CHAPTER X

FISHES

[Illustration]

SUGGESTIONS.--The behavior of a live fish in clear water, preferably in a glass vessel or an aquarium, should be studied. A skeleton may be prepared by placing a fish in the reach of ants. Skeletons of animals placed on ant beds are cleaned very thoroughly. The study of the perch, that follows, will apply to almost any common fish.

=Movements and External Features.=--What is the _general shape of the body_ of a fish? How does the dorsal, or upper, region differ in form from the ventral? Is there a narrow part or neck where the head joins the trunk? Where is the body thickest? What is the ratio between the length and height? (Fig. 209.) Are the right and left sides alike? Is the symmetry of the fish bilateral or radial?

The _body of the fish may be divided_ into three regions,--the head, trunk, and tail. The trunk begins with the foremost scales; the tail is said to begin at the vent, or anus. Which regions bear appendages? Is the head movable independently of the trunk, or do they move together? State the advantage or disadvantage in this. Is the body depressed (flattened vertically) or compressed (flattened laterally)? Do both forms occur among fishes? (See figures on pages 123, 124.)

How is the _shape of the body advantageous for movement_? Can a fish turn more readily from side to side, or up and down? Why? Is the head wedge-shaped or conical? Are the jaws flattened laterally or vertically? The fish swims in the water, the bird swims in the air. Account for the differences in the shape of their bodies.

[Illustration: FIG. 209.--WHITE PERCH (_Morone Americana_).]

Is the _covering of the body_ like the covering of any animal yet studied? The scales are attached in little pockets, or folds, in the skin. Observe the shape and size of scales on different parts of the body. What parts of the fish are without scales? Examine a single scale; what is its shape? Do you see concentric lines of growth on a scale? Sketch a few of the scales to show their arrangement. What is the use of scales? Why are no scales needed on the head? How much of each scale is hidden? Is there a film over the scale? Are the colors in the scale or on it?

=The Fins.=--Are the movements of the fish active or sluggish? Can it remain stationary without using its fins?

Can it move backward? How are the fins set in motion? What is the color of the flesh, or muscles, of a fish? Count the fins. How many are in pairs? (Fig. 209.) How many are vertical? How many are on the side? How many are on the middle line? Are the paired or unpaired fins more effective in balancing the fish? In turning it from side to side? In raising and lowering the fish? In propelling it forward? How are some of the fins useful to the fish besides for balancing and swimming?

The hard _spines_ supporting the fins are called the fin rays. The fin on the dorsal line of the fish is called the _dorsal_ fin. Are its rays larger or smaller than the rays of the other fins? The perch is sometimes said to have two dorsal fins, since it is divided into two parts. The fin forming the tail is called the tail fin, or _caudal_ fin. Are its upper and lower corners alike in all fishes? (Fig. 228.) On the ventral side, just behind the vent, is the ventral fin, also called the anal fin. The three fins mentioned are unpaired fins. Of the four-paired fins, the pair higher on the sides (and usually nearer the front) are the _pectoral_ fins. The pair nearer the ventral line are the _pelvic_ fins. They are close together, and in many fish are joined across the ventral line. The ventral fins are compared to the legs, and the pectoral fins to the arms, of higher vertebrates. (Fig. 244.) Compare fins of fish, pages 123, 124.

Make a =drawing= of the fish seen from the side, omitting the scales unless your drawing is very large.

Are the =eyes= on the top or sides of the head, or both? Can a fish shut its eyes? Why, or why not? Is the eyeball bare, or covered by a membrane? Is the covering of the eyeball continuous with the skin of the head? Is there a fold or wrinkle in this membrane or the surrounding skin? Has the eye a pupil? An iris? Is the eye of the fish immovable, slightly movable, or freely movable? Can it look with both eyes at the same object? Is the _range of vision_ more upward or downward? To the front or side? In what direction is vision impossible? Can a fish close its eyes in sleep? Does the eyeball appear spherical or flattened in front? The ball is really spherical, the lens is very convex, and fish are nearsighted. Far sight would be useless in a dense medium like water.

[Illustration: FIG. 210.--BLACKBOARD OUTLINE OF FISH.]

