In the warmest period of the year, certain lizards are found in the South of Europe, called geckos. They have a power of running, not only up walls, but across ceilings by means of a peculiar structure of their toes. They are types of a large family (Geckotidæ) widely spread over the world.
Another large family (Scincidæ) has also its type in the South of Europe in the skink (Scincus), which was formerly supposed to possess much medicinal value. This large family contains a number of species which exhibit a series of gradations in structure leading to forms which have the external aspect of serpents. One such form is the perfectly harmless slow-worm, or blind-worm, of our own country, which in spite of its scientific name,Anguis fragilis[14], is a legless lizard, and no snake.
Other lizards of a very different kind forming the familyAmphisbæidæare also legless, with the single exception of the genusChirotes, which has a pair of anterior limbs, but no posterior ones. The name of this family is derived from the similarity of appearance presented by both ends of the body, so that either end looks as if ready to take the lead as "head."
A family of lizards familiar by name to us all from our childhood is the family of chameleons (Chameleonidæ). There are many species of chameleons, but they are found in the Old World only; they are among the most exceptional and peculiar of all lizards, but there is one form which is yet more so.
This most exceptional of lizards is one found in New Zealand, and namedSphenodon. Its external aspect would not lead the ordinary observer at all to suspect that it is so remarkable a creature as its anatomy shows it really to be.
The orderCrocodiliacontains, of course, the true crocodiles which are found both in the Old and New Worlds. It contains besides the alligators (which are peculiar to America), as well as the long and slender-snouted gavials which are now found only in India and Australia. At one time the number of kinds of this order was very much greater than at present, and interesting structural modifications have taken place in it during the course of ages, as will be pointed out later.
On the whole, the order of crocodiles makes a much nearer approach to mammals and birds—especially (strange as it may seem) to birds, than is made by any other group of existing reptiles.
Reptiles, however, once existed have left their remains fossilized (inthe rocks of what is termed the "secondary" or "mesozoic" period), which reptiles in the structure of their skeleton approach much more closely to birds, and especially to birds of the ostrich order, than crocodiles do. Amongst these reptiles may be mentioned the huge Iguanododon (type of the, extinct orderDinosauria), which once roamed over the Weald of Kent, and has left its remains in the Isle of Wight and elsewhere. Such remains were collected by its discoverer, the late Dr. Mantell, and are now preserved in our British Museum.
The crocodilia and some of the lizards of our own day are aquatic, but none live constantly in the ocean, as do the cetacea amongst beasts. This was, however, by no means always the case. In the secondary period just adverted to, huge marine reptiles (IchthyosauriaandPlesiosauria) lorded it over the other then inhabitants of the deep, and presented some noteworthy resemblances to the whales and porpoises which have since succeeded them.
But other remains preserved in those same secondary rocks show us that in that period which has been so deservedly called "the age of reptiles," not only did many huge species of the class stalk over the land (either browsing on its foliage or preying on their fellows), and many others swarm in the then existing waters, but it shows us that the atmosphere also had its reptilian tenants. Flying reptiles which formed the now extinct order,Pterosauria, and which were some of small, some of very large size, as truly "flew" as do the bats of our own day fly, and by a very similar mechanism. Moreover, if theDinosauriapresent, as they do present, very noteworthy and interesting resemblances to birds of the ostrich order, no less noteworthy and interesting are the resemblances presented by these flying reptiles to ordinary—i.e., to "carinate"—birds.
The orders of extinct reptiles just referred to are not the only ones which formerly existed and have now passed away. There were reptiles with peculiarities in their teeth such as to have caused their order to be namedAmnodontia, and it is members of this extinct order that the lizardSphenodonmore or less resembles, and it is this resemblance which gives it that special interest before noted.
We may now return from these very various extinct forms to enumerate other kinds of reptiles which exist to-day. But before doing so the fact may be adverted to, that though amongst beasts many forms have become extinct, yet the proportion borne by the known extinct forms to the living kinds is much less than amongst reptiles, and that while it is the most highly-organized reptiles which have ceased to exist, the highest mammals which are in any way known to us are those which at present inhabit the earth's surface.
In passing from the orders of crocodiles and lizards to that of serpents—i.e., to the orderOphidia—we might select as first to be mentioned kinds which much resemble the legless lizards; but such kinds are not familiar ones in Europe.
The only serpents met with in England are but of three species—two harmless snakes and the common viper, which latter is the only really poisonous reptile in this country.
Of the harmless snakes, the ringed or collared snake (Tropidonotus) is much the commoner and more widely diffused. It ought to escape destruction on account of the ease with which it may be discriminated from the viper by means of the white collar-like mark which appears so conspicuously just behind its head.
Our viper is the type of a large and poisonous family, but by no means all poisonous snakes are vipers. The deadly cobras belong to a different group, having much more affinity with our own harmless snakes than with the vipers. The rattle-snakes again form a family (Crotalidæ) by themselves.
There are such things as true sea-serpents, and they are poisonous. They are not, however, allies of any "sea serpent," such as every now and again figures in startling paragraphs in our journals. The true sea-serpents are snakes of small or moderate size, which have their tails flattened from side to side, and which inhabit the Indian Ocean. Of other serpents which are not poisonous, the family of boas and pythons (which kill by crushing) is tolerably familiar to all who have visited zoological collections. There are many beautiful and harmless snakes, such as the families of tree-snakes and whip-snakes, but the snakes which more or less resemble legless lizards are burrowing forms which have the habits and more or less the appearance of earth-worms, such as those which form the families ofUropeltidæandTyphlopsidæ.
The last existing reptilian order (Chelonia) includes, besides the land tortoises of very various dimensions, a variety of aquatic forms.
The best known of these in this country, is the marine family (Chelonidæ), to which the edible and tortoise-shell turtles belong. The best known family in the United States and in the Continent of Europe, is theEmydæ, to which pertain the terrapins or ordinary river tortoises. Besides these, however, there is a very small family (Trionicidæ) of curious and exceptional forms, called mud-tortoises (Trionyx).
