The History of Creation Volume II Part 23

ii. p. 140. The egg of the Olynthus (Fig. 9), which represents the common ancestral form of all Calcareous Sponges, is a simple cell (Fig.

1). From this there arises, by repeated division (Fig. 2), a globular, mulberry-like heap of numerous equi-formal cells (Morula, Fig. 3; vol.

ii. p. 125.) As the result of the change of these cells into an outer series of clear ciliated cells (Exoderm) and an inner series of dark, non-ciliated cells (Entoderm), the ciliated larva, or Planula, makes its appearance. This is oval in shape, and forms a cavity in its centre (gastric cavity, or primitive stomach, Fig. 6 _g_, with an opening (mouth-opening, or primitive mouth, Fig. 6 _o_); the wall of the gastric cavity consists of two layers of cells, or germ-layers, the outer ciliated Exoderm (_e_) and the inner non-ciliated Entoderm (_i_). Thus arises the exceedingly important stomach-larva, or Gastrula, which reappears in the most different tribes of animals as a common larval form (Fig. 5, seen from the surface; Fig. 6, in long section. Compare, vol. ii. pp. 126 and 281). After the Gastrula has swum about for some time in the sea, it fastens itself securely to the sea-bottom, loses its outer vibratile processes, or cilia, and changes into the Ascula (Fig.

7, seen from the surface; Fig. 8, in long section; letters as in Fig.

6). This Ascula is the recapitulative form, according to the biogenetic fundamental law, the common ancestor of all Zoophytes, namely, the Protascus (vol. ii. pp. 129, 133). By the development of pores in the wall of the stomach and of three-rayed calcareous spicules, the Ascula changes into the Olynthus (Fig. 9.) In Fig. 9 a piece is cut out from the stomach-wall of the Olynthus in order to show the inside of the stomachal cavity, and the eggs which are forming on the surface (_g_).

From the Olynthus the most various forms of Calcareous Sponges can develop. One of the most remarkable is the Ascometra (Fig. 10), a stock or colony from which different species, and in fact different generic forms, grow (on the left Olynthus, in the middle Nardorus, on the right Soleniscus, etc., etc.). Further details as to these most interesting forms, and their high importance for the Theory of Descent, may be found in my "Monograph of the Calcareous Sponges" (1872), especially in the first volume. (Compare vol. ii. pp. 160, 167).

PLATE I. (_Between pages 184 and 185, Vol. I._)

_History of the Life of the most Simple Organism_, a Moneron (Protomyxa aurantiaca). Compare vol. i. p. 184, and vol. ii. p. 53. The plate is a smaller copy of the drawing in my "Monographie der Moneren" (Biologische Studien, 1 Heft, 1870; Taf. 1), of the developmental history of the Protomyxa aurantiaca; I have there also given a detailed description of this remarkable Moneron (pp. 11-30). I discovered this most simple organism in January, 1867, during a stay in Lanzarote, one of the Canary Islands; and moreover I found it either adhering to, or creeping about on the white calcareous sh.e.l.ls of a small Cephalopod (vol. ii. p. 162), the Spirula Peronii, which float there in ma.s.ses on the surface of the ocean, or are thrown up on the sh.o.r.e. The Protomyxa aurantiaca is distinguished from the other Monera by the beautiful and bright orange-red colour of its perfectly simple body, which consists merely of primaeval slime, or protoplasm. The fully developed Moneron is represented in Figs. 11 and 12, very much enlarged. When it is hungry (Fig. 11), there radiate from the surface of the globular corpuscule of plasm, quant.i.ties of tree-shaped, branching and mobile threads (pseudo-feet, or pseudopodia), which do not become retiformly connected. When, however, the Moneron eats (Fig. 12), the mucous threads become variously connected, form net-works and enclose the extraneous corpuscule which serves as food, which the threads afterwards draw into the interior of the Protomyxa. Thus in Fig. 12 (above on the right), a silicious and ciliated Whip-swimmer (Peridinium, vol. ii. pp. 51, 57), has just been caught by the extended mucous filaments, and has been drawn into the interior of the mucous globule, in which there already are several half digested silicious infusoria (Tintinoida), and Diatomeae (Isthmia). Now, when the Protomyxa has eaten and grown sufficiently, it draws in all its mucous filaments (Fig. 15), and contracts into the form of a globule (Fig. 16 and Fig. 1). In this state of repose the globule secretes a simple gelatinous covering (Fig. 2), and after a time subdivides into a large number of small mucous globules (Fig. 3). These soon commence to move, become pear-shaped (Fig. 4), break through the common covering (Fig. 5), and then swim about freely in the ocean by means of a delicate whip-shaped process, like the Flagellata (vol. ii.