In what direction are the =nostrils= from the eyes? (Fig. 211.) There are two pairs of nostrils, but only one pair of nasal cavities, with two nostrils opening into each. There are no nasal passages to the mouth, as the test with a probe shows that the cavities do not open into the mouth. What two functions has the nose in man? What function has it in the fish?

[Illustration: FIG. 211.--HEAD OF CARP.]

There are _no external_ =ears=. The ear sacs are embedded in the bones of the skull. Is hearing acute or dull? When fishing, is it more necessary not to talk or to step lightly, so as not to jar the boat or bank?

What is the use of the large openings found at the back of the head on each side? (Fig. 211.) Under the skin at the sides of the head are thin membrane bones formed from the skin; they aid the skin in protection. Just under these membrane bones are the gill covers, of true bone. Which consists of more parts, the membranous layer, or the true bony layer in the gill cover? (Figs. 211 and 212.)

[Illustration: FIG. 212.--SKELETON OF PERCH.]

Is the =mouth= large or small? Are the _teeth_ blunt or pointed? Near the outer edge, or far in the mouth? (Fig. 212.) Does the fish have lips? Are the teeth in one continuous row in either jaw? In the upper jaw there are also teeth on the premaxillary bones. These bones are in front of the maxillary bones, which are without teeth. Teeth are also found in the roof of the mouth, and the tongue bears horny appendages similar to teeth. Are the teeth of the fish better suited for chewing or for grasping? Why are teeth on the tongue useful? Watch a fish eating: does it chew its food? Can a fish taste? Test by placing bits of brown paper and food in a vessel or jar containing a live fish. Is the throat, or gullet, of the fish large or small?

The =skeleton= of a fish is simpler than the skeleton of other backboned animals. Study Fig. 212 or a prepared skeleton. At first glance, the skeleton appears to have two vertebral columns. Why? What bones does the fish have that correspond to bones in the human skeleton? Are the projections (processes) from the vertebræ long or short? The _ribs_ are attached to the vertebræ of the trunk, the last rib being above the vent. The tail begins at the vent. Are there more tail vertebræ or trunk vertebræ? Are there any neck (cervical) vertebræ (_i.e._ in front of those that bear ribs)? The first few ribs (how many?) are attached to the central body of the vertebræ. The remaining ribs are loosely attached to processes on the vertebræ. The ribs of bony fishes are not homologous with the ribs of the higher vertebrates. In most fishes there are bones called intermuscular bones attached to the first ribs (how many in the perch?) which are possibly homologous to true ribs; that is, true ribs in the higher vertebrates may have been developed from such beginnings.

[Illustration: FIG. 213.]

Which, if any, of the _fin skeletons_ (Fig. 214) are not attached to the general skeleton? Which fin is composed chiefly of tapering, pointed rays? Which fins consist of rays which subdivide and widen toward the end? Which kind are stiff, and which are flexible? Which of the fin rays are segmented, or in two portions? The outer segment is called the radial, the inner the basal segment. Which segments are longer? There is one basal segment that lacks a radial segment; find it (Fig. 212).

[Illustration: FIG. 214.--SOFT-RAYED AND SPINY-RAYED FINS.]

What is the advantage of the backbone plan of structure over the armor-plate plan? You have seen the spool-like body of the vertebra in canned salmon. Is it concave, flat, or convex at the ends?

[Illustration: FIG. 215.--CARP, with right gill cover removed to show gills.]

[Illustration: FIG. 216.--SKELETON AROUND THROAT OF FISH.]

The =gills= are at the sides of the head (Fig. 215) under the opercula, or gill covers. What is the color of the gills? Do the blood vessels appear to be very near the surface of the gills, or away from the surface? What advantage in this? Are the gills smooth or wrinkled? (Fig. 215.) What advantage? The bony supports of the gills, called the gill arches, are shown in Fig. 216 (_k_₁ to _k_₄). How many arches on each side? The gill arches have projections on their front sides, called gill rakers, to prevent food from being washed through the clefts between the arches. The fringes on the rear of the gill arches are called the gill filaments (_a_, Fig. 216). These filaments support the thin and much-wrinkled borders of the gills, for the gills are constructed on the plan of exposing the greatest possible surface to the water. Compare the plan of the gills and the human lungs. The gill opening on each side is guarded by seven rays (_kh_, Fig. 216) along the hinder border of the gill cover. These rays grow from the tongue bone. (_Zu_, Fig. 216. This is a rear view.)