The creatures which have next to be glanced at are those familiar forms, the frogs, toads and efts, which, together with their allies, form another class,—the classBatrachia. These animals were long confounded with reptiles but are really widely distinct from them. They are arranged in four orders, three of which have living representatives. The creatures of the first order (the order of tailless Batrachians orAnoura)—frogs and toads—exist over almost all the habitable globe; and though the number of their kinds is very great, yet they are all extremely alike in organization. Many kinds (of both frogs and toads) are found to live in trees, the ends of their fingers and toes being dilated to enable them to cling to the surfaces of leaves. The most exceptional species of the whole group are the two tongueless toads, thePipaof South America and theDaclytethraof Africa, the last-named kind being the lowest of all known animals provided with finger nails.
Closely related to the frogs and toads are the efts so common in our ponds. These familiar English forms are represented in other countries of the Northern Hemisphere by creatures, some of which (as we shall hereafter see) are of very great interest indeed. The whole group constitutes the second Batrachian order—the orderUrodela.
One of the most noteworthy forms of the order is the eftProteus, which inhabits the dark, subterranean caverns of Carniola and Istria. Allied to this is theMenobranchusof North America and the Axolotl of Mexico. Other forms of the order are the American eft-generaSpelerpesandAmblystoma, theMenopoma, and the gigantic Salamander (Cryptobranchus) of Japan and China, the eel-likeAmphiuma—with its very long body and minute legs—and the two-leggedSirenof the United States.
The third order of Batrachians is one which contains very few species, but these are very strange, for though allied to frogs they have the appearance of snakes, or rather perhaps of worms. With long and slender bodies (marked by many transverse wrinkles), devoid of every rudiment of limb, they remind us of the before-noticedAnguis,Typhlops, andUropeltisamongst reptiles. The Batrachians in question (which belong to the generaCæciliaandSiphonops) form the orderOphiomorpha.
The fourth order of Batrachians is one which has entirely passed away and become extinct. It is the orderLabyrinthodonta, and the species which composed it were, some of them, of large size, with great heads like those of crocodiles. Others bore more or less resemblance to enlargedOphiomorpha.
Every one knows that frogs begin their existence in the water as tadpoles, which have the habits and mode of life of fishes. Thus, the classBatrachianaturally conducts us to the classPisces, the class of true fishes. This class contains a prodigious variety of forms, and is far more rich in species than any other of the classes before enumerated—even that of birds.
The fishes most familiar to us—such as the perch, carp, mackerel, cod, herring, sole, turbot, salmon, pike, dory, and eel—all belong to one great order calledTeleostei, and which is made up of what are called "bony" fishes, though there are some bony fishes which do not belong to it. To the same order also belong the Murœna, the electric eel (Gymnotus), the flying fishes (ExocetusandDactyloptera), the sucking fish (Remora), the pipe-fish and sea-horse (Hippocampus), the diodon, the ostracion, the file-fish (Balistes), the largest of all fresh-water fishes (Sudis gigasof South America), with a multitude of other forms.
Certain more or less singular Teleosteans are classed together in a subordinate group of "Siluroids" (of which fish theSilurusis a type), and which group includes, amongst others, the singular, cuirassed fish Callichthys.
A group of fishes, which is now very small, but which at an earlier period of the world's history was very large, includes within it all those fishes which will be hereinafter occasionally spoken of as "Ganoids," as they compose the orderGanoidei. Of all the forms of this order, the sturgeon is that which is least unfamiliar to us. The Ganoids are mostly fresh-water fishes and consist of the spoonbill-fish (Polyodon), the bony-pike (Lepidosteus), the AfricanPolypterus, the mud fish (Lepidosiren), and the curious Australian fishCeratodus, which last is a singular instance of piscine survival.
Another order,Elosmobranchii, is made up of the sharks, together with the skates (or rays) and the curiousChimæra. Amongst the skates may be mentioned the celebrated torpedo or electric ray.
The three groups above enumerated contain almost all known fishes, but a few other kinds, all of lowly organization, constitute two other groups of very different structure.
One of these groups is calledMarsipo-branchii, and contains the lamprey, theMyxine(or Glutinous Hag), and theBdellestoma. They are fishes of parasitic habits and of relatively inferior structure.
Last of all comes a creature of such exceptional build, so widely different from, and so greatly inferior to, any kind of animal yet noticed, that it may but doubtfully be reckoned as a fish at all. The animal referred to is the lancelet (Amphioxus), which is a small, almost worm-like animal, living in the sand on our own coasts, and also widely distributed over other parts of the world. TheAmphioxushas no distinct head or heart, and its breathing apparatus—its gill structure—differs so much from that of all other fishes as to give a name to its "order" (which contains it alone)—the orderPharyngobranchii.
We have now, then, hastily surveyed no less than five "classes" of animals—(1) Mammalia, (2) Aves, (3) Reptilia, (4) Batrachia, and (5) Pisces.
But, as was said in the first beginning of this Essay,[15]"classes" are the groups into which "sub-kingdoms" are divided, and which, by their union, make up such "sub-kingdoms."
The five classes above-mentioned together constitute the highest of those sub-kingdoms into which the whole animal kingdom itself is divided. This highest sub-kingdom is named VERTEBRATA, and is called the vertebrate sub-kingdom, because every creature which belongs to it possesses a "spinal column," which is generally built up of bones, each of which is called a "Vertebra."
We ourselves are members of the genusHomo, of the familyHominidæ, of the orderPrimates, of the classMammalia, of the sub-kingdomVertebrata, and it is desirable to treat this sub-kingdom at considerable length, both because it is, to us who are members of it, the most interesting and important, and because, by treating it somewhat fully, a good example can be once for all given of biological classification.
