I. Protozoa.--Animals composed of a single cell, reproducing by cell division.
The following are the princ.i.p.al cla.s.ses of Protozoa, examples of which we may have seen or read about:--
CLa.s.s I. _Rhizopoda_ (Greek for _root-footed_). Having no fixed form, with pseudopodia. Either naked as _Amoeba_ or building limy (_Foraminifera_) or gla.s.slike skeletons (_Radiolaria_).
CLa.s.s II. _Infusoria (in infusions)._ Usually active ciliated Protozoa.
Examples, _Paramoecium_, _Vorticella_.
CLa.s.s III. _Sporozoa (spore animals)._ Parasitic and usually nonactive.
Example, _Plasmodium malariae_.
[Ill.u.s.tration: A h.o.r.n.y fiber sponge. Notice that it is a colony. One fourth natural size.]
II. Sponges.--Because the body contains many pores through which water bearing food particles enters, these animals are called _Porifera_. They are cla.s.sed according to the skeleton they possess into limy, gla.s.slike, and h.o.r.n.y fiber sponges. The latter are the sponges of commerce. With but few exceptions sponges live in salt water and are never free swimming.
[Ill.u.s.tration: Sea anemones. One half natural size. The right hand specimen is expanded and shows the mouth surrounded by the tentacles. The left hand specimen is contracted. (From model at the American Museum of Natural History.)]
III. Coelenterates.--The hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the _Coelenterata_. The word "coelenterate" (_coelom_ = body cavity, _enteron_ = food tube) explains the structure of the group. They are animals in which the real body cavity is lacking, the animal in its simplest form being little more than a bag. Some examples are the hydra, shown on page 179, salt-water forms known as hydroids, colonial forms which have part of their life free swimming as jellyfish; sea anemones and coral polyps, tiny colonial hydra like forms which build a living or secreted covering.
IV. Worms.--The wormlike animals are grouped into _flatworms_, _roundworms_, and segmented or _jointed_ worms.
(_a_) Flatworms are sometimes parasitic, examples being the tapeworm and liver fluke. They are usually small, ribbon- or leaf-like and flat and live in water.
(_b_) Roundworms, minute threadlike creatures, are not often seen by the city girl or boy. Vinegar eels, the horsehair worm, the pork worm or trichina and the dread hookworm are examples.
(_c_) Segmented worms are long, jointed creatures composed of body rings or segments. Examples are the earthworm, the sandworm (known to New York boys as the fishworm), and the leeches or bloodsuckers.
[Ill.u.s.tration: A jointed worm. The sandworm. Slightly reduced.]
[Ill.u.s.tration: The common starfish seen from below to show the tube feet.
About one half natural size.]
[Ill.u.s.tration: The crayfish, a crustacean. _A_, antenna; _M_, mouth; _E_, compound stalked eye; _Ch_, pincher claw; _C.P._, cephalothorax; _Ab_, abdomen; _C.F._, caudal fin. A little reduced.]
V. Echinoderms.--These are spiny-skinned animals, which live in salt water.
They are still more complicated in structure than the worms and may be known by the spines in their skin. They show radial symmetry. Starfish or sea urchins are examples.
VI. Arthropods.--These animals are distinguished by having jointed body and legs. They form two great groups. The higher forms of the _Crustacea_ have only two regions in the body, a fused head and thorax, called the _cephalothorax_, and an abdominal region. A second group is the _Insecta_, of which we know something already. Crustacea breathe by means of _gills_, which are structures for taking oxygen out of the water, while adult insects breathe through air tubes called _trachea_.
Two smaller groups of arthropods also exist, the _Arachnida_, consisting of spiders, scorpions, ticks, and mites, and the _Myriapoda_, examples being the "thousand leggers" found in some city houses.
[Ill.u.s.tration: A common snail, a mollusk. (From a photograph by Davison.)]
VII. Mollusca.--Another large group is the Mollusca. This phylum gets its name from the soft, unsegmented body (_mollis_ = soft). Mollusks usually have a sh.e.l.l, which may be of one piece, as a snail, or two pieces or _valves_, as the clam or oyster.
[Ill.u.s.tration: The skeleton of a dog; a typical vertebrate.]