p. 57, Fig. 11). When they meet a Spirula sh.e.l.l, or any other suitable object, they adhere to it, draw in their whip, and creep slowly about on it by means of form-changing processes (Figs. 6, 7, 8), like Protambae (vol. i. p. 186, vol. ii. p. 52). These small mucous corpuscules take food (Figs. 9, 10), and attain their full grown form (Figs. 11, 12), either by simple growth or by several of them fusing to form a larger protoplasmic ma.s.s (Plasmodium, Figs. 13, 14).

PLATES II. AND III. (_Between pages 294 and 295, Vol. I._)

_Germs or Embryos of four different Vertebrate Animals_, namely, Tortoise (_A_ and _E_), Hen (_B_ and _F_), Dog (_C_ and _G_), and Man (_D_ and _H_). Figs. _A_, _D_, an early stage of development; Figs. _E_, _H_, a later stage. All the eight embryos are represented as seen from the right side, the curved back turned to the left. Figs. _A_ and _B_ are seven times enlarged, Figs. _C_ and _D_ five times, Figs. _E_ and _H_ four times. Plate II. exhibits the very close blood relationship between birds and reptiles; Plate III. that between man and the other mammals.

PLATE IV. (_Between pages 34 and 35, Vol. II._)

_The Hand, or Fore Foot, of nine different Mammals._ This plate is intended to show the importance of Comparative Anatomy to Phylogeny, in as much as it proves how the internal skeleton of the limbs is continually preserved by _inheritance_, although the external form is extremely changed by _adaptation_. The bones of the skeleton of the hand are drawn in white lines on the brown flesh and skin which surrounds them. All the nine hands are represented in the same position, namely the wrist (where the arm would be joined to it) is placed above, whilst the ends of the fingers or toes are turned downwards. The thumb, or the first (large) fore-toe is on the left in every figure; the little finger, or fifth toe is to the right at the edge of the hand. Each hand consists of three parts, namely (i.) the _wrist_ (carpus), composed of two cross rows of short bones (at the upper side of the hand); (ii.) the _mid-hand_ (metacarpus), composed of five long and strong bones (marked in the centre of the hand by the numbers 1-5); and (iii.) the five _fingers_, or _fore toes_ (digiti), every one of which again consists of several (mostly from two to three), _toe-pieces_, or _phalanges_. The hand of _man_ (Fig. 1), in regard to its entire formation, stands midway between that of the two large human apes, namely, that of the _gorilla_ (Fig. 2), and that of the _orang_ (Fig. 3). The fore paw of the _dog_ (Fig. 4), is more different, and the hand or breast fin of the _seal_ (Fig. 5) still more so. The adaptation of the hand to the movement of swimming, and its transformation into a fin for steering, is still more complete in the _dolphin_ (Ziphius, Fig. 6). The extended fingers and bones of the central hand here have remained short and strong in the swimming membrane, but they have become extremely long and thin in the _bat_ (Fig. 7), where the hand has developed into a wing.

The extreme opposite of the latter formation is the hand of the _mole_ (Fig. 8), which has acquired a powerful spade-like form for digging, with fingers which have become extremely short and thick. What is far more like the human hand than these latter forms, is the fore paw of the lowest and most imperfect of all mammals, the Australian _beaked animal_ (Ornithorhynchus, Fig. 9), which in its whole structure stands nearer to the common, extinct, primary form of mammalia, than any known species.

Hence man differs less in the formation of the hand from this common primary form than from the bat, mole, dolphin, seal, and many other mammals.

PLATE V. (_Between pages 84 and 85, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Vegetable Kingdom_, representing the hypothesis of the common derivation of all plants, and the historical development of the different groups of plants during the palaeontological periods of the earth's history. The horizontal lines denote the different smaller and larger periods of the organic history of the earth (which are spoken of in vol. ii. p. 14), and during which the strata containing fossils were deposited. The vertical lines separate the different main-cla.s.ses and cla.s.ses of the vegetable kingdom from one another. The arboriform and branching lines indicate, in an approximate manner, by their greater or less number and thickness, the greater or less degree of development, differentiation, and perfecting which each cla.s.s probably attained in each geological period. (Compare vol. ii. pp. 82, 83.)