But the number of animal kinds which belong to other sub-kingdoms vastly exceeds the total number of vertebrate animals, and the structural contrasts found between different non-vertebrate species is very much greater than any such contrasts as can be found to exist between any two members of the highest, or vertebrate sub-kingdom. This is only what we might expect; for non-vertebrate animals—often spoken of collectively as "Invertebrata"—form several distinct sub-kingdoms, each of which has a rank approximatively co-ordinate with that sub-kingdom to which we ourselves belong. Nevertheless, since the members of the invertebrata sub-kingdoms are, speaking generally, much less known and familiar than are vertebrate animals, and as the structural differences between them cannot be pointed out till an initial acquaintance has been made with comparative anatomy, for these reasons we may treat the various animal sub-kingdoms which have yet to be noticed at much less length than we have treated the vertebrata. The details of their peculiarities and the various degrees of significance and interest which they present will begin to appear when we proceed to treat of "The Forms of Animals."
The last class of vertebrates is, as we have seen, constituted by the fishes, which are fishes properly so called. But there are many animals which are familiarly and improperly spoken of as "Fishes," but which are even more below true fishes than whales and porpoises are above them. Thus, we hear of cuttle-fishes, and a variety of creatures are spoken of as "shell-fish," which are not in the least related to true fishes. Indeed, the many so-called "shell-fish" are not even nearly related one to another. Thus, the oyster and the lobster are both commonly thus named, but they belong respectively to two altogether distinct sub-kingdoms of the world of animals.
The oyster is an animal which belongs to a vast assemblage of species, with much variety of form and structure, which, on account of their soft bodies (whether or not enclosed in shells), are called MOLLUSCA or "Mollusks." This assemblage ranks as a sub-kingdom and contains within it at least four subordinate great groups, or "classes." All snails and whelks, with their allies, and also all cuttle-fishes, belong to the sub-kingdom of "soft animals."
Amongst the most familiar of mollusks is the common snail, which may serve as a type of the "class" of mollusks to which it belongs—the classGasteropoda. The snail, with the slug, are representatives of land-forms of mollusca, but the bulk of the class and of the whole sub-kingdom are aquatic animals, such as the whelk (Buccinum), periwinkle (Littorina), limpet (Patella), &c. The Gasteropods generally possess spirally coiled shells (like the cowry or whelk), but some kinds have their shells in the form of simple cones—like a Chinaman's cap—as,e.g., the limpet. There are a few Gasteropods in which the shell consists of a series of similar segments as is the case withChiton, while many are altogether naked. In some kinds the soft body is drawn out into a number oftufted processes, as in Doris and Eolis, and sometimes the body is almost worm-like, as inPhylliroë, or provided with a pair of ring-like lateral processes and a rudimentary shell, as in the sea-hareAplysia.
Next above the Gasteropods comes a group of animals forming the classPteropoda. These pteropods are small, active, oceanic, surface-swimming creatures, many of which live in delicate glass-like shells, and some of which form a large part of the food of the whalebone whale. They flit through the water by the aid of lateral processes which much resemble those before-mentioned as existing in the sea-hare. Allied to these pteropods is a curious little animal, the shell of which resembles a miniature elephant's tooth and which is namedDentalium.
Highest of all the mollusca stand the cuttle-fishes, forming (with theNautilusand many extinct animals, such as ammonites and their allies) the great classCephalopoda. The Cephalopoda, such as the cuttle-fish (Sepia) and the Poulp (Octopus), have now become familiar objects through our aquaria, where their very eccentric forms and remarkable movements naturally attract attention. To this group also belongsSpirula, the coiled and chambered shell of which is found so abundantly, but its soft tenant so very rarely. To it also belongs the extinct Belemnite, which was provided with a dense, conical internal shell, specimens of which found in rocks were at one time taken for thunderbolts. Of a lower grade of organization is theNautilus, sole existing representative of a great group of Cephalopoda (including the ammonites and other forms) which has, with the above exception, long become entirely extinct.
The oyster is an animal which belongs to a much lower class of mollusca—namely, to the class calledLamellibranchiata, from the plate-like (or lamellar) structure of the gill. To that class also belongs the scallop (Pecten), the mussel (Magilus), the fresh-water mussel (Anodon), the razor-shell (Solen), the cockle (Cardium), species with a long fleshy tube such asMya, stone-perforating shells such asPholas, and the well-known wood-boring "ship-worm" (Teredo)—which is no "worm" at all—with a multitude of other forms.
Certain other animals (which, like the Lamellibranchs, all have a shell divided into two valves) form another still lower class calledBrachiopoda, a class which we may, at least provisionally, consider as belonging to the mollusca. TheseBrachiopodsare also called "Lamp-shells," from a certain resemblance which many of them show to the form of a classical lamp. They are interesting, because in very ancient times they seem to have held that place in the world's animal population which is now held by the Lamellibranchs, by which, as they died out, they have been gradually replaced till but comparatively few forms survive. Some of these, however, are of great antiquity, and one of them,Lingula, is, though still living, one of the most ancient of all known animals.
We may next pass to a small sub-kingdom which includes the curious and inert animals before referred to[16]as "Sea-squirts," Tunicariesor Ascidians, and which constitute the sub-kingdom TUNICATA. These are marine organisms of very simple but very peculiar structure which sometimes grow up in compound aggregations. Certain forms (e.g.,Pyrosoma) are luminous at night and may be seen swimming about in the ocean like so many red-hot urn-heaters. As we shall hereafter see, the reproductive processes and the earlier stages of existence of these creatures possess much interest, and have afforded strong grounds for regarding them, in spite of their lowly organization, as very close allies of the highest animals orVertebrata.
Returning now to the "lobster" (lately mentioned as one of those animals commonly called "shell-fish") we may regard it as an example of what is by far the most numerous of all the sub-kingdoms of animals. This sub-kingdom is made up of animals with jointed feet or "Arthropods," and the ARTHROPODA are subdivided into four classes—1,Crustacea; 2,Myriapoda; 3,Arachnida; and 4,Insecta; and it is to the first of these four classes that the lobster belongs.
