra { shaped stomach { 6. Lily Stars | 6. Crinastra { _Discogastra_ { | | { III. { 7. Plated Lilies with | 7. Phatnocrinida { Lilies with arms { arms | { _Brachiata_ { 8. Articulated Lilies | 8. Colocrinida { { with arms | | II. { IV. { 9. Regularly budding | 9. Pentremitida =Sea Lilies= { Lilies with buds { Lilies | +Crinoida+ { _Blastoidea_ { 10. Lilies budding on | 10. Eleutherocrina { { two sides | | { V. { 11. Bladder Lilies | 11. Agelacrinida { Bladder Lilies { without stalks | { _Cystidea_ { 12. Bladder Lilies | 12. Sphaeronitida { with stalks | | { VI. { 13. Palechinida with | 13. Melonitida { Older Sea Urchins { more than 10 | { (with more than { rows of ambulacral | { 20 rows of plates) { plates | III. { _Palechinida_ { 14. Palechinida with | 14. Eocidaria =Sea Urchins= { { 10 rows of | +Echinida+ { VII. { ambulacral plates | { More recent Sea { 15. Autechinida with | 15. Desmosticha { Urchins (with 20 { band-like ambulacra | { rows of plates) { 16. Autechnidia with | 16. Petalosticha { _Autechinida_ { leaf-like ambulacra | | { VIII. { 17. Eupodia with scuti- | 17. Aspidochirota { Sea Cuc.u.mbers { form tentacles | { with aquatic feet { 18. Eupodia with | 18. Dendrochirota IV. { _Eupodia_ { branching tentacles | =Sea Cuc.u.mbers= { { | { IX. { | +Holothuriae+ { Sea Cuc.u.mbers { 19. Apodia with water- | 19. Liodermatida { without aquatic { lungs | { feet { 20. Apodia without | 20. Synaptida { _Apodia_ { water-lungs |

Clypeastridae | Spatangidae | | | Dysasteridae | | | | | Aspidochirota | | | | | ------v------/ Synaptida | | | | | | Ca.s.sidulidae | | | +Petalosticha+ | | | | | | | | Liodermatida | | | +Apodia+ | ------v-------/ Echinonidae | | | | | | Galeritidae | | | | Echinometridae | ---------v-----------/ | | | | ------v---------/ Dendrochirota | +Eupodia+ Latistellae +Holothuriae+ Salenidae | ------v------/ | | | | | | ------v------------/ | | | Angustistellae | +Desmosticha+ | +Autechinida+ | | | | Colocrinae | | | | | Sphaeronitidae | Phytastra | | | | | | | | | | | Eocidaridae | | Eleutherocrina | | | | | | | | | | | | Ophiastra | | Agelacrinae | | +Discogastra+ | | +Cystidea+ | | | | | | | | | | | | | | | | Melonitida | Phatnocrinae | Brisingastra | Palechinida | +Brahiata+ Pentremitida | | +Echinida+ | | +Blastoidea+ | | | | | | | | | | | | Colastra ----------v-----------------/ -------v-----------/ | | +Brachiata+ | | Crinoida | | Crinastra | | | -------------------------v----------------------------------/ | Tocastra +Actinogastra+ +Asterida+ | Phracthelminthes | Clomati | Gastraea

Hence the Star-fishes would be compound stocks of worms which, by the radial formation of buds, have developed out of true articulated worms, or Annelids. This hypothesis is most strongly supported by the comparative anatomy, and by the ontogeny of some Star-fishes (Colastra), and of segmented worms. The many-jointed Ring-worms (Annelida) in their inner structure are closely allied to the individual arms or radii of the Star-fishes, that is to the original single worms, which each arm represents. Each of the five worms of the Star-fish is a chain composed of a great number of equi-formal members, or metamera, lying one behind the other, like every segmented Worm, and every Arthropod. As in the latter a central nervous cord, the ventral nerve cord runs along the central line of the ventral wall of each segment. On each metameron there is a pair of non-jointed feet, and besides these, in most cases, one or more hard thorns or bristles similar to those of many Ring-worms.

A detached arm of a Star-fish can lead an independent life, and can then, by the radially-directed growth of buds at one end, again become a complete star.