PLATE VI. (_Between pages 130 and 131, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Animal Kingdom_, representing the _historical growth of the six animal tribes_ during the palaeontological periods of the organic history of the earth. The horizontal lines _g h_, _i k_, _l m_, and _n o_ divide the five large periods of the organic history of the earth one from another. The field _g a b h_ comprises the archilithic, the field _i g h k_, the palaeolithic, the field _l i k m_ the mesolithic, and the field _n l o m_ the cenolithic period. The short, anthropolithic period is indicated by the line _n o_. (Compare vol. ii. p. 14.) The height of the separate fields corresponds with the relative length of the periods indicated by them, as they may approximately be estimated from the relative thickness of the neptunic strata deposited between them. (Compare vol. ii. p. 22.) The archilithic and primordial period alone, during which the Laurentian, Cambrian, and Silurian strata were deposited, was probably considerably longer than the four subsequent periods taken together.

(Compare vol. ii. pp. 10, 20.) In all probability the two tribes of worms and Zoophytes attained their full development during the mid-primordial period (in the Cambrian system); the star-fishes and molluscs probably somewhat later (in the Silurian system); whereas the articulata and vertebrata are still increasing in variety and perfection.

PLATE VII. (_Between pages 146 and 147, Vol. II._)

_Group of Animal-Trees_ (_Zoophytes, or Clenterata_) _in the Mediterranean_. On the upper half of the plate is a swarm of swimming medusae and ctenophora; on the lower half a few bunches of corals and hydroid polyps adhering to the bottom of the sea. (Compare the system of Zoophytes, vol. ii. p. 132, and on the opposite page their pedigree.) Among the adhering Zoophytes at the bottom of the ocean there is, below on the right hand, a large coral-colony (1), which is closely akin to the red precious coral (Eucorallium), and like the latter belongs to the group of corals with eight rays (Octocoralla Gorgonida); the single individuals (or persons) of the branching stock have the form of a star with eight rays, consisting of eight tentacles, which surround the mouth. (Octocoralla, vol. ii. p. 143.) Directly below and in front of it (quite below on the right), is a small bush of hydroid polyps (2), belonging to the group of bell-polyps, or Campanulariae (vol. ii. p.

146). A larger stock of hydroid polyps (3), belonging to the group of tube-polyps, or Tubullariae, rises, to the left, on the opposite side, with its long thin branches. At its base is spread a stock of silicious sponges (Halichondria) (4), with short, finger-shaped branches (vol.

ii. p. 139). Behind it, below on the left (5), is a very large marine rose (Actinia), a single individual from the cla.s.s of six-rayed corals (Hexacoralla, vol. ii. p. 143). Its low, cylindrical body has a crown of very numerous and large leaf-shaped tentacles. Below, in the centre of the ground (6), is a sea-anemone (Cereanthus) from the group of fourfold corals (Tetracoralla). Lastly, on a small hill on the bottom of the sea, there rises, on the right above the corals (1) a cup-polyp (Lucernaria), as the representative of the stalked-jellies.

(Podactinaria, or Calycozoa, vol. ii. p. 144.) Its cup-shaped, stalked body (7) has eight globular cl.u.s.ters of small, knotted tentacles on its rim.

Among the _swimming Zoophytes_ which occupy the upper half of Plate VII., the hydromedusae are especially remarkable, on account of their alteration of generation. (Compare vol. i. p. 206.) Directly above the Lucernaria (7) floats a small tiara jelly (Oceania), whose bell-shaped body has a process like a dome, the form of a papal tiara (8). From the opening of the bell there hangs a wreath of very fine and long tentacles. This Oceania is the offspring of a tube-polyp, resembling the adhering Tubularia below on the left (3). Beside this latter, on the left, swims a large but very delicate hair-jelly (aequorea). Its disc-shaped, slightly arched body is just drawing itself together, and pressing water out of the cavity of the cup lying below (9). The numerous, long, and fine hair-like tentacles which hang down from the rim of the cup are drawn by the ejected water into a conical bunch, which towards the centre turns upwards like a collar, and is thrown into folds. Above, in the middle of the cavity of the cup, hangs the stomach, the mouth of which is surrounded by four lobes. This aequorea is derived from a small bell-polyp, resembling the Campanularia (2). The small, slightly arched cap-jelly (Eucope), swimming above in the centre (10), is likewise derived from a similar bell-polyp. In these three last cases (8, 9, 10), as in the majority of the hydromedusae, the alternation of generation consists in the freely swimming medusa (8, 9, 10), arising by the formation of buds (therefore by non-s.e.xual generation, vol. i. p.