The classCrustaceacontains, besides the lobster (and its near allies, hermit-crabs, prawns, shrimps, and cray-fish), all crabs, including those very quaint-looking animals (now so often seen in our living collections), the king-crabs (Limulus), and a variety of more or less strangely different forms such as the following:—
Certain Crustaceans, of the group calledOstracods, have the hard outer coat of their body so peculiarly modified that they have quite the appearance of Lamellibranch Mollusks, and this resemblance is even more than skin deep, as we shall see later.
Some of another group, calledCopepoda, become, when adult, so degraded in structure as to have the appearance of mere worms, asLerneoceraandTracheliastes, and become strangely unlike the typical forms (crabs and lobsters) of their class.
Other animals of the classCrustacea, which animals form the orderCirripedia(barnacles and acorn-shells), bear such an external resemblance to mollusks that they were actually classed by Cuvier in the classMollusca. In some of them—the Barnacles which commonly attach themselves to the bottoms of ships—the head grows from above downwards to a relatively enormous degree, forming the long stalk or "peduncle," at the lower end of which the small body with its limbs hangs suspended.
In another group,Rhizocephala, the form of the adult becomes yet more strange. These creatures are parasitic on other crustacea. Having attached themselves to the surface of the soft abdomen of the Hermit crab, the head of the Rhizocephalon grows out into it as so many root-like processes, from which condition the group has received its name.
The numerous and long extinct group ofTrilobitesalso belongs to the classCrustacea.
The next class,Myriopoda, consists of the hundred-legs (centipedes), and thousand-legs (millipedes), which present us with some of the bestexamples of creatures the bodies of which are composed of a longitudinal series of similar segments. Allied to them is a very exceptional animal found in Africa and New Zealand, and calledPeripatus, the anatomy of which presents many significant peculiarities.
The third class of Arthropods (Arachnida) consists of the scorpions and spiders with their poor relations, the mites and tics, together with the very peculiarly-shapedPycnogonida(which present us with a good image of "no body"—being all legs and no body), and the singular worm-like parasiteLinguatula. Lastly, we come to the most zoologically important and numerous of all the classes of Arthropods—namely, to the "class" of insects—Insecta. Therein we meet with the power of flight in its most perfect form—i.e., in the Dragon-flies—and most of the species are aërial in their adult (orImago) condition. Some, however, are burrowers as, for example, the mole-cricket—an insect which presents some curious analogies in structure to the beast referred to in its name. Amongst insects may be mentioned the most familiar of all, the House-fly (which belongs to the orderDiptera), and Beetles of all kinds (which constitute the orderColeoptera), some of which latter are luminous, as is the well-known glow-worm, and the exotic beetlesPyrophorus. Another order (Orthoptera) is made up of the earwigs, cockroaches, crickets, grass-hoppers, and their allies the locusts, with Bamboo-insects and the curious walking-leaf (so-called from their resemblance to a Bamboo twig and a foliage leaf respectively), the praying mantis, and other curious kinds.
Bees and Ants, which belong to the orderHymenoptera, are, as every one knows, celebrated for their wonderfully complex instincts and community-life (which will occupy us later), and to the same order also belong the Ichneumon insects, which are provided with long appendages at the hinder ends of their bodies wherewith to pierce the bodies of animals in order to deposit their eggs within them, or to pierce the substance of plants, so producing "galls" which are structures of much interest from several points of view.
Butterflies and Moths form another order of insects calledLepidoptera, amongst which may be mentioned as (having to be referred to hereafter) the true butterflies (Papilio), and the hawkmoths (some of which in their flight so much resemble Humming-birds), the clear-wing moths, and those moths the grubs of which are known as "silk-worms," and certain moths of the generaSolenobiaandPsyche.
The numerous group of bugs is allied to the plant-lice (Aphides), which so often infest our Pelargoniums when kept in dwelling-rooms. Allied to them, again, are the small creatures the nature of which was so long disputed, though familiar to commerce as "Cochineal." Really, they are small, singularly inert, plant-lice, which adhere to the surface of certain "Cacti."
The Dragon-flies, before referred to, are the types of the orderNeuroptere.
All the insects above mentioned, save the House-fly, have four wings, or else none; but that familiar form may serve as the type of the two-winged order (Diptera) to which belong all flies and gnats—including, of course, the Mosquito—and the numerous "Bots," one of which (the Tsee-Tsee fly) is so fatal to cattle in Africa.
Finally, amongst insects may be mentioned the wingless, but active order of fleas (Aphaniptera), the wingless but sluggish lice (Aptera), and the jumping and wingless springtails (Thysanura).
In leaving the class of insects, we leave all the more highly-organized Invertebrata. But the next group to which we may direct our attention is one which is exceedingly numerous, and contains a very varied assemblage of forms. This group is the "sub-kingdom" of Worms, VERMES. First amongst its contents may be mentioned the higher or true "worms," such as the earth-worm (Lumbricus), the leech (Hirudo), the sea-mouse (Aphrodite), and their allies, together with the worms which live in tubes, which are calledTubicolous-"Annelids," because the whole class of these higher worms bears the nameAnnelida.
In this connexion may be mentioned certain exceptional vermiform creatures, about the affinities of which naturalists dispute.
One of these is a marine creature (calledSagitta, from the way in which it shoots like an arrow through the water), which has many affinities to Arthropods.
Another is a most remarkable worm, which has been found in the Bay of Naples, and is calledBalanoglossus. It is the type of a group calledEnteropneusta. To it reference will have again and again to be made on account of certain singularities in its structure.
