animal makes use of the organs in question. The crab always carries
them erect and elevated; and is incessantly striking the water with
them, with a very peculiar jerking action, now and then vibrating,
and, as it has been called, “twiddling” them. These antennæ,
therefore, appear to be always on the watch: let the animal be at
rest, let it be feeding, no matter, the superior antennæ are ever
elevated and on constant guard.
The lengthened and delicate setæ with which they are furnished are,
moreover, peculiarly adapted to receive and convey the most minute
vibratory sensations from the medium in which they are suspended;
and, on the whole, it seems to be satisfactorily settled by Mr.
Spence Bate (to whose excellent memoir I am indebted for these
explanatory details) that the inner antennæ are real _ears_.
Having thus taken our crab by the ears, we will endeavor next to
tweak his nose. But stay, we must find it first. We turn our horny
gentleman up, and in his flat ancient face we certainly discern
little sign of a nasal organ. Our friend Mr. Bate must assist us
again. He will tell us to look at the outer or lower antennæ. We will
look accordingly, magnifier in hand, while he makes it clear to us
that these are a pair of noses.
Each of these organs is formed of a stem consisting in general of
five joints, and a filament of many minute joints. In the prawn
and the lobster all the five joints of the stem are distinct; but
in the crab the whole are, as it were, soldered together into a
compact mass, with difficulty distinguishable into their constituent
articulations; while in some species their position can be indicated
only by the presence of the olfactory operculum.
This important little organ varies in its construction in the
different families of Crustacea. In the crab it is a small movable
appendage, situated at the point of junction between the second
and third joints; it is attached to a long, calcareous, lever-like
tendon, at the extreme limit of which is placed a set of muscles,
by which it is opened and closed; to assist in which operation, at
the angle of the operculum most distant from the central line of the
animal are fixed two small hinges. When the operculum is raised, the
internal surface is found to be perforated by a circular opening
protected by a thin membrane.
In the prawn, shrimp, and lobster, there is no operculum, but only
the orifice covered by a membrane, which is placed at the extremity
of a small protuberance, and it is not capable of being withdrawn
into the cavity of the antennæ, as in the crab.
In the latter animal, the little door, when it is raised, exposes
the orifice in a direction pointing to the mouth; and where there
is no door, still the direction of the opening is the same, inward
and forward, answering to the position of the nostrils in the higher
animals. In each case it is so situated that it is impossible for any
food to be conveyed into the mouth without passing under this organ;
and there most conveniently the animal is enabled to judge of the
suitability of any substance for food, by raising the little door,
and applying to the matter to be tested the sensitive membrane of the
internal orifice.
Thus it is concluded that this lower or outer pair of antennæ are
the proper organs of smell, as the upper and inner are of hearing.
The eyes, though constructed on the same general principles as those
of insects, yet present some particulars worthy of your notice. In
the crabs and lobsters they consist of numerous facets, behind each
of which is a conical or prismatic lens, the round extremity of which
is fitted into a transparent conical pit, corresponding to a vitreous
body, while the conical extremity of these lenses is received into
a kind of cup, formed by the filaments of the optic nerve. Each of
these filaments, together with its cup, is surrounded by pigment
matter in a sheath-like manner. To see this structure would require
anatomical skill; but you may here examine with a low power portions
of the cornea, or glassy exterior, of the eye of a crab and of a
lobster. In the former, you see that the facets into which the cornea
is divided are hexagonal, like those of most insects, but in the
latter they are square.
But Crustacea have a far greater faculty of circumspection than
insects have; for besides the extensive convexity and numerous facets
of their eyes, these organs are placed at the extremity of shelly
foot-stalks, which are themselves movable on hinges, capable of being
projected at pleasure, of being moved in different directions, and of
being packed snugly away, when not in active use, in certain grooves
hollowed out expressly for them in the front margin of the shell.
