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system is particularly connected with the organs of locomotion, which are different from those of all other animals, and are called ambulacra1 (a, a, a,

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Fig. 67.-Ambulacra of Star-fish,

As seen in a longitudinal and vertical section of one of the rays; and three of them in a separate figure on a larger scale, in which they are shewn in different conditions: a, a, a, tubular feet; b, b, b, internal vesicles; c, the organ which supplies the fluid with which they are filled.

in fig. 67). They are fleshy
and tubular, more or less
elongated, and terminated
by suckers. Their number
is often very great. They
pass through orifices in the
external integument of the
animal, and are generally
arranged in rows. The
movements of the ambu-
lacra are accomplished in
a very remarkable manner.
Each of these organs has
at its base a vesicle (b, b,
in fig. 67), supplied with a
watery fluid from a tube
which
springs from a

special secreting organ (c in fig. 67). By the contraction of the vesicles, the fluid is forced into the ambulacra, distending them to their utmost extent; whilst on their being contracted by another set of muscles, it returns into the vesicles. A star-fish, or even a sea-urchin, can climb a perpendicular rock or the side of a glass vessel by means of this apparatus. The mouths of the Echinodermata are variously furnished with masticating organs; that of the Sea-urchins has generally five flat calcareous teeth, moved by a very complex apparatus of muscles and bony sockets, which acts as a very powerful mill for grinding down food.

Articulata.

We come now to the great division of the animal kingdom called by Cuvier Articulata or Articulated Animals. The name refers not to the possession of articulated or jointed members, but to the articulated structure of the whole body. The Articulata are composed of segments jointed together in a line, each segment being formed of one or more rings, which in some appear externally as mere folds of a soft skin, as in worms, and in others are covered with a hard substance similar in composition to the bones of vertebrated animals, as in crabs, lobsters, &c. In some of the Articulata, the rings are almost equally developed, and the body is hardly otherwise divided into segments; in others, the rings differ

1 From Latin ambulare, to walk.

very much in their development, and the segments are very distinct. A few only of the lowest Articulata, of which the earthworm may be taken as an example, have no distinct head, and no eyes nor other organs of special sense. Some of them have the mouth adapted merely for suction; but others have jaws or mandibles, and frequently several pairs of them, for seizing their food and tearing it in pieces. Their jaws do not open vertically, as in vertebrated animals, but laterally. There is no proper heart, but instead of it a dorsal1 vessel, which runs along the central line of the body near the back or upper side. Respiration is effected by gills in those Articulata that live in water; but in those that live in air, by air-tubes, which ramify through the whole body, as in insects, or by mere air-sacs, as in the earthworm.

Annelida. Many of the lowest Articulata have no limbs, but locomotion is accomplished by mere contraction and extension of the body. An earthworm, pushing its way forward, draws in the hinder part of its body as much as possible, the rings closing together, and then stretches out the fore-part as far as the integuments will permit. The earthworm belongs to a class called Annelida or Annelids, from Latin annulus, a ring. They have all very much the same form, and none of them have limbs, but many, higher in the scale of organisation than the earthworm, have eyes, and some have feelers or tentacles. Most of them live in water and respire by gills.

Leeches belong to an order of Annelids differing from all the rest in their mode of locomotion, which is by means of suckers, one at each end of the body. The sucker at the fore-end of the body is also the mouth; and that of many species is admirably adapted not only for killing and eating the minute aquatic animals which constitute their ordinary food, but for making little wounds in the higher animals, when opportunity occurs, through which their blood may be sucked.

Myriapoda. Next to the Annelids may be ranked the class called Myriapoda, in which the same elongated form generally prevails, and the body is, in like manner, composed of a large number of almost equal rings. All the Myriapoda, however, have a distinct head, most of them have eyes, and they have antennæ like those of insects. The mouth is furnished with a complete masticating apparatus. The body is protected by a hard covering, or external skeleton. None of the Myriapoda have wings. They do not undergo so great transformations as insects, but issue from the egg somewhat like what they are ultimately to become. The higher kinds of Myriapoda, as Centipedes, feed on animal substances, or prey on small animals; the lower, as Gallyworms, generally feed on decaying vegetable matter.

1 From Latin dorsum, the back.

2 From Greek myrios, ten thousand, and pous, podos, a foot.

The organs called antennæ are jointed filaments with which the head is furnished, one on each side, and are evidently very delicate organs of touch. The creatures which possess them seem to feel their way by them, and to them is ascribed the bee's power of working in the dark. Some suppose that they are also organs of hearing, and by means of them it would appear that some insects, as bees and ants, have the power of communicating with each other. They possess great flexibility, but differ much in the number of joints of which they are composed, and also in their form, some being threadlike, some club-shaped, some feathered, &c. in endless variety.