A very distinct class of creatures is termedBryozoa(orPolyzoa), and is composed of very minute animals which live in compound aggregations, and often grow up in an arborescent manner. The common sea-mat (Flustra) is one example of the class, and another—a good type—is calledPlumatella. TheBryozoahave many affinities with theMollusca, to which some naturalists consider them to belong.
Other worms form the classNematoidea, of which many are parasitic and many not so. Amongst the better known of the former may be mentioned the worms which tease children (Ascarides), the guinea-worm (Filaria), the scourge of Germans who eat raw meat (Trichina), the deadly blood-parasite of the Nile (Bilharzia), and many others.
Another class (Trematoda) is made up of parasites called "Flukes," to some of which (e.g.,Monostomum) reference will have hereafter to be made with respect to their processes of development.
The classTurbellariacontains a variety of other worms of a lowly kind, one or two of which (e.g.,Borlesia) live coiled up in complex tangles which, if unravelled, would attain a length of forty feet. Amongst the commoner kinds may be mentioned the wormNemertes, and all worms calledPlanariæ(which are mostly fresh-water, though some live on land), allied to the flukes.
The class of tape-worms (Cestoidea) is one most numerous in its kinds, which are all completely parasitic in habit. Some of them are so fatal in their effects that they are estimated to occasion every seventh death which occurs in Iceland, and they cause mortality amidst our own flocks, producing in sheep the disease known as the "staggers."
Certain minute organisms, familiarly known as "Wheel-Animalcules," or Rotifers, form the "class"Rotifera. They have gained their name through an apparently (though, of course, not really) rotary motion, of that end of their bodies at which the mouth is situated. Here also may be mentioned certain curious aquatic worms calledGasterotricha, which are closely allied to the wheel animalcules.
Finally may be mentioned the classGephyrea, containing animals, worm-like indeed in form, but which have much apparent affinity to the group next to be spoken of—the group of star-fishes and their allies. Amongst theGephyreamay be mentioned the worms calledSipunculusandPriapulus.
This leads us to the sub-kingdom containing the star-fishes—the sub-kingdom ECHINODERMA, which includes, besides the star-fishes (orAsteridea), all sea-eggs or sea-urchins (Echinidea), the brittle-starsOphiuridea, as well as the elongated soft animals called sea-cucumbers, orHolothuridea, some of which latter are known as the Japanese edible, "Trepang."
Besides these groups there are still surviving a few creatures (ComatulaandPentacrinus) belonging to the class of "sea-lilies," orCrinoidea, creatures which once lived in countless multitudes, but have now almost entirely passed away. All these crinoids were like star-fishes on stalks, and of the existing forms,Pentacrinusstill passes the whole of its life, andComatulaits youth, in a stalked condition.
The next great primary division, or sub-kingdom of animals, is CŒLENTERA, and a good type of the cœlenterates, the sea anemone (Actinia), has now become a familiar object to us in our aquaria. These animals are plant-animals, or zoophytes, and some of them build up coral-reefs, or islands, and it is one kind which produces the red coral of commerce. Forms essentially similar, but the solid supporting framework of which is of a softer nature, are such asAlcyoniumandPennatula. All these belong to the "class"Actinozoa. There are other cœlenterates of an active free-swimming habit, such asBeröeandCydippe, which are balls of glassy transparency displaying iridescent hues as they move rapidly through the water by means of their peculiar locomotive organs.
Other cœlenterates, of the same essential type but of simpler structure, form the classHydrozoa. Amongst these may be mentioned the littleHydraof our ponds, which will often come before us in our survey of animal life. Some compound forms of Hydrozoa simulate the compound Actinozoa; such are the calcareous millipores, and those with a softerstructure, called "corallines," such asEudendriumand many others. The Portuguese man-of-war (Physalia) and the various forms of jelly-fish (Medusæ) all belong to theHydrozoa, as also does a very curious and very elementary form, to which the nameTetraplatiahas been given.
Next we come to the group of sponges, SPONGIDA, some of which—as the now well-knownEuplectella—are of marvellous beauty and delicacy of structure; while others, as the sponge of commerce, are of much greater simplicity of form. Simplest of all the sponges is the sponge calledAscetta Primordialis. Some sponges have a horny, some a calcareous, and some a siliceous skeleton, and (strange as it may appear) some have a habit of boring into shells, and living in the excavations they make.
An animal recently discovered,Dicyema, may at this initial stage of our inquiry be left with its place and affinities undetermined. It is a minute worm-like creature of most exceptionally simple structure, which lives parasitically within cuttle-fishes.
We now pass to animals (if so they are really to be considered) which are the lowest and simplest of all, and which are mostly microscopic in size, and may be grouped together under the term HYPOZOA, or under the generally employed nameProtozoa. With very few exceptions these animals are aquatic, and if terrestrial they are found in damp localities. Some are marine, others are fresh-water organisms.
The highest of the group are the animalcules, which are namedInfusoria, most of which are freely swimming organisms, though a certain number of them live fixed to some supporting body.
Another group ofHypozoais that termedGregarinida, a group made up of very lowly parasites, such as are often found tenanting the intestines of insects as well as those of higher animals. Finally, we have the group ofRhizopoda, animals which have the faculty of projecting and retracting (so to say, at will) filamentary or conical processes of their semi-fluid substance, such processes being thePseudopodia, which were referred to earlier.[17]
Amongst theRhizopoda, the most complex and beautiful are the delicate and symmetrical creatures known asRadiolaria,[18]the siliceous skeletons of which are amongst the most remarkable of microscopic objects.
Allied to them are the simplerHeliozoa, of which the after-mentionedActinophrysmay be taken as a type.
Next come theFlagellata, or minute creatures which swim about by means of one or two whip-like processes, whence the name of the group.