REPTILES
--PETER MARK ROGET
The order of Batrachia, or Amphibious Reptiles, constitutes the first
step in the transition from aquatic to terrestrial vertebrata. It
is more particularly the function of respiration that requires to
be modified, in consequence of the change of element in which the
animal is to reside; and as if it had been necessary, conformably
to the laws of animal creation, that this change should not be
abruptly made, we find that Batrachian reptiles, with which this
series commences, are constructed, at first, on the model of fishes;
breathing the atmospheric air contained in the water by means of
gills, and moving through the fluid by the same instruments of
progression as fishes, which, indeed, they exactly resemble in
every part of their mechanical conformation. The tadpole, which is
the young of the frog, is, at first, not distinguishable in any
circumstance of its internal skeleton, or in the disposition of
its vital organs, from the class of fishes. The head, indeed, is
enlarged, but the body immediately tapers to form a lengthened tail,
by the prolongation of the spinal column, which presents a numerous
series of coccygeal vertebræ, furnished with a vertical expansion of
membrane to serve as a caudal fin, and with appropriate muscles for
executing all the motions required in swimming.
Yet, with all this apparent conformity to the structure of a strictly
aquatic animal, the tadpole contains within its organization the
germs of a higher development. Preparations are silently making
for a change of habitation, for the animal’s emerging from the
waters, for the reception of atmospheric air into new cavities, for
the acquisition of limbs suited to new modes of progression; in a
word, for a terrestrial life, and for all the attributes and powers
which belong to quadrupeds. The succession of forms, which these
metamorphoses present, are in themselves exceedingly curious, and
bear a remarkable analogy with the progress of the transformations
of those insects, which in the first stages of their existence are
aquatic. To this philosophic inquirer into the marvelous plans of
creation, the series of changes which mark these singular transitions
can not fail to be deeply interesting; and occurring, as we here find
them, among a tribe of animals allied to the more perfect forms of
organization, they afford us a better opportunity of exploring the
secrets of their development by tracing them from the earlier stages
of this complicated process, so full of mystery and of wonder.
The egg of the frog is a round mass of transparent nutritive jelly,
in the centre of which appears a small black globule. By degrees
this shapeless globule exhibits the appearance of a head and tail,
and in this form it emerges from its prison, and moves briskly in
the water. From the sides of the neck there grow out feathery tufts,
which float loosely, and without protection, in the surrounding
fluid. These, however, are mere temporary organs, for they serve
the purposes of respiration only until the proper gills are formed,
and they then shrink and become obliterated. The true gills, or
_branchiæ_, are contained within the body, and are four in number on
each side, constructed on a plan very similar to those of fishes.
Retaining this aquatic constitution, the tadpole rapidly increases
in size and in activity for several weeks. In the meantime the
legs, of which no trace was at first apparent, have commenced their
growth. The hindlegs are the first to make their appearance, showing
their embryo forms within the transparent coverings of the hinder
part of the trunk, just at the origin of the tail. These are soon
succeeded by the forelegs, which exactly follow the hindlegs in all
the stages of their development, until they have acquired their due
proportion to the size of the trunk. The animal at this period wears
a very ambiguous appearance, partaking of the forms both of the
frog and of the lizard, and swimming both by the inflexions of the
tail and the irregular impulses given by the feet. This interval is
also employed by this amphibious being in acquiring the faculty of
respiring atmospheric air. We observe it rising every now and then to
the surface, and cultivating its acquaintance with that element, into
which it is soon to be raised; occasionally taking in a mouthful of
air, which is received into its newly developed lungs, and afterward
discharging it in the form of a small bubble. When the necessary
internal changes are at length completed, preparations are made for
getting rid of the tail, which is now a useless member, and which,
ceasing to be nourished, diminishes by degrees, leaving only a short
stump, which is soon removed. The gills are by this time shrunk,
and rapidly disappear, their function being superseded by the lungs,
which have been called into play; and the animal now emerges from the
water and begins a new mode of existence, having become a perfect
frog. It still, however, retains its aquatic habits, and swims with
great ease in the water by means of its hindfeet, which are very long
and muscular, and of which the toes are furnished with a broad web,
derived from a thin extension of the integuments.