[Illustration: FIG. 217.--CIRCULATION IN GILLS.]

[Illustration: FIG. 218.--NOSTRILS, MOUTH, AND GILL OPENINGS OF STING-RAY.]

Watch a live fish and determine how the water is forced between the gills. Is the mouth opened and closed in the act of breathing? Are the openings behind the gill covers opened and closed? How many times per minute does fresh water reach the gills? Do the mouth and gill covers open at the same time? Why must the water in contact with the gills be changed constantly? Why does a fish usually rest with its head up stream? How may a fish be kept alive for a time after it is removed from the water? Why does drying of the gills prevent breathing? If the mouth of a fish were propped open, and the fish returned to the water, would it suffocate? Why, or why not?

[Illustration: FIG. 219.--GILL OPENINGS OF EEL.]

=Food Tube.=--The gullet is short and wide. The stomach is elongated (Fig. 220). There is a slight constriction, or narrowing, where it joins the intestine. Is the intestine straight, or does it lie in few or in many loops? (Fig. 220.) The liver has a gall bladder and empties into the intestine through a bile duct. Is the liver large or small? Simple or lobed? The spleen (_mi_, Fig. 220) lies in a loop of the intestine. The last part of the intestine is straight and is called the rectum. Is it of the same size as the other portions of the intestine? The fish does not possess a pancreas, the most important digestive gland of higher vertebrates.

[Illustration: FIG. 220.--ANATOMY OF CARP. (See also colored figure 4.)

_bf_, barbels on head (for feeling); _h_, ventricle of heart; _as_, aortic bulb for regulating flow to gills; _vk_, venous sinus; _ao_, dorsal aorta; _ma_, stomach; _l_, liver; _gb_, gall cyst; _mi_, spleen; _d_, small intestine; _md_, large intestine; _a_, vent; _s_, _s_, swim bladder; _ni_, _ni_, kidney; _hl_, ureter; _hb_, bladder; _ro_, eggs (roe); _mhe_, opening of ducts from kidney and ovary.

=Questions:= Are the kidneys dorsal or ventral? The swim bladder? Why? Why is the swim bladder double? Does blood enter gills above or below?]

The _ovary_ lies between the intestine and the air bladder. In Fig. 220 it is shown enlarged and filled with egg masses called roe. It opens by a pore behind the vent. The silver lining of the body cavity is called the peritoneum. (See Chap. VII, Human Biology.)

Is the _air bladder_ simple or partly divided in the perch? In the carp? (Fig. 220.) Is it above or below the center of the body? Why? The air bladder makes the body of the fish about as light as water that it may rise and sink with little effort. When a fish dies, the gases of decomposition distend the bladder and the abdomen, and the fish turns over. Why?

Where are the _kidneys_? (Fig. 220.) Their ends unite close under the spinal column. The ureters, or tubes, leading from them, unite, and after passing a small urinary bladder, lead to a tiny urinary pore just behind the opening from the ovary. (Colored figure 4.)

=The Circulation.=--The fish, unlike other vertebrates, has its breathing organs and its heart in its head. The gills have already been described. The heart of an air-breathing vertebrate is near its lungs. Why? The _heart_ of a fish is near its gills for the same reason. The heart has one auricle and one ventricle. (Colored figure 1.)

[Illustration: FIG. 221.--PLAN OF CIRCULATION.

_Ab_, arteries to gills; _Ba_, aortic bulb; _V_, ventricle.]

Blood returning to the heart comes through several veins into a _sinus_, or antechamber, whence it passes down through a valve into the _auricle_; from the auricle it goes forward into the _ventricle_. The ventricle sends it into an _artery_, not directly, but through a _bulb_ (as, Fig. 220), which serves to maintain a steady flow, without pulse beats, into the large artery (_aorta_) leading to the gills. The arteries leading from the gills join to form a _dorsal aorta_ (_Ao_, Fig. 221), which passes backward, inclosed by the lower processes of the spinal column. After going through the _capillaries_ of the various organs, the blood returns to the heart through veins.