192), from adhering hydroid polyps (2, 3). These latter, however, originate out of the fructified eggs of the medusae (therefore by s.e.xual generation, vol. i. p. 195). Hence the non-s.e.xual, adhering generation of polyps (I., III., V., etc.) regularly alternates with the s.e.xual, freely swimming generation of medusae (II., IV., VI., etc.). This alteration of generation can only be explained by the Theory of Descent.

The same remark applies to a kindred form of propagation, which is still more remarkable, and which I discovered in 1864, near Nice, in the Elephant-jellies (Geryonida), and called _allogony_, or _allogenesis_.

In this case two completely distinct forms of medusa are descended from one another; the larger and more highly developed generation (11), Geryonia, or Carmarina, is six-rayed, with six foliated s.e.xual organs, and six very movable marginal filaments. From the centre of its bell-shaped cup, like the tongue of a bell, hangs a long proboscis, at the end of which is the opening of the mouth and stomach. In the cavity of the stomach is a long, tongue-shaped bunch of buds (which on Plate VII. (_n_) is extended from the mouth on the left like a tongue). On this tongue, when the Geryonia is s.e.xually ripe, there bud a number of small medusae. They are, however, not Geryoniae, but belong to an entirely distinct but very different form of medusa, namely, to the genus Cunina, of the family of the _aeginida_. This Cunina (12) is very differently constructed; it has a flat, semi-globular cup without proboscis, consists in early life of six divisions, later of sixteen, and has sixteen bag-shaped s.e.xual organs, and sixteen short, stiff, and strongly curved tentacles. A further explanation of this wonderful allogenesis may be found in my "Contributions to the Natural History of the Hydromedusae." (_Leipzig_, Englemann, 1865), the first part of which contains a monograph of the Elephant-jellies, or Geryonida, ill.u.s.trated by six copper-plates.

Even more interesting and instructive than these remarkable relations are the vital phenomena of the _Siphonophora_, whose wonderful polymorphism I have frequently spoken of, and described in a popular manner in my lecture on "Differentiation in Nature and Human Life."(37) (Compare vol. i. p. 270, and vol. ii. p. 140.) An example of this is given in Plate VII. in the drawing of the beautiful Physophora (13).

This swimming stock or colony of hydromedusae is kept floating on the surface of the sea by a small swimming bladder filled with air, which in the drawing is seen rising above the surface of the water. Below it is a column of four pairs of swimming bells, which eject water, and thereby set the whole colony in motion. At the lower end of the column of swimming bells is a crown-shaped wreath of curved spindle-shaped _sensitive polyps_, which also serve as a covering, under the protection of which the other individuals of the stock (the eating, catching, and reproductive persons) are hidden. The _ontogenesis_ of the Siphonophora (and especially of this Physophora), I first observed in Lanzerote, one of the Canary Islands, in 1866, and described in my "History of the Development of the Siphonophora," and added fourteen plates for its explanation. (Utrecht, 1869). It is rich in interesting facts, which can only be explained by the Theory of Descent.

Another circ.u.mstance, which is also only explicable by the Theory of Descent, is the remarkable change of generation in the higher medusae, the disc-jellies (Discomedusae, vol. ii. p. 136), a representative of which is given at the top of Plate VII., in the centre (rather in the background), namely, a Pelagia (14). From the bottom of the bell-shaped cup, which is strongly arched and the rim of which is neatly indented, there hang four very long and strong arms. The non-s.e.xual polyps, from which these disc-jellies are derived, are exceedingly simple primaeval polyps, differing very little from the common fresh-water polyp (Hydra).

The alternation of generation in these Discomedusae has also been described in my lecture on Differentiation,(37) and there ill.u.s.trated by the Aurelia by way of example.