No less curious are the changes which take place in all the other
organs for the purpose of effecting the transformations rendered
necessary by this entire alteration in all the external circumstances
of that animal--this total reversal of its wants, of its habits, of
its functions, and of its very constitution.
There are five toes in the foot, with sometimes the rudiment of a
sixth: the anterior extremity has only four toes, which are without
claws.
The necessity of employing the same instruments for progression in
the water and on land is probably the cause which prevents their
having the form best adapted for either function. The hindfeet of
the frog, being well constructed for striking the water backward
in swimming, are, in consequence, less capable of exerting a force
sufficient to raise and support the weight of the body in walking:
and this animal accordingly is exceedingly awkward in its attempt to
walk. On a short level plane it can proceed only by leaps; an action
which the length and great muscularity of the hindlegs particularly
fit them for performing. The toad, on the other hand, whose hindlegs
are short and feeble, walks better, but does not jump or swim so well
as the frog.[5] The Hyla, or tree-frog, has the extremities of each
of its toes expanded into a fleshy tubercle, approaching in the form
of its concave surface to that of a sucker, and by the aid of which
it fastens itself readily to the branches of trees, which it chiefly
inhabits, and along which it runs with great agility.
The salamander is an animal of the same class as the frog, undergoing
the same metamorphoses from the tadpole state. It differs much,
however, in respect to the development of particular parts of the
skeleton. The anterior extremities of the salamander make their
appearance earlier than the hindlegs, and the tail remains as a
permanent part of the structure. The land salamanders have a rounded
tail, but the aquatic species, or Tritons, have it compressed
vertically; thus retaining the fish-like form of the tadpole, and the
same radiated disposition of the muscles.
In the class of serpents we see exemplified the greatest possible
state of simplicity to which a vertebrated skeleton can be reduced;
for it consists merely of a lengthened spinal column, with a head
but little developed, and a series of ribs; but apparently destitute
of limbs, and of the bones which usually connect those limbs with
the trunk. In the conformation of the skull and bones of the face,
they present strong analogies with Batrachian reptiles, and also
with fishes, one tribe of which, namely, the apodous or anguilliform
fishes, they greatly resemble by the length and flexibility of the
spine. These peculiarities of conformation may be in a great measure
traced to the mode of life for which they are destined. The food
assigned to them is living prey, which they must attack and vanquish
before they can convert it into nourishment. The usual mode in which
the boa seizes and destroys its victims is by coiling the hinder
part of its body round the trunk or branch of a tree, keeping the
head and anterior half of the body disengaged; and then, by a sudden
spring, fasten upon the defenceless object of its attack, and twining
round its body so as to compress its chest and put a stop to its
respiration. Venomous serpents, on the other hand, coil themselves
into the smallest possible space, and suddenly darting upon the
unsuspecting or fascinated straggler, inflict the quickly fatal
wound.[6]
It is evident, from these considerations, that, in the absence of all
external instruments of prehension and of progressive motion, it is
necessary that the spine should be rendered extremely flexible, so as
to adapt itself to a great variety of movements. This extraordinary
flexibility is given, first, by the sub-division of the spinal column
into a great number of small pieces; secondly, by the great freedom
of their articulations; and thirdly, by the peculiar mobility and
connections of the ribs.
Numerous as are the vertebræ of the eel, the spine of which consists
of above a hundred, that of serpents is in general formed of a still
greater number. In the rattlesnake (Crotalus horridus) there are
about two hundred; and above three hundred have been counted in the
spine of the Coluber natrix. These vertebræ are all united by ball
and socket joints, as in the adult batrachia; the posterior rounded
eminence of each vertebra being received into the anterior surface of
the next.