The _color of the blood_ is given by red corpuscles. These are nucleated, oval, and larger than the blood corpuscles of other vertebrates. The blood of the fish is slightly above the temperature of the water it inhabits.

[Illustration: FIG. 222.--BRAIN OF PERCH, from above.

_n_, end of nerve of smell; _au_, eye; _v_, _z_, _m_, fore, mid, and hind brain; _h_, spinal bulb; _r_, spinal cord.]

Notice the general shape of the =brain= (Fig. 222). Are its subdivisions distinct or indistinct? Are the lobes in pairs? The middle portion of the brain is the widest, and consists of the two _optic lobes_. From these lobes the optic nerves pass beneath the brain to the eyes (_Sn_, Fig. 223). In front of the optic lobes lie the two cerebral lobes, or the _cerebrum_. The small _olfactory lobes_ are seen (Fig. 224) in front of the cerebrum. The olfactory nerves may be traced to the nostrils. Back of the optic lobes (mid brain) is the cerebellum (hind brain), and back of it is the _medulla oblongata_, or beginning of the spinal cord.

[Illustration: FIG. 223.--BRAIN OF PERCH, side view.]

[Illustration: FIG. 224.--BRAIN OF PERCH, from below.]

Taking the eyeball for comparison, is the whole brain as large as one eyeball? (Fig. 222.) Judging from the size of the parts of the brain, which is more important with the fish, thinking or perception? Which is the most important sense?

The scales along a certain line on each side of the fish, called the lateral line, are perforated over a series of lateral line sense organs, supposed to be the chief organs of _touch_ (see Fig. 209).

[Illustration: FIG. 225.--THE STICKLEBACK. Instead of depositing the eggs on the bottom, it makes a nest of water plants--the only fish that does so--and bravely defends it.]

=Questions.=--Which of the fins of the fish have a use which corresponds to the keel of a boat? The rudder? A paddle for sculling? An oar? State several reasons why the head of the fish must be very large, although the brain is very small. Does all the blood go to the gills just after leaving the heart?

Make a list of the different species of fish found in the waters of your neighborhood; in the markets of your town.

[Illustration: FIG. 226.--ARTIFICIAL FECUNDATION. The egg-cells and sperm-cells are pressed out into a pan of water.]

[Illustration: FIG. 227.--NEWLY HATCHED TROUT, with yolk-sac adhering, eyes large, and fins mere folds of the skin. (Enlarged.)]

=Reproduction.=--The female fish deposits the unfertilized eggs, or ova, in a secluded spot on the bottom. Afterward the male fish deposits the sperms in the same place (see Fig. 225). The eggs, thus unprotected, and newly hatched fish as well, are used for food by fish of the same and other species. To compensate for this great destruction, most fish lay (spawn) many thousands of eggs, very few of which reach maturity. Higher vertebrates (_e.g._ birds) have, by their superior intelligence, risen above this wasteful method of reproduction. Some kinds of marine fish, notably cod, herring, and salmon, go many miles up fresh rivers to spawn. It is possible that this is because they were originally fresh-water species; yet they die if placed in fresh water except during the spawning season. They go because of _instinct_, which is simply an inherited habit. Rivers may be safer than the ocean for their young. They are worn and exhausted by the journey, and never survive to lay eggs the second time.

[Illustration: FIG. 228.--A SHARK (_Acanthias vulgaris_).]

The _air bladder is developed from the food tube_ in the embryo fish, and is homologous with lungs in the higher vertebrates. Are their functions the same?

Fish that _feed on flesh have a short intestine_. Those that eat plants have a long intestine. Which kind of food is more quickly digested?

There are _mucous glands in the skin_ of a fish which supply a secretion to facilitate movement through the water; hence a freshly caught fish, before the secretion has dried, feels very slippery.