The most important proofs, however, of the truth of my hypothesis are furnished by the ontogeny or the individual development of the Echinoderma. The most remarkable facts of this ontogeny were first discovered in the year 1848 by the great zoologist, Johannes Muller of Berlin. Some of its most important stages are represented on Plates VIII. and IX. (Compare their explanation in the Appendix.) Fig. _A_ on Plate IX. shows us a common Sea-star (Uraster), Fig. _B_, a Sea-lily (Comatula), Fig. _C_, a Sea-urchin (Echinus), and Fig. _D_, a Sea-cuc.u.mber (Synapta). In spite of the extraordinary difference of form manifested by these four representatives of the different cla.s.ses of Star-fishes, yet the beginning of their development is identical in all cases. Out of the egg an animal-form develops which is utterly different from the fully developed Star-fish, but very like the ciliated larvae of certain segmented Worms (Star-worms and Ring-worms). This peculiar animal-form is generally called the "larva," but more correctly the "nurse" of these Star-fish. It is very small and transparent, swims about by means of a fringe of cilia, and is always composed of two equal symmetrical halves or sides. The fully grown Echinoderm, however-which is frequently more than a hundred times larger, and quite opaque-creeps at the bottom of the sea, and is always composed of at least five co-ordinate pieces, or antimera, in the form of radii. Plate VIII. shows the development of the "nurses" of the four Echinoderms represented on Plate IX.

The fully developed Echinoderm arises by a very remarkable process of budding in the interior of the "nurse," of which it retains little more than the stomach. The nurse, erroneously called the "larva," of the Echinoderm, must accordingly be regarded as a solitary worm, which by internal budding produces a second generation, in the form of a stock of star-shaped and connected worms. The whole of this process is a genuine alternation of generations, or metagenesis, not a "metamorphosis," as is generally though erroneously stated. A similar alternation of generations also occurs in many other worms, especially in some star worms (Sipunculidae), and cord worms (Nemertinae). Now if, bearing in mind the fundamental law of biogeny, we refer the ontogeny of Echinoderma to their phylogeny, then the whole historical development of the Star-fishes suddenly becomes clear and intelligible to us, whereas without this hypothesis it remains an insoluble mystery. (Compare Gen.

Morph. ii. pp. 95-99.)

Besides the reasons mentioned, there are many other facts (princ.i.p.ally from the comparative anatomy of Echinoderma) which most distinctly prove the correctness of my hypothesis. I established this hypothesis in 1866, without having any idea that _fossil articulated worms_ still existed, apparently answering to the hypothetical primary forms. Such have in the mean time, however, really been discovered. In a treatise "On the Equivalent of the North American Taconic Schist in Germany,"[3] Geinitz and Liebe, in 1867, have described a number of articulated Silurian worms, which completely confirm my suppositions. Numbers of these very remarkable worms are found in an excellent state of preservation in the slates of Wurzbach, in the upper districts of Reusz. They are of the same structure as the articulated arm of a Star-fish, and evidently possessed a hard coat of mail, a much denser, more solid cutaneous skeleton than other worms in general. The number of body-segments, or metamera, is very considerable, so that the worms, although no more than a quarter or half an inch in breadth, attained a length of from two to three feet. The excellently preserved impressions, especially those of the Phyllodocites thuringiacus and Crossopodia Henrici, are so like the arms of many Star-fish (Colastra) that their true blood relationship seems very probable. This primaeval group of worms, which are most probably the ancestors of Star-fish, I call Mailed worms (Phracthelminthes, p. 150.)

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

STAR FISHES. FIRST GENERATION. WORM PERSON.]

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

STAR FISHES. SECOND GENERATION. WORM STOCK.]

The three other cla.s.ses of Echinoderma evidently arose at a later period out of the cla.s.s of Sea-stars which have most faithfully retained the original form of the stellate colony of worms. The Sea-lilies, or Crinoida, differ least from them, but having given up the free, slow motion possessed by other Sea-stars, they have become adherent to rocks, etc., and form for themselves a long stalk. Some Encrinites, however (for example, the Comatulae, Fig. _B_, on Plates VIII. and IX.), afterwards detach themselves from their stalk. The original worm individuals in the Crinoida are indeed no longer preserved in the same independent condition as in the case of the common star-fish; but they nevertheless always possess articulated arms extending from a common central disc. Hence we may unite the Sea-lilies and Sea-stars into a main-cla.s.s, or branch, characterized as possessing articulated arms (Colobrachia).

In the other two cla.s.ses of Echinoderma, the Sea-urchins and Sea-cuc.u.mbers, the articulated arms are no longer present as independent parts, but, by the increased centralization of the stock, have completely fused so as to form a common, inflated, central disc, which now looks like a simple box or capsule without arms. The original stock of five individuals has apparently degenerated to the form-value of a simple individual, a single person. Hence we may represent these two cla.s.ses as a branch characterized as being without arms (Lipobrachia), equivalent to those which possess articulated arms. The first of these two cla.s.ses, that of Sea-urchins (Echinida) takes its name from the numerous and frequently very large thorns which cover the hard sh.e.l.l, which is itself artistically built up of calcareous plates. (Fig. _C_, Plates VIII. and IX.) The fundamental form of the sh.e.l.l itself is a pentagonal pyramid. The Sea-urchins probably developed directly out of the group of Sea-stars. The different cla.s.ses and orders of marine lilies and stars which are given in the following table, ill.u.s.trate the laws of progress and differentiation in a striking manner. In each succeeding period of the earth's history we see the individual cla.s.ses continually increasing in variety and perfection. (Gen. Morph. ii. Plate IV.)