While provision has thus been made for extent of motion,
extraordinary care has at the same time been bestowed upon the
security of the joints. Thus, we find them effectually protected from
dislocation by the locking in, above and below, of the articular
processes, and by the close investment of the capsular ligaments.
The direction of the surfaces of these processes, and the shape
and length of the spinous processes, are such as to allow of free
lateral flexion, but to limit the vertical and longitudinal motions:
and whatever degree of freedom of motion may exist between the
adjoining vertebræ, that motion being multiplied along the column,
the flexibility of the whole becomes very great, and admits of its
assuming every degree and variety of curvature.
The mode in which the boa exerts a powerful pressure on the bodies
of the animals it has seized, and which it has encircled within
its folds, required the ribs to be movable laterally, as well as
backward, in order to elude the force thus exerted. The broad convex
surfaces on which they play give them, in this respect, an advantage
which the ordinary mode of articulation would not have afforded. The
spinous processes in this tribe of serpents are short and widely
separated, so as to allow of flexion in every direction. In the
rattlesnake, on the other hand, their length and oblique position are
such as to limit the upward bending of the spinal column, although,
in other respects, its motion is not restricted. The vertebræ at the
end of the tail are furnished with broad transverse processes for the
attachment of the first joints of the rattle.
But of whatever variety of flexions we may suppose the lengthened
body of a serpent to be capable, it will, at first view, be difficult
to conceive how these simple actions can be rendered subservient
to the purposes of progression on land: and yet experience teaches
us that few animals advance with more celerity on the surface of
the ground, or dart upon their prey with greater promptitude and
precision. They raise themselves without difficulty to the tops of
the highest trees, and escape to their hiding-places with a quickness
which eludes observation and baffles the efforts of their pursuers.
The solution of this enigma is to be sought for partly in the
structure of the skin, which, in almost every species, is covered
with numerous scales: and partly in the peculiar conformation of
the ribs. The edges of the scales form rough projections, which are
directed backward, so as to catch the surfaces of the bodies to which
they are applied, and to prevent any retrograde motion. In some
species, the integument is formed into annular plates, reminding us
of the structures so prevalent among worms and myriapode animals.
Each scale is connected with a particular set of muscular fibres,
capable of raising or depressing it, so that, in this way, it is
converted into a kind of toe; and thus the body rests upon the ground
by numerous fixed points of support.
This support is further strengthened by the connection of the ribs
with the abdominal _scuta_, or the scales on the under side of the
body. The mode in which the ribs become auxiliary instruments of
progressive motion was first noticed by Sir Joseph Banks. While he
was watching the movements of a Coluber of unusual size which was
exhibited in London, and was moving briskly along the carpet, he
thought he saw the ribs come forward in succession, like the feet of
a caterpillar. Sir Everard Home, to whom Sir Joseph Banks pointed
out this circumstance, verified the fact by applying his hand below
the serpent, and he then distinctly felt the ends of the ribs moving
upon the palm, as the animal passed over it. The mode in which the
ribs are articulated with the spine is peculiar, and has evidently
been employed with reference to this particular function of the ribs,
which here stand in place of the anterior and posterior extremities,
possessed by most vertebrated animals, and characterizing the type
of their osseous fabric. In the ordinary structure, the head of
each rib has a convex surface, that plays either on the body of a
single vertebra with which it is connected, or upon the two bodies
of adjacent vertebræ: but in serpents the extremity of the head of
the rib has two slightly concave articular surfaces, which play on
a convex protuberance of the vertebra. This structure is attended
with the advantage of preventing the ribs from interfering with the
motions of the vertebræ upon one another. At their lower ends the
ribs of one side have no connection with those of the other, nor are
they joined to any bone analogous to a sternum: for, except in the
Ophiosaurus and the blind-worm (Anguis fragilis), there is no vestige
either of a sternum or scapula, in any animal of this class. Each rib
terminates in a slender cartilage, tapering to a point, which rests,
for its whole length, upon the upper surface of one of the scuta,
or broad scales on the lower side of the body. These scuta, which
are thus connected with the ends of the ribs, and which are moved
by means of short muscles, may be compared to hoofs, while the ribs
themselves may be considered as performing the office of legs. The
ribs move in pairs; and the scutum under each pair, being carried
along with it in all its motions, and laying hold of the ground by
its projecting edge, becomes a fixed point for the advance of the
body. This motion, Sir E. Home observes, is beautifully seen when a
snake is climbing over an angle to get upon a flat surface. When the
animal is moving on a plane, it alters its shape from a circular
or oval form, to one that approaches to a triangle, of which the
surface applied to the ground forms the base. Five sets of muscles
are provided for the purpose of giving to the ribs the motions
backward and forward, by which, as levers, they effect this species
of progression. These muscles are disposed in regular layers; some
passing over one or two ribs to be attached to the succeeding rib.