Crustacea. The remaining classes of Articulata-namely, Crustacea, Arachnida, and Insecta—all have the body generally less elongated than the Annelida and Myriapoda, the division into segments quite distinct, and the circulating and nervous systems much concentrated in those segments with which the organs of special sense and of locomotion are connected. All of them have articulated limbs, and Insects are also very generally furnished with wings, which neither Crustaceans nor Arachnids possess. The Crustacea derive their name from their external skeleton, the hard armour which in most of them covers the whole body, and which, in those of highest organisation, is very complex in its structure. The body in Crustaceans, as in Arachnids and Insects, consists of three segments, the head, thorax, and abdomen. In Crustaceans and Arachnids, the head and thorax are very much combined into one piece, whilst in Insects they are perfectly distinct. All the organs of special sense are connected with the head, and the limbs with the thorax; but in many Crustaceans, as the lobster, prawn, and shrimp, the principal organ of locomotion is the abdomen, which terminates in fan-like appendages. By bending the abdomen suddenly down under the thorax, they dart backward in the water. The limbs of some are adapted for swimming; those of others are used for walking at the bottom of the water or on dry ground. The first pair of legs is not unfrequently transformed into powerful claws or pincers, as in crabs and lobsters. The limbs of the first thoracic rings are, in many Crustaceans, organs not of locomotion, but connected with the mouth, and employed for tearing food. The respiratory organs of all the Crustaceans are adapted to an aquatic life; even those which live on land being generally inhabitants of damp places and breathing by means of gills. Crustaceans are all oviparous ; and they undergo remarkable metamorphoses after issuing from the egg, before they attain their adult form, after which, however, they still increase in size, moulting or casting the shell frequently.

The minute Crustaceans, which abound in lakes, ponds, and rivers, as well as in the sea, are of great use in the economy of nature, consuming

organic matter which would otherwise pollute the water by its decay, and affording food for fish.

Arachnida.—The Arachnida are commonly regarded as intermediate between Crustaceans and Insects. They have generally eight legs; although some, like insects, have only six. None of them have wings. The higher kinds-spiders and scorpions-breathe by means of pulmonary cavities; the lower-mites and ticks-by air-tubes. All of them have two or more eyes; many have eight. Some of the lowest Arachnida are parasitic upon insects, and a few live on decaying animal or vegetable substances, of which we have an example in the cheesemite. In this and other mites and ticks, the mouth is a mere proboscis formed for suction; but spiders and scorpions have a mouth fitted for tearing and masticating their prey. Scorpions are remarkable for the sting at the tip of the tail, the tail itself being a prolongation of the abdomen. Spiders also subdue their prey by means of poison; but it is emitted through the mandibles. Both spiders and scorpions prey chiefly on insects; but there are very large spiders in the tropical parts of South America which occasionally prey on small birds. Many spiders catch their prey by stealthily approaching it, and suddenly springing upon it; but others employ webs for this purpose, the spider lurking in a corner of the web till the vibrations of its threads announce that an insect is entangled in its meshes. Spiders' webs are formed of a substance exuding from small protuberances called spinnerets, at the extremity of the abdomen. This substance is at first glutinous, but dries into thread as soon as it comes into contact with the air.

Insecta.-Insects,1 so named from the extremely marked division of the three segments which form the body, are the most important and numerous class of Articulata. The segments are often so deeply divided that the slenderness to which the body is reduced between them cannot be contemplated without admiration, as in the neck of a wasp, or in the link which connects its abdomen with its thorax. The first segment is the head, which may be regarded as formed of several rings, modified and condensed together till their character as distinct rings is lost. The second segment is the thorax, which is always formed of three rings, closely combined, but easily distinguishable. The third segment is the abdomen, usually consisting of nine rings. All insects in their adult state have six legs; some are destitute of wings; others have two, and others four; but no insect, and indeed no animal, has any other number, and the only animals having four wings are insects. The external covering of the body of insects is of a horn-like substance, in most parts hard, but more or less flexible. It is the principal framework of the body, and to it the muscles are attached. Respiration is extremely

1 Latin insecta, cut into, from in, into, and seco, to cut.

active in insects; and they display, in general, an extraordinary degree of activity and muscular energy. The flight of many kinds is far more rapid, in proportion to their size, than that of birds; others display a similar superiority in running, swimming, and digging or burrowing; whilst the leaping of many, as fleas and grasshoppers, and the springing of others, as cheese-hoppers, greatly exceed that of which any vertebrate animal is capable.

Insects feed on very different kinds of food. Some prey on other insects; some devour animal, and some vegetable substances; some suck the juices of animals, some the juices of plants or the honey of their flowers. The mouth of many is adapted for gnawing, cutting, or tearing; that of others, for sucking. An insect's mouth of the former kind is very complex in structure; and although the mouth adapted for sucking is apparently much more simple, it in reality consists of the same parts, but very much modified and united. The eyes of insects are of two kinds-simple and compound. Some have simple eyes only, others have only compound eyes; but the greater number have two large compound eyes on the sides of the head, and three small simple eyes

Fig. 68.-Section of the Eye of a
Cockchafer (highly magnified):

A, section: a, optic ganglion, into which
the optic nerve swells; b, nerves arising
from its surface, and proceeding to the
general retina; c, general retina; d,
layer of pigment, in front of the general
retina; e, optic nerves of the individual
eyes which form the compound eye. B,
a group of these, much magnified: , bulb
of optic nerve; g, layer of pigment; h,
vitreous humour; i, cornea.

In

between them. A compound eye is an
organ of most remarkable structure.
When examined through the micro-
scope, its surface is seen to be divided
into a great number of hexagonal
facets, which are in fact corneœ.
the ant, there are only about fifty
of these facets in each eye; but in the
common house-fly, there are about four
thousand, in butterflies, upwards of
seventeen thousand, and in some
beetles, more than twenty-five thou
sand. Each cornea may be regarded
as belonging to a distinct eye, provided
with a nervous apparatus, lens, iris,
and pupil of its own.
The eyes of
insects cannot be turned to one side,
like those of vertebrate animals, and
for the want of this power, compensa-
tion is made by the number of eyes,
each looking in its own particular
direction.

Insects, in general, take no care of their eggs after depositing them; but they are guided by wonderful instincts to select the proper situations for the deposition of them, so that the young, when hatched, may

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