VIII. The Vertebrates.--All of the animals we have studied thus far agree in having whatever skeleton or hard parts they possess on the outside of the body. Collectively, they are called _Invertebrates_. This exoskeleton differs from the main or axial skeleton of the higher animals, the latter being inside of the body. The exoskeleton is dead, being secreted by the cells lining the body, while the endoskeleton is, in part at least, alive and is capable of growth, _e.g._ a broken arm or leg bone will grow together. But a man has certain parts of the skeleton, as nails or hair, formed by the skin and in addition possesses inside bones to which the muscles are attached. Some of the bones are arranged in a flexible column in the _dorsal_ (the back) side of the body. This _vertebral column_, as it is called, is distinctive of all _vertebrates_. Within its bony protection lies the delicate central nervous system, and to this column are attached the big bones of the legs and arms. The vertebrate animals deserve more of our attention than other forms of life because man himself is a vertebrate.
[Ill.u.s.tration: The sand shark, an elasmobranch. Note the slits leading from the gills. (From a photograph loaned by the American Museum of Natural History.)]
Five groups or cla.s.ses of vertebrates exist. _Fishes_, _Amphibians_, _Reptiles_, _Birds_, and _Mammals_. Let us see how to distinguish one cla.s.s from another.
[Ill.u.s.tration: The sturgeon, a ganoid fish.]
Fishes.--Fishes are familiar animals to most of us. We know that they live in the water, have a backbone, and that they have fins. They breathe by means of gills, delicate organs fitted for taking oxygen out of the water.
The heart has two chambers, an auricle and a ventricle. They have a skin in which are glands secreting mucus, a slimy substance which helps them go through the water easily. They usually lay very many eggs.
CLa.s.sIFICATION OF FISHES
ORDER I. _The Elasmobranchs._ Fishes which have a soft skeleton made of cartilage and exposed gill slits. Examples: sharks, skates, and rays.
ORDER II. _The Ganoids._ Fishes which once were very numerous on the earth, but which are now almost extinct. They are protected by platelike scales. Examples: gars, sturgeon, and bowfin.
[Ill.u.s.tration: A bony fish.]
ORDER III. _The Teleosts, or Bony Fishes._ They compose 95 per cent of all living fishes. In this group the skeleton is bony, the gills are protected by an operculum, and the eggs are numerous. Most of our common food fishes belong to this cla.s.s.
ORDER IV. _The Dipnoi, or Lung Fishes._ This is a very small group. In many respects they are more like amphibians than fishes, the swim bladder being used as a lung. They live in tropical Africa, South America, and Australia, inhabiting the rivers and lakes there.
Characteristics of Amphibia.--The frog belongs to the cla.s.s of vertebrates known as Amphibia. As the name indicates (_amphi_, both, and _bia_, life), members of this group live both in water and on land. In the earlier stages of their development they take oxygen into the blood by means of gills.
When adult, however, they breathe by means of lungs. At all times, but especially during the winter, the skin serves as a breathing organ. The skin is soft and unprotected by bony plates or scales. The heart has three chambers, two auricles and one ventricle. Most amphibians undergo a complete metamorphosis, or change of form, the young being unlike the adults.
[Ill.u.s.tration: Newt. (From a photograph loaned by the American Museum of Natural History.) About natural size.]
[Ill.u.s.tration: The leopard frog, an amphibian.]
CLa.s.sIFICATION OF AMPHIBIA
ORDER I. _Urodela._ Amphibia having usually poorly developed appendages. Tail persistent through life. Examples: mud puppy, newt, salamander.
ORDER II. _Anura._ Tailless Amphibia, which undergo a metamorphosis, breathing by gills in larval state, by lungs in adult state.
Examples: toad and frog.
Characteristics of Reptilia.--These animals are characterized by having scales developed from the skin. In the turtle they have become bony and are connected with the internal skeleton. Reptiles always breathe by means of lungs, differing in this respect from the amphibians. They show their distant relationship to birds in that their large eggs are incased in a leathery, limy sh.e.l.l.
[Ill.u.s.tration: Box tortoise, a land reptile. (From photograph loaned by the American Museum of Natural History.) About one fourth natural size.]
[Ill.u.s.tration: The gila monster, a poisonous lizard. About one twelfth natural size.]
CLa.s.sIFICATION OF REPTILES
ORDER I. _Chelonia_ (turtles and tortoises). Flattened reptiles with body inclosed in bony case. No teeth or sternum (breastbone).
Examples: snapping turtle, box tortoise.
ORDER II. _Lacertilia_ (lizards). Body covered with scales, usually having two-paired appendages. Breathe by lungs. Examples: fence lizard, horned toad.
[Ill.u.s.tration: The common garter snake. Reduced to about one tenth natural size.]