Cold-blooded Vertebrates (p. 19), which breathe by gills during their
whole life. The heart consists only of a single ventricle and a single
auricle (p. 18). The head passes immediately into the body without any
intervening neck. Fishes move chiefly by means of the tail, at the end
of which there is a tail-fin. This and the dorsal and anal fins lie in
the median plane of the fish, while the pectoral fins, which are
attached to the skull in bony fishes, and also the ventral fins, are
paired structures more or less comparable to the fore and hind limbs of
higher vertebrates. The skeleton of most fishes (pike, perch, carp, eel,
plaice) is bony; but in a few subdivisions of fishes (sharks, skates,
sturgeons, lampreys) it is cartilaginous. The skin is usually covered
with thin translucent scales; but there are fishes with a smooth skin
(lampreys), with prickle-like dermal ossifications (sharks), and with
rhomboidal bony plates (sturgeons). As Fishes are without exception
aquatic, there is no need for me to treat more specially of them here.
[Illustration:
FIG. 53.—The Perch (_Perca fluviatilis_).
]
Second Sub-Kingdom: ARTHROPODA (JOINTED-LIMBED ANIMALS).
[Illustration:
FIG. 54.—Wood-borer (_Sirex_); 1, larva; 2, adult insect.
]
[Illustration:
FIG. 55.—The Large Centipede (_Scolopendra morsitans_).
]
[Illustration:
FIG. 56.—Ground Beetle, with nervous system drawn in white.
]
The body of an Arthropod is bilaterally symmetrical (cp. p. 16), and
consists of a number of joints (Fig. 55), not equally numerous in all
members of the sub-kingdom. These joints or segments lie one behind the
other, and are at first alike; but as in the course of further
development they become adapted to various functions, the difference
between them becomes greater. Compare the Wood-borer represented in Fig.
54, 2, with the young form of the same animal (Fig. 54, 1). The segments
often fuse together, which brings about the formation of a smaller
number of subdivisions to the body; or even all the segments may become
united (mites). In the last case the Arthropod characters are only to be
seen in the jointing of the limbs. Segmented animals (_e.g._ the common
earthworm) are also found among the worms (Sub-kingdom III. of the
Animal Kingdom); but these worms have either no limbs, or only small
unjointed foot-stumps, never jointed limbs as in the Arthropods. These
last are limbless or with unjointed foot-stumps only when young; when
adult they always have jointed limbs. The head of Arthropods bears
several successive pairs of jaws, which move to and fro from one side
towards the other. The covering of the body consists of hard protective
pieces; it is only in the young condition that the skin of several
species is soft. Arthropods have no internal skeleton; the muscles are
attached to the skin. The central part of the nervous system (p. 10),
which consists in Vertebrates of brain and spinal cord, lies in
Arthropods almost entirely on the ventral side. In the head is situated
the cerebral ganglion, a large nervous mass resting on the gullet, and
giving off nerves to the eyes and feelers. Besides this, there is a
ventral nervecord on the ventral side of the animal, running below the
gut, and made up of several pairs of ganglia, united with one another by
means of nerve fibres. The ganglia of the ventral cord send their nerves
to the jaws, limbs, muscles, viscera, etc. The cerebral ganglion is
connected with the first ventral ganglion by means of a cord on each
side of the gullet (Fig. 56), so that a ring-like structure is formed.
The breathing organs are gills in some Arthropods (the Crustacea, _e.g._
crayfish, crabs); insects and centipedes breathe by air-tubes or
tracheæ, while the respiratory organs of spider-like animals, when these
do not breathe entirely by means of the skin, are more or less strongly
modified tracheæ. The structure of the tracheal system is generally as
follows: on each side of the body there is a row of breathing-holes, or
stigmata, through which the air can enter the tracheæ; these are very
much branched, so that they finally become very fine tubes investing the
various organs, which in this way obtain the requisite amount of oxygen.
The stigmata on one side of a caterpillar are clearly shown in Fig. 60,
and those of a hornet larva in Fig. 61.
Four classes belong to this sub-kingdom: Insecta (Insects), Myriapoda
(centipedes, etc.), Arachnoidea (spiders, scorpions, etc.), Crustacea
(crayfish, crabs, lobsters, etc.).