The history of three of these cla.s.ses of Star-fish is very minutely recorded by numerous and excellently preserved fossils, but on the other hand, we know almost nothing of the historical development of the fourth cla.s.s, that of the Sea-cuc.u.mbers (Holothuriae). These curious sausage-shaped Star-fish manifest externally a deceptive similarity to worms. (Fig. _D_, Plates VIII. and IX.) The skeletal structures in their skin are very imperfect, and hence no distinct remains of their elongated, cylindrical, worm-like body could be preserved in a fossil state. However, from the comparative anatomy of the Holothuriae, we can infer that they have arisen, by the softening of the cutaneous skeleton, from members of the cla.s.s of Sea-urchins.

From the Star-fish we turn to the fifth and most highly developed tribe of the invertebrate animals, namely, the phylum of Articulata, or those with _jointed feet_ (Arthropoda). As has already been remarked, this tribe corresponds to Linnaeus' cla.s.s of Insects. It contains four cla.s.ses: (1) the genuine six-legged Insects, or Flies; (2) the eight-legged Spiders; (3) the Centipedes, with numerous pairs of legs; and (4) the Crabs, or Crustacea, whose legs vary in number. The last cla.s.s breathe water through gills, and may therefore be contrasted as the main-cla.s.s of gill-breathing Arthropoda, or Gilled Insects (Carides), with the three first cla.s.ses. The latter breathe air by means of peculiar wind-pipes, or tracheae, and may therefore appropriately be united to form the main-cla.s.s of the trachea-breathing Arthropoda, or Tracheate Insects (Tracheata).

In all animals with articulated feet, as the name indicates, the legs are distinctly articulated, and by this, as well as by the strong differentiation of the separate parts of the body, or metamera, they are sharply distinguished from Ringed worms, with which Bar and Cuvier cla.s.sed them. They are, however, in every respect so like the Ringed worms that they can scarcely be considered altogether distinct from them. They, like the Ringed worms, possess a very characteristic form of the central nervous system, the so-called ventral marrow, which commences in a gullet-ring encircling the mouth. From other facts also, it is evident that the Arthropoda developed at a late period out of articulated worms. Probably either the Wheel Animalcules or the Ringed worms are their nearest blood relations in the Worm tribe. (Gen. Morph.

ii. Plate V. pp. 85-102.)

Now, although the derivation of the Arthropoda from ringed Worms may be considered as certain, still it cannot with equal a.s.surance be maintained that the whole tribe of the former has arisen out of one branch of the latter. For several reasons seem to support the supposition that the Gilled Arthropods have developed out of a branch of articulated worms, different from that which gave rise to the Tracheate Arthropods. But on the whole it remains more probable that both main-cla.s.ses have arisen out of one and the same group of Worms. In this case the Tracheate Insects-Spiders, Flies, and Centipedes-must have branched off at a later period from the gill-breathing Insects, or Crustacea.

The pedigree of the Arthropoda can on the whole be clearly made out from the palaeontology, comparative anatomy, and ontogeny of its four cla.s.ses, although here, as everywhere else, many details remain very obscure. Not until the history of the individual development of all the different groups has become more accurately known than it is at present, can this obscurity be removed. The history of the cla.s.s of Gilled Insects, or Crabs (Carides), is at present that best known to us; they are also called encrusted animals (Crustacea), on account of the hard crust or covering of their body. The ontogeny of these animals is extremely interesting and, like that of Vertebrate animals, distinctly reveals the essential outlines of the history of their tribe, that is, their phylogeny. Fritz Muller, in his work, "Fur Darwin,"(16) which has already been referred to, has explained this remarkable series of facts in a very able manner.

[Ill.u.s.tration: _Nauplius. Youth-form of six Crab-fish._ _Pl. X._

A. Limnetis.

B. Cyclops.

C. Lernacocera.

D. Lepas.

E. Sacculina.

F. Peneus.]

[Ill.u.s.tration: _Adult form of the same six Crab-fish._ _Pl. XI._

A. Limnetis.

B. Cyclops.

C. Lernacocera.

D. Lepas.

E. Sacculina.

F. Peneus.]

The common primary form of all Crabs, which in most cases is even now the first to develop out of the egg, is originally one and the same, the so-called _Nauplius_. This remarkable primaeval crab represents a very simple form of articulated animal, the body of which in general has the form of a roundish, oval, or pear-shaped disc, and has on its ventral side only three pairs of legs. The first of these is uncloven, the two subsequent pairs are forked. In front, above the mouth, lies a simple, single eye. Although the different orders of the Crustacean cla.s.s differ very widely from one another in the structure of their body and its appendages, yet the early Nauplius form always remains essentially the same. In order to be convinced of this, let the reader look attentively at Plates X. and XI., a more detailed explanation of which is given in the Appendix. On Plate XI. we see the fully developed representatives of six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. _A c_); a Stalked Crab (Lepas, Fig. _D c_); a Root Crab (Sacculina, Fig.

