------------------+-------------------------+----------------------+------------------ _Cla.s.ses_ | | _Systematic_ | _of the_ | _Orders of the_ | _Name of the_ | _Name of a Genus_ _Worm Tribe._ | _Worm Tribe._ | _Orders of Worms_. | _as example._ | | | ------------------+-------------------------+----------------------+------------------ 1. _Flat_ { 1. Primaeval worms | 1. Archelminthes | Prothelmis _Worms_ { 2. Gliding-worms | 2. Turbellaria | Planaria Platyhel- { 3. Sucker-worms | 3. Trematoda | Distoma minthes { 4. Tape-worms | 4. Cestoda | Taenia | | 2. _Round_ { 5. Arrow-worms | 5. Chaetognatha | Sagitta _Worms_ { 6. Thread-worms | 6. Nematoda | Trichina Nemathel- { 7. Hook-headed | 7. Acanthocephala | Echinorhynchus minthes { worms | | | | 3. _Moss_ } 8. Horse-shoe-lipped | 8. Lophopoda | Alcyonella _Polyps_ } 9. Circle-lipped | 9. Stelmopoda | Retepora Bryozoa } | | | | 4. _Sea-sacs_ { 10. Sea-squirts | 10. Ascidia | Phallusia Tunicata { 11. Sea-barrels | 11. Thaliacea | Salpa | | 5. _Proboscideans_ } 12. Tongue-worms | 12. Enteropneusta | Balanoglossus Rhynchocla } 13. Cord-worms | 13. Nemertina | Borlasia | | { 14. Star-worms without | 14. Sipunculida | Sipunculus 6. _Star-Worms_ { bristles | | Gephyrea { 15. Star-worms with | 15. Echiurida | Echiurus { bristles | | | | 7. _Wheel_ } | | _Animalcule_ } 16. Wheel-worms | 16. Rotatoria | Hydatina Rotifera } | | | | { 17. Bear-worms | 17. Arctisca | Macrobiotus { 18. Worms with claws | 18. Onychophora | Peripatus 8. _Ring_ { 19. Leeches | 19. Hirudinea | Hirudo _Worms_ { 20. Land-worms | 20. Drilomorpha | Lumbricus Annelida { 21. Mailed worms | 21. Phracthelminthes | Crossopodia { 22. Bristle-footed | 22. Chaetopoda | Aphrodite { worms | |

Chaetopoda | Drilomorpha | | -------v-------/ | Phracthelminthes | | | Echiurida| | Hirudinea Sipunculida | | | | | | | | | Onychophora ----v---/ | | | | +Gephyrea+ | | | | Arctisca | | | | | | | | | | --v--/ Chaetognatha | --v-----/ | | | | | -----v-----/ Nematoda | | | | | | | ---------v---------/ --v--/ | +Annelida+ | Acantho- | | | cephala | Stelmopoda | | | | | | Enteropneusta --v--/ | | | | +Nemathelminthes+ | Lophopoda | Ascidia | | | +Bryozoa+ | Thalicea | | Nemertina | | | | | | | | | | | +Rotifera+ | | | | | | | | | | | | ---v-/ | | | | | | | +Rhynchocla+ | | | | | --v---/ | | | | ----v---/ +Tunicata+ | | | | | | | -----------------------v-----------------------------/ +Clomati+ (_worms with body-cavity_) | Cestoda | | | | | Trematoda | | | | | Turbellaria | +Platyhelminthes+ | | | | | -------v---------/ +Aclomi+ (_worms without body-cavity_) | | Archelminthes +Prothelmis+ | | +Gastraea+

The cla.s.s of Sea-sacs (Tunicata) is one of the most remarkable among Worms. They all live in the ocean, where some of the Ascidiae 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 Clomatous 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 Aclomi, on the other hand, show close relationships to the Zoophytes, and to the Primaeval animals. The great phylogenetic interest of the Worm tribe rests upon this peculiar intermediate position.

CHAPTER XIX.

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

II. MOLLUSCA, STAR-FISHES, AND ARTICULATED ANIMALS.

Tribe of Molluscs.-Four Cla.s.ses of Molluscs: Lamp-sh.e.l.ls (Spirobranchia); Mussels (Lamellibranchia); Snails (Cochlides); Cuttle-fish (Cephalopoda).-Tribe of Star-fishes, or Echinoderma.-Their Derivation from Ringed Worms (Mailed Worms, or Phracthelminthes).-The Alternation of Generation in the Echinoderma.-Four Cla.s.ses of Star-fish: Sea-stars (Asteridea); Sea-lilies (Crinoidea); Sea-urchins (Echinidea); Sea-cuc.u.mbers (Holothuridea).-Tribe of Articulated Animals, or Arthropoda.-Four Cla.s.ses of Articulated Animals: Branchiata, or Crustacea, breathing through gills; Jointed Crabs; Mailed Crabs; Articulata Tracheata, breathing through Air Tubes.-Spiders (Long Spiders, Round Spiders).-Myriopods.-Insects.-Chewing and Sucking Insects.-Pedigree and History of the Eight Orders of Insects.