Last of all is the group ofForaminifera, animals which are well worthy of note, seeing that, though they are each but as it were a minute particle of structureless jelly, they manage to build most complexly-formed, generally calcareous, shells, or to pick up from the sand of thesea minute particles, which they agglutinate around them with marvellous neatness and precision. Their calcareous shells are generally pierced by a multitude of minute pores, through which the little creatures protrude theirpseudopodia. It is from these pores (orforamina) that the group receives its name. AllForaminifera, however, are not provided with shells. Some, as theAmœba, are naked, and the simplest of all animals,ProtogenesandProtamœba, consist of but a minute particle of semi-fluid jelly, or protoplasm, naked and as devoid of every external protection as it is of internal organization.
We have thus descended to the bottom of the animal kingdom, and passing from these rudimentary forms, which are generally reckoned as animals, we may next survey in ascending order the different organisms which together compose the kingdom of Plants, a group much less rich in species than is the animal kingdom.
At the bottom of that kingdom are very simple creatures, but little different, to all appearance, from the lowest animals. As an example of such we may take the minute plantProtococcus, which is an humble member of the great group ofAlgæ, to which all sea-weeds belong. Not all of this important tribe, however, are marine. Many are found in fresh water—such as the protococcus itself, and many of the green vegetable threads known asConferræ. Some even live on land, and draw their moisture from the atmosphere. TheAlgæare exceedingly varied in their structure; some, like the protococcus, being of extreme simplicity; others attaining a large size, and presenting the appearance of a stout stem with branches and leaves.
The Algæ are divisible into the green-spored[19](Chlorospermeæ), the rose-spored (Florideæ), and the olive-spored (Melanospermeæ).
It is in the first division that theProtococcusmay be placed, as also those microscopic plants calledDiatomsandDesmids. The former, theDiatomaceæ, are a very numerous group of minute organisms, some of which are used as test objects for microscopes. They contain in their outer coat or case a relatively large portion of silex, and their remains here and there form deposits—vast beds many feet in thickness—known as "tripoli," and used for polishing. The minute particle of their protoplasm is contained within the siliceous case. They may be entirely free, or cohere in aggregations, or be attached to a supporting surface by a slender stalk, which may ramify and bear a little siliceous case or "frustule" at the end of each branch.
The desmids (orDesmidiaceæ) are green and devoid of silex, though their protoplasm is enclosed in hard or flexible cases, often marked with beautiful and characteristic patterns.
Both diatoms and desmids may cohere together, forming more complex masses; but another creature allied toProtococcusis noted for its mode of cohesion. This is the microscopic plantVolvox, the individuals of which cohere so as to form spheroidal aggregations, which swim aboutby the action of filamentary prolongations of their protoplasm, such prolongations reminding us of the pseudopodia of radiolarians and other rhizopods.
Amongst these simplest plants may be also mentioned the curious thread-like organisms, which, on account of their remarkable and as yet unexplained movements, are calledOscillatoriæ.
Another curious vegetable organism which may here be mentioned isVaucheria. It is a green, thread-like plant, which may be several inches long, and which at one stage of its existence (when it is what is called a "spore") swims about by pseudopodial prolongations of its protoplasm.
Some few of theChlorospermeæare large and conspicuous organisms. Such,e.g., isCaulerpa, which abounds on warm, sandy coasts, and on which turtles browse. Though, as we shall hereafter see, it is really as simple in structure as a particle of yeast, it yet presents a very complicated external figure.
Some of the great group ofAlgæattain enormous dimensions. Thus,Macrocystis(one of theMelanospermæ), of the Southern Ocean, may be even 700 feet in length. Another kind,Lessonia, forms submarine forests, with stems like the trunks of trees.
The group ofFloridiæincludes the delicate and elegant sea-weeds, which are amongst the most admired vegetable productions of our coasts. They are of interest, on account of various peculiarities in their reproductive processes.
Other lowly plants may, at least provisionally, be placed in the great group to which mushrooms and truffles belong—the group ofFungi—a group the members of which agree in certain exceptional phenomena of function,[20]as well as of structure and composition—as they are exceptionally nitrogenous.
Amongst the lowest which we may for convenience provisionally include in this group may be mentioned minuteVibrios, such as theBacteriaso much talked of in connexion with spontaneous generation, and the small plant which by its growth produces fermentation—the yeast-plant (Saccharomyces).[21]Closely allied to the yeast-plant are the "moulds" which grow on organic matters such asPenicillium,Mucor,Saprolegna,Phytophthora, the last of which is the potato disease.
A singular group of organisms goes by the name ofMyxomycetes. These enigmatical creatures have been classed in turn as animals and as plants, and, indeed, at one period of their existence they seem to have more resemblance to the former, while at another stage of their life history they must unquestionably be ranked as plants. When young, they are in a semi-fluid condition, and so move that they seem, as it were, to flow over the body on which they rest. They growupon the bark of trees or on leaves and decayed wood. They exhibit movements like those of the amæbæ and are said to engulph nutritious matters which come in their way.
The dry-looking, green, grey, red or yellow vegetable structures which encrust our rocks, walls, and trees, and which are calledLichens, form a group of plants curiously intermediate between Fungi andAlgæ.
Plants somewhat higher in the scale of vegetable life are those which are termed liverworts (Hepaticæ), including the scale-mosses (Jungermanniaceæ) andMarchantia. These plants, as we shall see, are interesting on account of the variations to be found in the forms of different genera. In many, there is no stem, but only a connected series of green disk-like expansions, while others have a distinct stem with leaf-like outgrowths.
Two genera of aquatic plants (CharaandNitella) constitute another group of plants calledCharaceæ. These will be hereafter referred to both on account of peculiarities in their structure and on account of a peculiar motion of protoplasm which is easily to be seen[22]in them.
Mosses (Musci) are familiar objects to every one in this country, and allied to them are the so-called "club-mosses" orLycopods, which form a sort of green sward in so many parts of the warmer regions of the earth. To one of the lycopods, calledSelaginella, reference will hereafter be made in connexion with its very instructive reproductive process.