The air bladder, although homologous to lungs, is not a breathing organ in common fishes. It is filled by the formation of gases from the blood, and can be made smaller by the contraction of muscles along the sides of the body; this causes the fish to sink. In the gar and other ganoids, the air bladder contains blood vessels, is connected with the gullet, and is used in breathing. Organs _serving the same purpose_ in different animals are said to be _analogous_. To what in man are the gills of the fish analogous? Organs having _a like position and origin_ are said to be _homologous_. The air bladders of a fish are homologous with the lungs of man; but since they have not the same use they are not analogous.

How does the tail of a shark or a gar differ from the tail of common fishes? (Fig. 228.) Do you know of fish destitute of scales? Do you know of fish with whiplike feelers on the head? (Figs.) Why are most fishes white on the under side?

=Comparative Review.=--(Copy table on one page or two facing pages of notebook.)

=========+===========+===========+===========+===========+=========== | IS THERE | | DIGESTIVE | | | A HEAD? | METHOD OF |ORGANS AND | RE- | | A NECK? | FEEDING | DIGESTION |PRODUCTION | SENSES ---------+-----------+-----------+-----------+-----------+----------- | | | | | Ameba | | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Sponge | | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Hydra | | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Starfish | | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Earthworm| | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Wasp | | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Mussel | | | | | | | | | | ---------+-----------+-----------+-----------+-----------+----------- | | | | | Fish | | | | | | | | | | =========+===========+===========+===========+===========+===========

[Illustration: FIG. 229.--DRAWING THE SEINE.]

[Illustration: FIG. 230.--SUNFISH.

FIG. 231.--TUNNY.

FIG. 232.--SWORDFISH.

FIG. 233.--SWELLFISH.

FIG. 234.--TURBOT.

FIG. 235.--CARP.

FIG. 236.--HERRING.

FIG. 237.--SPECKLED TROUT.

FIG. 238.--PERCH.

FIG. 239.--SALMON.

=Seven Food Fish. Three Curious Fish.=

SPECIAL REPORTS. (Encyclopedia, texts, dictionary.)]

[Illustration: FIG. 240.--SEA HORSE (_hippocampus_), with incubating pouch, _Brt._

FIG. 241.--BAND FISH.

FIG. 242.--TORPEDO. Electrical organs at right and left of brain.

FIG. 243.--LANTERN FISH (_Linophryne lucifer_). (After Collett.)

FIG. 244.--LUNG FISH of Australia (_Ceratodus miolepis_).

FIG. 245.--TRUNK FISH.

FIG. 246.--SEAWEED FISH. × ¹⁄₅ (_Phyllopteryx eques_).

=Remarkable Fish.= SPECIAL REPORTS. (Encyclopedia, texts, dictionary.)]

KEY TO THE BRANCHES, OR SUB-KINGDOMS

A₁ =One-celled Animals= (_Protozoans_) I. PROTOZOANS

A₂ =Many-celled Animals= (_Metazoans_)

B₁ RADIATE (around a center). Without head; all aquatic, resembling plants, and often fixed to bottom

C₁ Walls of body serving as digestive organs

D₁ Many openings, no tentacles II. SPONGES (_Porifera_)

D₂ One opening, which is both mouth and III. POLYPS vent; tentacles for seizing prey (_Cœlenterata_)

C₂ Digestive tube distinct from body wall, IV. ECHINODERMS spiny skin

B₂ BILATERAL. With anterior and posterior end; dorsal and ventral surface

C₁ Body of successive segments; legs V. VERMES without joints

C₂ External skeleton of successive rings; VI. ARTHROPODS jointed legs

C₃ Body soft; no skeleton; usually bearing VII. MOLLUSKS a limy shell

C₄ Internal jointed skeleton, attached to VIII. VERTEBRATES an axis or vertebral column

=Examples.=--Tell the branch to which each of the following animals belongs: crayfish, earthworm, thousand leg, white grub, sea anemone, ameba, tapeworm, caterpillar, beetle, sparrow, snake, oyster, starfish, fish. Be prepared to state the reason for each classification.

The =classes= in the branch =vertebrata= are: 1. Fishes (_pisces_). 2. Frogs and Salamanders (_batrachia_). 3. Reptiles (_reptilia_). 4. Birds (_aves_). 5. Mammals (_mammalia_).

[Illustration: FIG. 247.--A SNAIL. (Which branch? Why?)]