Finally, the last cla.s.s of Zoophytes, the group of comb-jellies (Ctenophora, vol. ii. p. 142), has two representatives on Plate VII. To the left, in the centre, between the aequorea (9), the Physophora (13), and the Cunina (12), is a long and thin band like a belt (15), winding like a snake; this is the large and splendid _Venus' girdle_ of the Mediterranean (Cestum), the colours of which are as varied as those of the rainbow. The actual body of the animal, which lies in the centre of the long belt, is very small, and constructed exactly like that of the _melon-jelly_ (Cydippe), which floats above to the left (16). On the latter are visible the eight characteristic fringed bands, or ciliated combs, of the ctenophora, and also two long tentacles which extend right across the page, and are fringed with still finer threads.

PLATES VIII. AND IX. (_Between pages 170 and 171, Vol. II._)

_History of the Development of Star-fishes_ (_Echinoderma_, or _Estrella_). The two plates exhibit their alternation of generation (vol. ii. p. 168), with an example from each of the four cla.s.ses of Star-fishes. The sea-stars (Asterida) are represented by Uraster (_A_), the sea-lilies (Crinoida) by Comatula (_B_), the sea-urchins (Echinida) by Echinus (_C_), and finally, the sea-cuc.u.mbers (Holothuriae) by Synapta (_D_). (Compare vol. ii. pp. 166 and 176.) The successive stages of development are marked by the numbers 1-6.

Plate VIII. represents the individual development of the first and non-s.e.xual generation of Star-fishes, that is, of the _nurses_ (usually, but erroneously, called larvae). These nurses possess the form-value of a simple, unsegmented worm-individual. Fig. 1 represents the egg of the four Star-fishes; and it, in all essential points, agrees with that of man and of other animals. (Compare vol. i. p. 297, Fig. 5.) As in man, the protoplasm of the egg-cell (the yolk) is surrounded by a thick, structureless membrane (zona pellucida), and contains a globular, cell-kernel (nucleus), as clear as gla.s.s, which again encloses a nucleolus. Out of the fertilised egg of the Star-fish (Fig. _A_ 1) there develops in the first place, by the repeated sub-division of cells, a globular ma.s.s of h.o.m.ogeneous cells (Fig. 6, vol. i. p. 299), and this changes into a very simple nurse, which has almost the same shape as a wooden shoe (Fig. _A_ 2-_D_ 2). The edge of the opening of the shoe is bordered by a fringe of cilia, the ciliary movements of which keep the microscopically small and transparent nurse swimming about freely in the sea. This fringe of cilia is marked in Fig. _A_ 2-_A_ 4, on Plate VII., by the narrow alternately light and dark seam. The nurse then, in the first place, forms a perfectly simple intestinal ca.n.a.l for nutrition, mouth (_o_), stomach (_m_) and a.n.u.s (_a_). Later, the windings of the fringe of cilia become more complicated, and there arise arm-like processes (Fig. _A_ 3-_D_ 3). In sea-stars (_A_ 4) and sea-urchins (_C_ 4) these arm-like processes, which are fringed with cilia, afterwards become very long. But in the case of sea-lilies (_B_ 3) and sea-cuc.u.mbers (_D_ 4), instead of this, the fringe of cilia, which at first, through winding in and out, forms one closed ring, changes subsequently into a succession of separate ciliated girdles, one lying behind the other.

In the interior of this curious nurse there then develops, by a non-s.e.xual process of generation, namely, by the formation of internal buds or germ-buds (round about the stomach), the second generation of Star-fishes, which later on become s.e.xually ripe. This second generation, which is represented on Plate IX. in a fully developed condition, exists originally as a stock or cormus of five worms, connected at one end in the form of a star, as is most clearly seen in the sea-stars, the most ancient and original form of the star-fishes.

The second generation, which grows at the expense of the first, appropriates only the stomach and a small portion of the other organs of the latter, but forms for itself a new mouth and a.n.u.s. The fringe of cilia, and the other parts of the body of the nurse, afterwards disappear. The second generation (_A_ 5-_D_ 5), is at first smaller or not much larger than the nurse, whereas, by growth, it afterwards becomes more than a hundred times, or even a thousand times, as large.

If the ontogeny of the typical representatives of the four cla.s.ses of Star-fishes be compared, it is easily seen that the original kind of development has been best preserved in sea-stars (_A_) and sea-urchins (_C_) by inheritance, whereas in sea-lilies (_B_) and sea-cuc.u.mbers it has been suppressed according to the laws of abbreviated inheritance (vol. i. p. 212).