The great natural main groups of the animal kingdom, which we have distinguished as TRIBES, or PHYLA ("types" according to Bar and Cuvier), are not all of equal systematic importance for our phylogeny or history of the pedigree of the living world. They can neither be cla.s.sed in a single series of stages, one above another, nor be considered as entirely independent stems, nor as equal branches of a single family-tree. It seems rather (as we saw in the last chapter) that the tribe of Protozoa, the so-called primaeval animals, is the common radical group of the whole animal kingdom. Out of the Gastraeada-which we cla.s.s among the Protozoa-the Zoophytes and the Worms have developed, as two diverging branches. We must now in turn look upon the varied and much-branching tribe of Worms as the common primary group, out of which (from perfectly distinct branches) arose the remaining tribes, the four higher phyla of the animal kingdom. (Compare the Pedigree, p. 133.)

Let us now take a genealogical look at these four higher tribes of animals, and try whether we cannot make out the most important outlines of their pedigree. Even should this attempt prove defective and imperfect, we shall at all events have made a beginning, and paved the road for subsequent and more satisfactory attempts.

It does not matter in what succession we take up the examination of the four higher tribes. For these four phyla have no close relationship whatever among one another, but have grown out from entirely distinct branches of the group of Worms (p. 133). We may consider the tribe of Molluscs as the most imperfect and the lowest in point of morphological development. We nowhere meet among them with the characteristic articulation or segmented formation of the body, which distinguishes even the Ring-worms, and which in the other three higher tribes-the Echinoderma, Articulata, and Vertebrata-is most essentially connected with the high development of their forms, their differentiation, and perfection. The body in all Molluscs-in mussels, snails, etc.-is a simple non-jointed sack, in the cavity of which lie the intestines. The nervous system consists not of a cord but of several distinct (generally three) pairs of knots loosely connected with one another. For these and many other anatomical reasons, I consider the tribe of Molluscs (in spite of the high physiological development of its most perfect forms) to be morphologically the lowest among the four higher tribes of animals.

Whilst, for reasons already given, we exclude the Moss-polyps, and Tunicates-which have hitherto been generally cla.s.sed with the tribe of Molluscs-we retain as genuine Molluscs the following four cla.s.ses: Lamp-sh.e.l.ls, Mussels, Snails, and Cuttles. The two lower cla.s.ses of Molluscs, the Lamp-sh.e.l.ls and Mussels, possess neither head nor teeth, and they can therefore be comprised under one main cla.s.s, or branch, as headless animals (Acephala), or toothless animals (Anodontoda). This branch is also frequently called that of the clam-sh.e.l.ls (Conchifera, or Bivalvia), because all its members possess a two-valved calcareous sh.e.l.l. In contrast to these the two higher cla.s.ses of Molluscs, the snails and cuttles, may be represented as a second branch with the name of Head-bearers (Cephalophora), or Tooth-bearers (Odontophora), because both head and teeth are developed in them.

The soft, sack-shaped body in most Molluscs is protected by a calcareous sh.e.l.l or house, which in the Acephala (lamp-sh.e.l.ls and mussels) consists of two valves, but in the Cephalophora (snails and cuttles) is generally a spiral tube (the so-called snail's house). Although these hard skeletons are found in large quant.i.ties in a petrified state in all the neptunic strata, yet they tell us but little of the historical development of the tribe, which must have taken place for the most part in the primordial period. Even in the Silurian strata we find fossil remains of all the four cla.s.ses of Molluscs, one beside the other, and this, conjointly with much other evidence, distinctly proves that the tribe of Molluscs had then obtained a strong development, when the higher tribes, especially the Articulates and Vertebrates, had scarcely got beyond the beginning of their historical development. In subsequent periods, especially in the primary and secondary periods, these higher tribes increased in importance more and more at the expense of Molluscs and Worms, which were no match for them in the struggle for life, and accordingly decreased in number. The still living Molluscs and Worms must be considered as only a proportionately small remnant of the vast molluscan fauna, which greatly predominated in the primordial and primary periods over the other tribes. (Compare Plate VI. and explanation in the Appendix.)

