are also legless, with the single exception of the genus _Chirotes_,
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 named
_Sphenodon_. 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 order _Crocodilia_ contains, 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 (in
the 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 order _Dinosauria_), 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 (_Ichthyosauria_ and
_Plesiosauria_) 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 the _Dinosauria_ present,
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 named _Amnodontia_, and it is members of this extinct order that the
lizard _Sphenodon_ more 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 order _Ophidia_--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 of _Uropeltidæ_ and _Typhlopsidæ_.
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 the _Emydæ_, 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 class _Batrachia_. 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 or
_Anoura_)--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, the
_Pipa_ of South America and the _Daclytethra_ of 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 order _Urodela_.
One of the most noteworthy forms of the order is the eft _Proteus_,
which inhabits the dark, subterranean caverns of Carniola and Istria.
Allied to this is the _Menobranchus_ of North America and the Axolotl of
Mexico. Other forms of the order are the American eft-genera _Spelerpes_
and _Amblystoma_, the _Menopoma_, and the gigantic Salamander
(_Cryptobranchus_) of Japan and China, the eel-like _Amphiuma_--with its
very long body and minute legs--and the two-legged _Siren_ of 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-noticed _Anguis_, _Typhlops_, and
_Uropeltis_ amongst reptiles. The Batrachians in question (which belong
to the genera _Cæcilia_ and _Siphonops_) form the order _Ophiomorpha_.
The fourth order of Batrachians is one which has entirely passed away
and become extinct. It is the order _Labyrinthodonta_, 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
enlarged _Ophiomorpha_.
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
class _Batrachia_ naturally conducts us to the class _Pisces_, 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 called _Teleostei_, 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 Muroena, the electric eel
(_Gymnotus_), the flying fishes (_Exocetus_ and _Dactyloptera_), 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 gigas_ of 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 the _Silurus_ is 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 order _Ganoidei_. 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 African _Polypterus_,
the mud fish (_Lepidosiren_), and the curious Australian fish
_Ceratodus_, 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 curious _Chimæra_. 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 called _Marsipo-branchii_, and contains the
lamprey, the _Myxine_ (or Glutinous Hag), and the _Bdellestoma_. 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. The _Amphioxus_ has 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 order
_Pharyngobranchii_.
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 genus _Homo_, of the family _Hominidæ_,
of the order _Primates_, of the class _Mammalia_, of the sub-kingdom
_Vertebrata_, 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
class _Gasteropoda_. 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
with _Chiton_, while many are altogether naked. In some kinds the soft
body is drawn out into a number of tufted processes, as in Doris and
Eolis, and sometimes the body is almost worm-like, as in _Phylliroë_, or
provided with a pair of ring-like lateral processes and a rudimentary
shell, as in the sea-hare _Aplysia_.
Next above the Gasteropods comes a group of animals forming the class
_Pteropoda_. 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 named _Dentalium_.
Highest of all the mollusca stand the cuttle-fishes, forming (with the
_Nautilus_ and many extinct animals, such as ammonites and their allies)
the great class _Cephalopoda_. 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 belongs
_Spirula_, 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 the _Nautilus_, 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 called _Lamellibranchiata_, 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 as _Mya_, stone-perforating shells
such as _Pholas_, 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 called
_Brachiopoda_, a class which we may, at least provisionally, consider as
belonging to the mollusca. These _Brachiopods_ are 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," Tunicaries or
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 or _Vertebrata_.
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 class _Crustacea_ contains, 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 called _Ostracods_, 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, called _Copepoda_, become, when adult, so
degraded in structure as to have the appearance of mere worms, as
_Lerneocera_ and _Tracheliastes_, and become strangely unlike the
typical forms (crabs and lobsters) of their class.
Other animals of the class _Crustacea_, which animals form the order
_Cirripedia_ (barnacles and acorn-shells), bear such an external
resemblance to mollusks that they were actually classed by Cuvier in the
class _Mollusca_. 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 of _Trilobites_ also belongs to the
class _Crustacea_.
The next class, _Myriopoda_, consists of the hundred-legs (centipedes),
and thousand-legs (millipedes), which present us with some of the best
examples 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 called _Peripatus_, the anatomy of
which presents many significant peculiarities.