_E c_); a Boatman Crab (Cyclops, Fig. _B c_); a Fish Louse (Lernaeocera, Fig. _C c_); and, lastly, a highly developed Shrimp (Peneus, Fig. _F c_). These six crabs vary very much, as we see, in the entire form of body, in the number and formation of the legs, etc. When, however, we look at the earliest stages, or "nauplius," of these six different cla.s.ses, after they have crept out of the egg-those marked with corresponding letters on Plate X. (Fig. _A n-F n_)-we shall be surprised to find how much they agree. The different forms of Nauplius of these six orders differ no more from one another than would six different "good species" of one genus. Consequently, we may with a.s.surance infer a common derivation of all those orders from a common Primaeval Crab, which was essentially like the Nauplius of the present day.

SYSTEMATIC SURVEY

_Of the 7 Legions and 20 Orders of Crabs, or Crustacea._

-------------------------+--------------------------+----------------------+-------------- _Legions of the_ | _Orders of the_ | _Systema Name_ | _Name of a_ _Crustaceae._ | _Crustaceae._ | _of the Orders._ | _Genus as an_ | | | _example._ -------------------------+--------------------------+----------------------+-------------- I. ENTOMOSTRACA, _Lower Crustacea_, or Segmented Crabs (not pa.s.sing through the actual Zoea form in youth).

+I. Branchiopoda+ { 1. Primaeval Crabs 1. Archicarida Nauplius Gill-footed Crabs { 2. Leaf-foot Crabs 2. Phyllopoda Limnetis { 3. Trilobites 3. Trilobita Paradoxides { 4. Water Fleas 4. Cladocera Daphnia { 5. Bivalve Crabs 5. Ostracoda Cypris

+II. Pectostraca+ { 6. Barnacle Crabs 6. Cirripedia Lepas Fixed Crabs { 7. Root Crabs 7. Rhizocephala Sacculina

+III. Copepoda+ { 8. Boatmen Crabs 8. Eucopepoda Cyclops Oar-footed Crabs { 9. Fish Lice 9. Siphonostoma Lernaeocera

+IV. Pantopoda+ {10. No-body Crabs 10. Pycnogonida Nymphon No-body Crabs {

+V. Pcilopoda+ {11. Spear-tails 11. Xiphosura Limulus Shield Crabs {12. Giant Crabs 12. Gigantostraca Eurypterus

------------------------------------------------------------------------------------------ II. MALACOSTRACA, _Higher Crustacea_, or Mailed Crabs (pa.s.sing through the Zoea form in youth).

+VI. Podophthalma+ {13. Zoea Crabs 13. Zoepoda Zoea Stalk-eyed Mailed {14. Split-legged Crabs 14. Schizopoda Mysis Crabs {15. Mouth-footed Crabs 15. Stomatopoda Squilla {16. Ten-footed Crabs 16. Decapoda Peneus

+VII. Edriophthalma+ {17. c.u.ma Crabs 17. c.u.macea c.u.ma Mailed Crabs with {18. Flea Crabs 18. Amphipoda Gammarus sessile eyes {19. Wizard Crabs 19. Laemodipoda Caprella {20. Louse Crabs 20. Isopoda Oniscus

Brachyura Isopoda | | | Laemodipoda | | | | Anomura | | | | | | Amphipoda | | | | | -----v----/ Macrura | Decapoda Stomatopoda c.u.macea | | +Edriophthalma+ | | | ----v----/ | | | | | ---------------------------v------------/ | Schizopoda +Podophthalma+ | Zoepoda +Malacostraca+ Gigantostraca | Rhizocephala | | Siphonostoma | | Xiphosurae Zoea | Cirripediae | | | | +Pectostraca+ | | | | | ------v------/ | | | | | | | +Pcilopoda+ | | | | Nebaliae | | | | | | Eucopepoda | Belinur | | +Copepoda+ | Pycnogonida | | | | | +Pantopoda+ Trilobita | | | | | | Phyllopoda | | Ostracoda | | Cladocerae | | | | | | | | | | | | | | | -----------------v-------/ | | ---v---/ | | | | | | +Branchiopoda+ | | | | | | | | | | | | | | ----------------------v-------------------------/ | Nauplius | +Archicaridae+

(Articulated Worms)