No tribe of animals shows more distinctly than do the Molluscs, how very different the value of fossils is in geology and in phylogeny. In geology the different species of the fossil sh.e.l.ls of Molluscs are of the greatest importance because they serve as excellent marks whereby to characterize the different groups of strata, and to fix their relative ages. As far as relates to the genealogy of Molluscs, however, they are of very little value, because, on the one hand, the sh.e.l.ls are parts of quite subordinate morphological importance, and because the actual development of the tribe belongs to the earlier primordial period, from which no distinct fossils have been preserved. If therefore we wish to construct the pedigree of Molluscs, we are mainly dependent upon the records of ontogeny and comparative anatomy from which we obtain something like the following result. (Gen. Morph. ii. Plate VI. pp.

102-116.)

The lowest stage of the four cla.s.ses of genuine Molluscs known to us, is occupied by the Lamp-sh.e.l.ls or Spiral-gills (Spirobranchia), frequently but inappropriately called Arm-footers (Brachiopoda), which have become attached to the bottom of the sea. There now exist but few forms of this cla.s.s; for instance, some species of Lingula, Terebratula, and others akin to them, which are but feeble remnants of the great variety of forms which represented the Lamp-sh.e.l.ls in earlier periods of the earth's history. In the Silurian period they const.i.tuted the princ.i.p.al portion of the whole Mollusc tribe. From the agreement which, in many respects, their early stage of development presents with the Moss animals, it has been concluded that they have developed out of Worms, which were nearly related to this cla.s.s. Of the two sub-cla.s.ses of Lamp-sh.e.l.ls, the Hinge-less (Ecardines) must be looked upon as the lower and more imperfect, the Hinged (Testicardines) as the higher and more fully developed group.

The anatomical difference between the Lamp-sh.e.l.ls and the three other cla.s.ses of Molluscs is so considerable that the latter may be distinguished from the former by the name of Otocardia. All the Otocardia have a heart with chamber (ventricle) and ante-chamber (auricle), whereas Lamp-sh.e.l.ls do not possess the ante-chamber.

Moreover, the central nervous system is developed only in the former (and not in the latter) in the shape of a complete pharyngeal ring.

Hence the four cla.s.ses of Molluscs may be grouped in the following manner:-

{ 1. Lamp-sh.e.l.ls } I. Haplocardia I. Molluscs { (Spirobranchia). } (with simple heart).

without head. { _Acephala._ { 2. Mussels } { (Lamellibranchia). } II. Otocardia } (with chamber II. Molluscs { 3. Snails } and ante-chamber with head. { (Cochlides). } to the heart).

_Cephalophora._ { 4. Cuttles } { (Cephalopoda). }

The result of these structural dispositions for the history of the pedigree of Molluscs, which is confirmed by palaeontology, is that Lamp-sh.e.l.ls stand much nearer to the primaeval root of the whole tribe of Molluscs than do the Otocardia. Probably Mussels and Snails developed as two diverging branches out of Molluscs, which were nearly akin to the Lamp-sh.e.l.ls.

Mussels, or Plate-gills (Lamellibranchia), possess a bivalved sh.e.l.l like the Lamp-sh.e.l.ls. In the latter, one of the two valves covers the back, the other the belly of the animal; whereas in Mussels the two valves lie symmetrically on the right and left side of the body. Most Mussels live in the sea, only a few in fresh water. The cla.s.s is divided into two sub-cla.s.ses, Asiphonia and Siphonida, of which the latter were developed at a later period out of the former. Among the Asiphonia are Oysters, mother-of-pearl Sh.e.l.ls, and fresh water Mussels; among the Siphonida, which are characterized by a respiratory tube, are the Venus-sh.e.l.ls, Razor-sh.e.l.ls, and Burrowing Clams. The higher Molluscs seem to have developed at a later period out of those without head and teeth; they are distinguished from the latter by the distinct formation of the head, and more especially by a peculiar kind of tooth apparatus. Their tongue presents a curious plate, armed with a great number of teeth. In our common Vineyard Snail (Helix pomatia) the number of teeth amount to 21,000, and in the large Garden Slug (Limax maximus) to 26,800.

SYSTEMATIC SURVEY

_Of the 4 Cla.s.ses, 8 Sub-cla.s.ses, and 21 Orders of Molluscs._

------------------+-----------------------+------------------------+------------------- _Cla.s.ses of_ | _Sub-cla.s.ses of_ | _Orders of_ | _Systematic Name_ _Molluscs._ | _Molluscs._ | _Molluscs._ | _of the Orders._ ------------------+-----------------------+------------------------+------------------- I. _Molluscs without head or teeth_: ACEPHALA _or_ ANODONTODA.