Certain humble plants, in some of which the foliage leaves present a superficial resemblance to those of a four-leaved clover, are popularly called pepperworts; by botanists,RhizocarpeæorMarsiliaceæ. They are creeping or floating stemless plants which inhabit ditches or inundated places. They are scattered over both the Old and New Worlds, but are chiefly found in temperate latitudes.
The horse-tails (Equisetaceæ) are also found in most parts of the world, though wanting in Australia and New Zealand. They inhabit wet and sandy places, and sometimes are of a considerable size even in the present day, but in ancient geological periods they attained the proportions of trees.
This group leads us on to their allies the ferns which form a very large natural groupFilicesorPteridophytes—a group now familiar to every one interested in plants. Common as ferns are in our own country, they are far more abundant and attain to a much greater size in southern latitudes—notably in New Zealand and various Pacific islands.
All the plants hitherto enumerated, from the protococcus to the tree-ferns inclusive, together form what is commonly regarded as one great primary division or "sub-kingdom" of vegetals called CRYPTOGAMIA. In no plant belonging to this sub-kingdom—in no single cryptogam—isany flower ever developed. These form the great group which is often spoken of as "flowerless plants."
The other primary division of vegetable organisms consists of all plants with flowers, and is termed PHANEROGAMIA, and is subdivided into two sections,[23]very unequally numerous. To the first section of phanerogams—a section containing comparatively few kinds—belong all firs, pines, yews, junipers, araucarias, and a most remarkable African plant,Welwitschia, which has never more than two leaves, though these attain enormous dimensions. All these plants are collectively spoken of as conifers, orConiferæ. Besides these, certain curious southern forms called Cycads are also associated in this section. To this section, thus composed of conifers and cycads, the nameGymnospermsis given, from the naked mode of development of their young seeds. These gymnosperms are also characterized by having such peculiar and inconspicuous flowers that the ordinary observer would hardly apply that term to denote their floral organs.
All the plants which yet remain to be noticed, and which belong to the second and very much larger section of thePhanerogamiaare spoken of asAngiosperms. Their seeds are, from their first appearance, in a very different condition from those of gymnosperms, and their flowers are generally conspicuous. To this group, therefore, belong all the familiar ornamental plants of our gardens, and all the brightly coloured natural ornaments of our fields, as well as a number of herbs and trees, the flowers of which, though truly flowers, are not commonly recognized as such.
This group of Angiospermous flowering plants is divided into a great number of natural groups or "orders." Of these there are about 275, and they are grouped in two sets or classes, which are separated one from another, as we shall hereafter see, by differences as to their modes of growth, the structure of their seeds, the numbers of the parts of their flowers, and the course of the veins in their leaves.
First amongst the Angiospermous flowering plants may be mentioned the grasses forming the orderGramineæ, including under that term the tree-like bamboos (of multitudinous uses), with the rice plant, and all the grain-bearing herbs, all of which are grasses. Thus, with much reason may it be said of man, that "all flesh is grass;" for with the exception of the piscivorous Esquimaux, the exclusively flesh-eating Gouchos, the population of Australia, and the people of the Molluccas who nourish themselves on sago—which is the produce of a palm—with these and a few more exceptions, the staple food of the human race is one or another form of grass. It is, indeed, a remarkable fact that men of such varied races so widely spread should have thus selected as their food objects so little tempting in appearance, and so small and so inconspicuous as the seeds of grasses!
Allied to the grasses are the sedges (forming the orderCyperaceæ), and the rushes (Juncaceæ). The apparently insignificant, but really interesting duckweeds (Pistiaceæ) should also be noted with the bullrushes (Typheæ), and the arums (Aroideæ). This last-mentioned order, familiar to us by the kind known as "Lords and Ladies," presents some climbing forms in tropical countries. Generally acrid, some species, when in flower, even produce headache and vomiting; at least an explorer was attacked with these symptoms after gathering forty specimens ofArum dracunculus. The order is also interesting from experiments as to vegetable heat, which have been made with the flowers of some of its species.
The screw-pines (Pandanaceæ) are not "pines" at all, any more than "pine-apples" are pines. They are, indeed, trees or shrubs, which, from one point of view, may be regarded as gigantic bulrushes. The flowers of certain species are in some places eaten as the solid equivalent of a love potion. Allied to the plants of the last-mentioned order are the palms (Palmaceæ), which are the first really large trees we come to after leaving the tree-ferns and the gymnosperms. Amongst the more noteworthy palms may be mentioned the palmetto (Chamærops) of Southern Europe (a summer ornament of our public gardens), the date palm, the areca palm, the sago palm, the cocoa palm, the rattan palm—a natural cordage—andSeaforthia, so remarkable for its graceful and elegant form.
Next may be enumerated the great order of lilies (Liliaceæ), to which the homely and useful onion, leek, garlic, chive, and asparagus belong, no less than a multitude of lovely flowers.
The New Zealand flax (Phormium tenax), and all the magnificent yuccas and aloes, together with our English butcher's broom (Ruscus aculeatus), which has not a little botanical interest (as being the only British shrub which belongs to the group called "Monocotyledons") also belong to this order. Closely allied to the lilies are the amaryllids (Amaryllidaceæ), amongst which are the agaves, with their gigantic flower stems, sometimes forty feet high, supporting a multitudinous crop of flowers, the product and termination of a life.
To these follow the pine-apples (Bromeliaceæ) all originally from America, the useful bananas and plantains (Musaceæ), and the ginger-plants (Zingiberaceæ), tropical herbs, generally of great beauty.
The underground parts of certain tropical plants (Dioscoreaceæ) are known as "yams." A representative of this order exists in England in the climbing black bryony (Tamus) of our hedges, and to the same group belongs the very singularly stemmed elephant's foot, or tortoise-tree (Testudinaria elephantipes). The last-named plant is a native of the Cape of Good Hope, where it has been known as Hottentot's bread, because the soft interior of its swollen base was at one time eaten by the natives of that region, who have, however, now abandoned it to the baboons.