Plate IX. shows the fully developed and s.e.xually mature animals of the second generation from the mouth side, which, in the natural position of Star-fishes (when creeping at the bottom of the sea), in sea-stars (_A_ 6) and sea-urchins (_C_ 6), is below, in sea-lilies (_B_ 6) above, and in sea-cuc.u.mbers (_D_ 6) in front. In the centre we perceive, in all the four Star-fishes, the star-shaped, five-pointed opening of the mouth. In sea-stars, from each arm there extend several rows of little sucking feet, from the centre of the under-side of each arm to the end. In sea-lilies (_B_ 6), each arm is split and feather-like from its base upwards. In sea-urchins (_C_ 6) the five rows of sucking feet are divided by broader fields of spines. In sea-cuc.u.mbers, lastly (_D_ 6), on the worm-like body it is sometimes only the five rows of little feet, sometimes only the feathery tentacles surrounding the mouth, from five to fifteen (in this case ten), that are externally visible.

PLATES X. AND XI. (_Between pages 174 and 175, Vol. II._)

_Historical Development of the Crab-fish_ (Crustacea).-The two plates ill.u.s.trate the development of the different Crustacea from the nauplius, their common primaeval form. On Plate XI. six Crustacea, from six different orders, are represented in a fully developed state, whereas on Plate X. the early nauplius stages are given. From the essential agreement between the latter we may, on the ground of the fundamental law of biogeny, with full a.s.surance maintain the derivation of the different Crustacea from a single, common primary form, a long since extinct Nauplius, as was first shown by Fritz Muller in his excellent work "Fur Darwin."(16)

Plate X. represents the _early nauplius stages_ from the ventral side, so that the three pairs of legs, on the short, three-jointed trunk are distinctly visible. The first of these pairs of legs is simple and unsegmented, whereas the second and third pairs are forked. All three pairs are furnished with stiff bristles, which, through the paddling motion of the legs, serve as an apparatus for swimming. In the centre of the body, the perfectly simple, straight intestinal ca.n.a.l is visible, possessing a mouth in front, and an a.n.a.l orifice behind. In front, above the mouth, lies a simple, single eye. All the six forms of nauplius entirely agree in all these essential characteristics of organization, whereas the six fully developed forms of Crustacea belonging to them, Plate XI., are extremely different in organisation. The differences of the six nauplius forms are confined to quite subordinate and unessential relations in regard to size of body, and the formation of the covering of the skin. If they could be met with in this form in a s.e.xually mature condition, no zoologist would hesitate to regard them as six different species of one genus. (Compare vol. ii. p. 175.)

Plate XI. represents those fully developed and s.e.xually mature forms of Crustacea, as seen from the right side, which have ontogenetically (hence also phylogenetically) developed out of the six kinds of nauplius. Fig. _A c_ shows a freely swimming fresh-water crab (Limnetis brachyurus) from the order of the Leaf-foot Crabs (Phyllopoda), slightly enlarged. Of all the still living Crustacea, this order, which belongs to the legion of Gill-foot Crabs (Branchiopoda), stands nearest to the original, common primary form of nauplius. The Limnetis is enclosed in a bivalved sh.e.l.l, like a mussel. Our drawing (which is copied from Grube) represents the body of a female animal lying in the left sh.e.l.l; the right half of the sh.e.l.l has been removed. In front, behind the eye, we see the two feelers (antennae), and behind them the twelve leaf-shaped feet of the right side of the body, behind on the back (under the sh.e.l.l), the eggs. Above, in front, the animal is fixed to the sh.e.l.l.

Fig. _B c_ represents a common, freely swimming fresh-water crab (Cyclops quadricornis) from the order of Oar-legged crabs (Eucopepoda), highly magnified. In front, below the eye, we see the two feelers of the right side, the foremost of which is longer than the hinder one. Behind these are the gills, and then the four paddling legs of the right side.

Behind these are the two large egg-sacks, which, in this case, are attached to the end of the hinder part of the body.