--------------------------------------------------------------------------------------- I. { I. Ecardines { 1. Stalked 1. Lingulida =Lamp-sh.e.l.ls= { _Hinge-less_ { 2. Flattened 2. Craniada { +Spirobranchia+ { or { +Brachiopoda+ { II. Testicardines { 3. Fleshy armed 3. Sarcobrachia { _Hinge-less_ { 4. Calcareous-armed 4. Sclerobrachia

II. { III. Asiphonia { 5. One-muscled 5. Monomya =Mussels= { _Mussels without_ { 6. Uneven-muscled 6. Heteromya or { _respiratory tubes_ { 7. Even-muscled 7. Isomya =Plate-gills= { { +Lamellibranchia+ { IV. Siphonida { 8. Round-mantled 8. Integripallia or { _Mussels with_ { 9. Ray-mantled 9. Sinupalliata +Phyllobranchia+ { _respiratory tubes_ { 10. Tube-mussels 10. Inclusa

--------------------------------------------------------------------------------------- II. _Molluscs with head and teeth_: CEPHALOPHORA _or_ ODONTOPHORA.

{ V. Stump-headed { 11. Tube-snails 11. Scaphopoda III. { _Perocephala_ { 12. b.u.t.terfly-snails 12. Pteropoda =Snails= { { { 13. With hind gills 13. Opisthobranchia +Cochlides+ { { 14. With fore gills 14. Prosobranchia or { VI. Large-headed { 15. Swimming-snails 15. Heteropoda +Gasteropoda+ { _Delocephala_ { 16. Beetle-snails 16. Chitonoida { { 17. Snails with lungs 17. Pulmonata

VII. Chamber-Poulps { 18. Pearl boats 18. Nautilida IV. with four gills { 19. Ammon's horns 19. Ammonitida =Cuttles= } _Tetrabranchia_ { or } =Poulps= } VIII. Ink-Poulps with { 20. Ten-armed 20. Decabrachiones } two gills { +Cephalopoda+ } _Dibranchia_ { 21. Eight-armed 21. Octobrachiones

Dibranchia Heteropoda | | _Prosobranchia_ | _Pulmonata_ | | Tetrabranchia | Lipobranchia | | Cephalopoda | | | | (=Cuttles= or =Poulps=) | Gymn.o.branchia | | | | | -------v------/ | | | | | | Pleurobranchia | Chitonides | | _Opisthobranchia_ | | | | | | | | ---------------------------------v---------/ | | | _Delocephala_ | | | | | --------v----------/ | | _Pteropoda_ Inclusa | | | | | | ------v-----/ Sinupalliata | | Scaphopoda | | | | Integripalliata ---v--/ Sclerobrachia _Siphoniata_ | | | _Perocephala_ | | Cochlides | | (=Snails=) Sarcobrachia _Asiphonia_ | _Testicardines_ Lamellibranchia | | (=Mussels=) | | | | _Ecardines_ ----------v-------------/ Spirobranchia | (=Lamp-sh.e.l.ls=) Otocardia | (Molluscs with chamber and ante-chamber | to the heart) | | | | -----------------v--------------/ | Promollusca (Primaeval Molluscs) Molluscs with simple heart | (Worms) | Gastraea

We distinguish two sub-cla.s.ses among the Snails (Cochlides, or Gasteropoda), namely, the Stump-headed and the Large-headed Snails. The Stump-headed Snails (Perocephala) are very closely allied to Mussels (through the Tooth-sh.e.l.ls), and also to the Cuttle-fish (through the b.u.t.terfly-snails). The more highly developed Snails, with large heads (Delocephala), can be divided into Snails with gills (Branchiata) and Snails with lungs (Pulmonata). Among the latter are the Land-snails, the only Molluscs which have left the water and become habituated to a life on land. The great majority of Snails live in the sea, only a few live in fresh water. Some River-snails in the tropics (the Ampullaria) are amphibious, living sometimes on land, sometimes in water, and at one time they breathe through gills, at another through lungs. They have both kinds of respiratory organs, like the Mud-fish and Gilled Newts among the Vertebrata.

The fourth and last cla.s.s, and at the same time the most highly developed cla.s.s of Molluscs, is that of the Cuttles, or Poulps, also called Cephalopoda (foot attached to the head). They all live in the sea, and are distinguished from Snails by eight, ten, or more long arms, which surround the mouth in a circle. The Cuttles existing in our recent oceans-the Sepia, Calamary, Argonaut, and Pearly Nautilus-are, like the few Spiral-gill Lamp-sh.e.l.ls of the present time, but a poor remnant of the host which represents this cla.s.s in the oceans of the primordial, primary, and secondary periods. The numerous fossil "Ammon's horns"

(Ammonites), "pearl boats" (Nautilus), and "thunderbolts" (Belemnites) are evidences of the long since extinct splendour of the tribe. The Poulps, or Cuttles, have probably developed out of a low branch of the snail cla.s.s, out of the b.u.t.terfly-snails (Pteropoda) or kindred forms.