In all snakes the ribs are continued backward much beyond the region
occupied by the lungs; and although the anterior set are subservient
to respiration, as well as to progressive motion, it is evident that
all those posterior to the lungs must be employed solely for the
latter of these purposes.
It is easy to understand how the serpent can slowly advance, by this
creeping, or vermicular motion, consisting in reality of a succession
of very short steps. But its progress is accelerated by the
curvatures into which it throws its body; the fore part being fixed,
and the hind part brought near to it; then, by a reverse process,
the hind part is fixed, and the head projected forward. By an
alternation of these movements, assisted by the actions of the ribs,
the serpent is enabled to glide onward with considerable rapidity,
and without attracting observation. But where greater expedition is
necessary, they employ a more hurried kind of pace, although one
which exposes them more to immediate view. The body, instead of being
bent from side to side, is raised in one great arch, of which the
two extremities alone touch the ground; and these being alternately
employed as points of support, are made successively to approach and
to separate from each other, the body being propelled by bringing it
from a curved to a straight line.
There is yet a third kind of motion, which serpents occasionally
resort to, when springing upon their prey, or when desirous of making
a sudden escape from danger. They coil themselves into a spiral,
by contracting all the muscles on one side of the body, and then,
suddenly throwing into violent action all the muscles on the opposite
side, the whole body is propelled, as if by the release and unwinding
of a powerful spring, with an impulse which raises it to some height
from the ground, and projects it to a considerable distance.
Thus these animals, to which nature has denied all external
members, are yet capable, by the substitution of a different kind
of mechanism, still constructed from the elements belonging to the
primitive type of vertebrated animals, of silently gliding along
the surface of the earth, of creeping up trees, of striding rapidly
across the plain, and of executing leaps with a vigor and agility
which astonish the beholder, and which, in ages of ignorance and
superstition, were easily ascribed to supernatural agency.
The conformation of those parts of the frame which are subservient
to progressive motion becomes more perfect in the class of Saurian
reptiles, which includes all the lizard tribes. Several links of
connection with the preceding class may still be noticed, marking
the progress of development, as we follow the ascending series of
animals. Rudiments of the bones of the extremities, and, also, of
the sternum, make their appearance very visibly in the Ophiosaurus,
and in the blind-worm (Anguis fragilis). The Siren lacertina has
two diminutive forefeet, placed close to the head. The Lacerta
lumbricoides of Linnæus, or the Bipes canaliculatus of Lacépède,
which is found in Mexico, and of which a specimen is preserved in the
collection at Paris, has a pair of very short feet, also placed near
the head, and divided into four toes, with the rudiment of a fifth.
The Lacerta bipes (Linn.), or Sheltopusic of Pallas, has, on the
other hand, a pair of hindfeet only, but extremely small, together
with rudiments of a scapula and clavicle concealed under the skin.
Next in order must be placed the Chalcides, or snake-lizard, and the
Lacerta seps, animals frequently met with in the south of France, and
which have four minute feet, totally inefficient for the support of
the body, and only remotely useful in contributing to its progressive
undulations.