Fig. _C c_ is a parasitic Oar-legged crab (Lernaeocera esocina), from the order of fish lice (Siphonostoma). These peculiar crabs, which were formerly regarded as worms, have originated, by adaptation to a parasitical life, out of freely swimming, Oar-legged crabs (Eucopepoda), and belong to the same legion (Copepoda, vol. ii. p. 176). By adhering to the gills on the skin of fish or other crabs, and feeding on the juice of these creatures, they forfeited their eyes, legs, and other organs, and developed into formless, inarticulated sacks, which, on a mere external examination, we should never suppose to be animals. On the ventral side only there exist, in the shape of short, pointed bristles, the last remains of legs which have now almost entirely disappeared. Two of these rudimentary pairs of legs (the third and fourth) are seen in our drawing on the right. Above, on the head, we see thick, shapeless appendages, the lower ones of which are split. In the centre of the body is seen the intestinal ca.n.a.l, which is surrounded by a dark covering of fat. At its posterior end is the ovary, and the cement-glands of the female s.e.xual apparatus. The two large egg-sacks hang externally (as in the Cyclops, Fig. _B_). Our Lernaeocera is represented in half profile, and is copied from Claus. (Compare Claus, "Die Copepoden-Fauna von Nizza. Ein Beitrag zur Characteristik der Formen und deren Abanderungen im Sinne Darwins." Marburg, 1866).

Fig. _D c_ represents a so-called "duck mussel" (Lepas anatifera), from the order of the Barnacle crabs (Cirripedia). These crabs, upon which Darwin has written a very careful monograph, are, like mussels, enclosed in a bivalved, calcareous case, and hence were formerly (even by Cuvier) universally regarded as a kind of mussel, or mollusc. It was only from a knowledge of their ontogeny, and their early nauplius form (_D n_, Plate VIII.), that their crustacean nature was proved. Our drawing shows a "duck mussel" of the natural size, from the right side. The right half of the bivalved sh.e.l.l has been removed, so that the body is seen lying in the left half of the sh.e.l.l. From the rudimentary head of the Lepas there issues a long, fleshy stalk (curving upwards in our drawing); by means of it the Barnacle crab grows on rocks, ships, etc. On the ventral side are six pairs of feet. Every foot is forked and divided into two long, curved, or curled "tendrils" furnished with bristles. Above and behind the last pair of feet projects the thin cylindrical tail.

Fig. _E c_ represents a parasitic sack-crab (Sacculina purpurea) from the order of Root-crabs (Rhizocephala). These parasites, by adaptation to a parasitical life, have developed out of Barnacle crabs (Fig. _D c_), much in the same way as the fish-lice (_C c_), out of the freely swimming Oar-legged crabs (_B c_). However, the suppression, and the subsequent degeneration, of all of the organs, has gone much further in the present case than in most of the fish-lice. Out of the articulated crab, possessing legs, intestine, and eye, and which in an early stage as nauplius (_E n_, Plate VIII.), swam about freely, there has developed a formless, unsegmented sack, a red sausage, which now only contains s.e.xual organs (eggs and sperm) and an intestinal rudiment. The legs and the eye have completely disappeared. At the posterior end is the opening of the genitals. From the mouth grows a thick bunch of numerous tree-shaped and branching root-like fibres. These spread themselves out (like the roots of a plant in the ground) in the soft hinder part of the body of the hermit-crab (Pagurus), upon which the root-crab lives as a parasite, and from which it draws its nourishment. Our drawing (_E c_), a copy of Fritz Muller's, is slightly enlarged, and shows the whole of the sausage-shaped sack-crab, with all its root-fibres, when drawn out of the body upon which it lives.

Fig. _F c_ is a _shrimp_ (Peneus Mulleri), from the order of _ten-foot_ crabs (Decapoda), to which our river cray-fish, and its nearest relative, the lobster, and the short-tailed sh.o.r.e-crabs also belong.

This order contains the largest and, gastronomically, the most important crabs, and belongs, together with the mouth-legged and split-legged crabs, to the legion of the stalk-eyed mailed crabs (Podophthalma). The shrimp, as well as the river crab, has in front, on each side below the eye, two long feelers (the first much shorter than the second), then three jaws, and three jaw-feet, then five very long legs (the three fore ones of which, in the Peneus, are furnished with nippers, and the third of which is the longest). Finally, on the first five joints of the hinder part of the body there are other five pairs of feet. This shrimp, which is one of the most highly developed and perfect crabs, originates (according to Fritz Muller's important discovery) out of a nauplius (_F n_ Plate VIII.), and consequently proves that the higher Crustacea have developed out of the same form as the lower ones, namely, the nauplius.