The different sub-cla.s.ses and orders, distinguished in the four cla.s.ses of Molluscs, whose systematic succession is given on the Table (p.

160), furnish various proofs of the validity of the law of progress by their historical development and by the systematic development corresponding to it. As however these subordinate groups of Molluscs are in themselves of no further special interest, I must refer to the sketch of their pedigree on p. 161, and to the detailed pedigree of Molluscs which I have given in my General Morphology, and I shall now at once turn to the consideration of the tribe of Star-fishes.

The Star-fishes (Echinoderma, or Estrellae) among which are the four cla.s.ses of Sea-stars, Sea-lilies, Sea-urchins, and Sea-cuc.u.mbers are one of the most interesting divisions of the animal kingdom, and yet we know less about them than about any. They all live in the sea. Every one who has been at the sea sh.o.r.e must have seen at least two of their forms, the Sea-stars and the Sea-urchins. The tribe of Star-fishes must be considered as a completely independent tribe of the animal kingdom on account of its very peculiar organization, and must be carefully distinguished from the Animal-plants-Zoophytes, or Clenterata, with which it is still frequently but erroneously cla.s.sed under the name Radiata (as for example, by Aga.s.siz, who even to this day defends this error of Cuvier's, together with many others).

All Echinoderma are characterized, and at the same time distinguished from all other animals, by a very remarkable apparatus for locomotion, which consists of a complicated system of ca.n.a.ls or tubes, filled with sea water from without. The sea water in these aqueducts is moved partly by the strokes of the cilia, or vibratile hairs lining their walls, and partly by the contractions of the muscular walls of the tubes themselves, which resemble india-rubber bags. The water is pressed from the tubes into a number of little hollow feet, which thereby become widely distended, and are then employed for walking and suction. The Sea-stars are moreover characterized by a peculiar calcareous formation in the skin, which in most cases forms a firm, well-closed coat of mail, composed of a number of plates. In almost all Echinoderma the body consists of five radii (counterparts, or antimera) standing round the main axis of the body, where they meet. It is only in some species of Sea-stars that the number of these radii amount to more than five-to 6-9, 10-12, or even to 20-40; and in this case the number of radii is generally not constant, but varies in different individuals of one species.

The historical development and the pedigree of the Echinoderma are completely revealed to us by their numerous and, in most cases, excellently preserved fossil remains, by their very remarkable individual developmental history, and by their interesting comparative anatomy; this is the case with no other tribe of animals, even the Vertebrata themselves are not to be excepted. By a critical use of those three archives, and by a careful comparison of the results derived from their study, we obtain the following genealogy of the Star-fishes, which I have already published in my General Morphology (vol. ii. Plate IV.

pp. 62-77.)

The most ancient and original group of the Star-fishes, the primary form of the whole phylum, consists of the cla.s.s of the true Sea-stars (Asterida). This is established by numerous and important arguments in anatomy and the history of development, but above all by the irregular and varying number of the radii, or antimera, which in all other Echinoderma is limited, without exception, to five. Every Star-fish consists of a central, small, body-disc, all round the circ.u.mference of which are attached five or several long articulated arms. _Each arm of the Star-fish essentially corresponds in its organisation with an articulated worm_ of the cla.s.s of Ring-worms, or Annelida (p. 149). I therefore consider the Star-fish as a genuine _stock or cormus of five or more articulated worms_, which have arisen by the star-wise growth of a number of buds out of a central mother-worm. The connected members, thus grouped like the rays of a star, have inherited from the mother-worm the common opening of the mouth, and the common digestive cavity (stomach) lying in the central body-disc. The end by which they have grown together, and which fuses in the common central disc, probably corresponds to the posterior end of the original independent worms.

In exactly the same way several individuals of certain kinds of worms are united so as to form a star-like cormus. This is the case in the _Botryllidae_, compound Ascidians, belonging to the cla.s.s of the Tunicata. Here also the posterior ends of the individual worms have grown together, and have formed a common outlet for discharges, a central cloaca; whereas at the anterior end each worm still possesses its own mouth. In Star-fishes the original mouths have probably become closed in the course of the historical development of the cormus, or colony, whereas the cloaca has developed into a common mouth for the whole cormus.

SYSTEMATIC SURVEY