Lastly, in this connexion may be mentioned the very interesting and beautiful group of orchids (Orchidaceæ), many of which live high up inthe air, supported on the branches of trees, from which their roots hang freely down. Such orchids are sometimes spoken of as "air-plants."
All the Angiosperms as yet mentioned, from the grasses to the orchids inclusively, belong to the lower of the two great groups or classes into which, as was lately said, the whole mass of Angiosperms is divided.
This great group is namedMonocotyledones(on account of the structure of the seed), and it is sometimes spoken of asEndogens, in reference to a generally prevalent habit of growth. The members of this whole class will then hereinafter be spoken of as "Monocotyledons."
All the plants which yet remain to be enumerated belong to the other and still greater group of Angiosperms called (also in reference to their seeds)Dicotyledons, a group sometimes spoken of as "Exogens," in reference to the habit of growth prevalent amongst its species.
All our familiar trees which are not conifers, and most of our flowering shrubs and herbs, are "Dicotyledons."
Amongst the many orders which compose the Dicotyledonous group the few following may be selected for enumeration, either on account of the general interest they possess, or because they will have to be more or less referred to hereafter.
We may thus note the singular order of vegetable parasites, theLoranthaceæ, an order containing some thirty genera with four hundred species, and including the mistletoe, which is traditionally venerable in our island. The great group of catkin-bearing trees (Amentaceæ), contains a great assemblage of plants, familiar in England, such as the hornbeam, hazel, oak, beech, Spanish chestnut, birch, willow, poplar, &c.[24]
The largest and one of the most remarkable flowers in the world,Rafflesia—a parasite found in Java and Sumatra by Sir Stamford Raffles—is the type of the small orderRafflesiaceæ. The eccentric pitcher-bearing plants form the orderNepenthaceæ. The English herb called "Spurge" (with its milky juice), belongs to the order (Euphorbiaceæ), which is a large[25]cosmopolitan group, some species of the plants belonging to which attain, in hot countries, the size of trees. Certain African species strangely resemble different kinds ofCactus. The elm order (Ulmaceæ) may come next. The hop, the hemp, the mulberry, the fig, and the dorstenia are all nearly allied, the first two belonging to the orderCannabinaceæ, the last three to theMoraceæ. The bread-fruit of the South-Sea Islands belongs to the same order (Artocarpaceæ) as does the deadly upas-tree of Java. Garments made of the inner bark of this plant are like the shirt of Nessus, and will produce intolerable irritation; and even climbing the tree to obtain its flowers is said to have produced severe effects on the climber. In proximity to the last-mentioned plant comes appropriately (as also in its proper botanical order) thegroup of stinging-nettles (Urticaceæ). The curious Australian plants which delighted the eyes of Captain Cook's botanical companions belong to the orderProteaceæ. Besides these may be mentioned the dead-nettle order (Labiatæ); the broom-rapes (Orobanchaceæ); the order of snap-dragons and foxgloves (Scrophularineæ); the potato group (Solanaceæ), which includes the deadly nightshade and the dulcamara of our hedges; the parasitic order (Cuscutaceæ); the beautiful group of convolvuluses (Convolvulaceæ); the gentians (Gentianaceæ); the primrose group (Primulaceæ); the heaths (Ericaceæ); the graceful hair-bell and its allies (Campanulaceæ); the very large group to which belong the daisy, dandelion, and thistle (Compositæ); the honeysuckle order (Caprifoliaceæ); the ivy (Araliaceæ); the large order containing the fennel, hemlock, and a multitude of other forms which, though mostly ranking as herbs, attain gigantic dimensions in some species found in Africa and Kamskatka (Umbelliferæ); the very singularly-shaped group of cactuses (Cactaceæ), with leafless fleshy stems, which sometimes look like dry columns and sometimes are globular; the begonias (Begoniaceæ); the cucumbers, melons, and vegetable marrows (Cucurbitaceæ); the singularly-formed passion-flowers (Passifloraceæ); the myrtles (Myrtaceæ); the carnivorous group containing the sundew and Venus's flytrap (Droseracæ); the fleshy houseleek and stonecrops (Crassulaceæ); the Saxifrages (Saxifragaceæ); the rose group (Rosaceæ), which includes within it most of our fruits, such as the apple, pear, strawberry, cherry, peach, plum, almond, and others; the very large order which contains the peas, beans, and their allies (Leguminoseæ); the horse-chestnut order (Hippocastaneæ); the maples (Acerineæ); the hollies (Ilicineæ); the oranges and citrons (Aurantiaceæ); the cranesbills and pelargoniums (Geraniaceæ); the flaxes (Linaceæ); the limes (Tiliaceæ), in which the useful jute is included; the mallows (Malvaceæ); the St. John's worts (Hypericaceæ); the order of pinks (Caryophylleæ); the pansies (Violaceæ); the rock-roses (Cistaceæ); the mignonette group (Resedaceæ); the great wall-flower and cabbage group (Cruciferæ); the poppies (Papaveraceæ); the water-lilies (Nymphaceæ); the berberries (Berberideæ); the custard-apples (Anonaceæ); the magnolias (Magnoliaceæ); and, finally, the great group (Ranunculaceæ) containing the anemones, the clematis, hellebore, monkshood, and the buttercup, which last is of great use to the student of Botany because it is an excellent type of all flowers.
The above may serve as a brief enumeration of the more generally known or more interesting orders of flowering plants, as also of the most noteworthy forms of cryptogams. The much more numerous and complex groups of animals have also been catalogued in the earlier and larger part of this Essay, which may thus, it is hoped, answer the purpose of an introduction to those multitudinous forms of organic life, the leading points in the structure and functions of which are hereafter to occupy us.
The main groups of Animals and Plants may be provisionally tabulated as follows:—
St. George Mivart.