(Compare vol. ii. p. 175.)

PLATES XII. AND XIII. (_Between pages 200 and 201, Vol. II._)

_Blood relationship between the Vertebrata and the Invertebrata._ (Compare vol. ii. pp. 152 and 201.) It is definitely established by Kowalewski's important discovery, which was confirmed by Kupffer, that the ontogeny of the lowest vertebrate animal-the Lancelet, or Amphioxus-agrees in all essential outlines completely with that of the invertebrate Sea-squirts, or Ascidiae, from the cla.s.s of Sea-sacks, or Tunicata. On our two plates, the ascidia is marked by _A_, the amphioxus by _B_. Plate XIII. represents these two very different animal-forms in a _fully developed_ state, as seen from the _left side_, the end of the mouth above, the opposite end below. Hence, in both figures the dorsal side is to the right, the ventral to the left. Both figures are slightly magnified, and the internal organisation of the animals is distinctly visible through the transparent skin. The full-grown ascidia (Fig. _A_ 6) grows at the bottom of the ocean, from whence it cannot move, and clings to stones and other objects by means of peculiar roots (_w_) like a plant. The full-grown amphioxus, on the other hand (Fig. _B_ 6), swims about freely like a small fish. The letters on both figures indicate the same parts: (_a_) orifice of the mouth; (_b_) orifice of the body, or porus abdominalis; (_c_) dorsal rod, or chorda dorsalis; (_d_) intestine; (_e_) ovary; (_f_) oviduct (same as the sperm-duct); (_g_) spinal marrow; (_h_) heart; (_i_) blind-sac of the intestine; (_k_) gill basket (respiratory cavity); (_l_) cavity of the body; (_m_) muscles; (_n_) t.e.s.t.i.c.l.e (in the ascidia united with the ovary into a hermaphrodite gland); (_o_) a.n.u.s; (_p_) genital orifice; (_q_) well-developed embryos in the body cavity of the ascidia; (_r_) rays of the dorsal fin of the amphioxus; (_s_) tail-fin of the amphioxus; (_w_) roots of the ascidia.

Plate XII. shows the _Ontogenesis_, or the individual development of the _Ascidia_ (_A_) and the _Amphioxus_ (_B_) in five different stages (1-5). Fig. 1 is the egg, a simple cell like the egg of man and all other animals (Fig. _A_ 1 the egg of the ascidia, Fig. _B_ 1 the egg of the amphioxus). The actual cell-substance, or the protoplasm of the egg-cell (_z_), the so-called yolk, is surrounded by a covering (cell-membrane, or yolk-membrane), and encloses a globular cell-kernel, or nucleus (_y_), the latter, again, contains a kernel-body, or nucleolus (_x_); when the egg begins to develop, the egg-cell first subdivides into two cells. By another sub-division there arise four cells (Fig. _A_ 2, _B_ 2), and out of these, by repeated sub-division, eight cells (vol. i. p. 190, Fig. 4 _C_, _D_). By fluid gathering in the interior these form a globular bladder bounded by a layer of cells. On one spot of its surface the bladder is turned inwards in the form of a pocket (Fig. _A_ 4, _B_ 4). This depression is the beginning of the intestine, the cavity (_d_ 1) of which opens externally by the provisional larval-mouth (_d_ 4). The body-wall, which is at the same time the stomach-wall, now consists of two layers of cells-the germ-layers. The globular larva (Gastrula), now grows in length. Fig.

_A_ 5 represents the larva of the ascidia, Fig. _B_ 5 that of the amphioxus, as seen from the left side in a somewhat more advanced state of development. The orifice of the intestine (_d_ 1) has closed. The dorsal side of the intestine (_d_ 2) is concave, the ventral side (_d_ 3) convex. Above the intestinal tube, on its dorsal side, the neural tube, the beginning of the spinal marrow, is being formed, its cavity still opens externally in front (_g_ 2). Between the spinal marrow and the intestine has arisen the spinal rod, or chorda dorsalis (Notochord) (_c_), the axis of the inner skeleton. In the larva of the ascidia this rod (_c_) proceeds along the long rudder-tail, a larval organ, which is cast off in later transformation. Yet there still exist some very small ascidiae (Appendicularia) which do not become transformed and attached, but which through life swim about freely in the sea by means of their rudder-tail.