The Fresh-water Mussel

Suggestions.—The mussel is usually easy to procure from streams and lakes by raking or dredging. In cities the hard-shelled clam, or quahog, is for sale at the markets, and the following descriptions apply to the anodon, unio, or quahog, with slight changes in regard to the siphons. Mussels can be kept alive for a long time in a tub with sand in the bottom. Pairs of shells should be at hand for study.

External Features.—The shell is an elongated oval, broader and blunter at one end (Fig. [188]). Why does the animal close its shell? Does it open the shell? Why? Does it thrust the foot forward and pull up to it, or thrust the foot back and push? (Mussels and clams have no bones.) Does it go with the blunt end or the more tapering end of the shell forward? (Fig. [188].) Can a mussel swim? Why, or why not?

Fig. 188.—Anodon, or fresh-water mussel.

Lay the shells, fitted together, in your hand with the hinge side away from you and the blunt end to the left (Fig. [188]). Is the right or the left shell uppermost? Which is the top, or dorsal, side? Which is the front, or anterior, end? Is the straight edge at the top or at the bottom? Our word “valve” is derived from a word meaning shell, because the Romans used shells for valves in pumps. Is the mussel a univalve or a bivalve? Which kind is the oyster? The snail?

Does the mussel have bilateral symmetry? Can you find a horny covering, or epidermis, over the limy shell of a fresh specimen? Why is it necessary? Does water dissolve lime? Horn? Find a bare spot. Does any of the shell appear to be missing there?

Fig. 189.—Diagram of Shell open and closed, showing muscle, m, and ligament, b.

The bare projection on each shell is called the umbo. Is the umbo near the ventral or the dorsal line? The posterior or anterior end? Is the surface of the umbones worn? Do the umbones rub against the sand as the mussel ploughs its way along? How are the shells held together? Where is the ligament attached? (Fig. [189].) Is it opposite the umbones or more to the front or to the rear? (Fig. [189].) Is the ligament of the same material as the shell? Is the ligament in a compressed condition when the shell is open or when it is closed? (Fig. [189].) When is the muscle relaxed?

Fig. 190.—Mussel crawling in sand.

Notice the lines on the outside of the shell (Figs. [188] and [190]). What point do they surround? They are lines of growth. Was each line once the margin of the shell? If the shell should increase in size, what would the present margin become? (Fig. [191].) Does growth take place on the margin only? Did the shell grow thicker as it grew larger? Where is it thinnest?

Draw the outside of the shell from the side. Draw a dorsal view. Near the drawings write the names of the margins of the shell (p. [98]) and of other parts learned, using lines to indicate the location of the parts.

Study the surface of the shell inside and out. The inside is called mother-of-pearl. Is it of lime? Is the deeper layer of the shell of lime? (When weak hydrochloric acid or strong vinegar is dropped on limy substances, a gas, carbon dioxide, bubbles up.) Compare the thickness of the epidermal layer, the middle chalky layer, and the inner, pearly layer.

Fig. 191.—Diagram.
Change of points of attachment of muscles as mussel enlarges. (Morgan.)

Anatomy of the Mussel.—What parts protrude at any time beyond the edge of the shell? (Fig. [190].) The shell is secreted by two folds of the outer layer of the soft body of the mussel. These large, flaplike folds hang down on each side, and are called the mantle. The two great flaps of the mantle hang down lower than the rest of the body and line the shell which it secretes (Fig. [192]). The epidermis of the mantle secretes the shell just as the epidermis of the crayfish secretes its crust. Can you find the pallial line, or the line to which the mantle extended on each shell when the animal was alive? A free portion of the mantle extended like a fringe below the pallial line.

Fig. 192.—Cross Section of Mussel. (Diagram, after Parker.)

The shells were held together by two large adductor muscles. The anterior adductor (Fig. [193]) is near the front end, above the foot. The posterior adductor is toward the rear end, but not so near the end as the anterior. Can you find both muscle scars in the shells? Are they nearer the ventral or the dorsal surface? The points of attachment travelled downward and farther apart as the animal grew (see Fig. [191]). Higher than the larger scars are small scars, or impressions, where the protractor and retractor muscles that extend and draw in the foot were attached.

Fig. 193.—Anatomy of Mussel. (Beddard.)

The muscular foot extends downward in the middle, halfway between the shells (Fig. [193]). On each side of the foot and behind it hang down the two pairs of gills, the outer pair and the inner pair (Fig. [192]). They may be compared to four V-shaped troughs with their sides full of holes. The water enters the troughs through the holes and overflows above. Is there a marked difference in the size of the two pairs of gills? A kind of chamber for the gills is made by the joining of the mantle flaps below, along the ventral line. The mantle edges are separated at two places, leaving openings called exhalent and inhalent siphons.

Fig. 194.—Mussel.
A, left shell and mantle flap removed.
B, section through body.
Question: Guided by other figures, identify the parts to which lines are drawn.

Fresh water with its oxygen, propelled by cilia at the opening and on the gills, enters through the lower or inhalent siphon, passes between the gills, and goes to an upper passage, leaving the gill chamber by a slit which separates the gills from the foot. For this passage, see arrow (Fig. [194]). The movement of the water is opposite to the way the arrow points. After going upward and backward, the water emerges by the exhalent siphon. The gills originally consisted of a great number of filaments. These are now united, but not completely so, and the gills still have a perforated or lattice structure. Thus they present a large surface for absorbing oxygen from the water.

The mouth is in front of the foot, between it and the anterior adductor muscle (Fig. [194]). On each side of the mouth are the labial palps, which are lateral lips (Fig. [195]). They have cilia which convey the food to the mouth after the inhalent siphon has sent food beyond the gill chamber and near to the mouth. Thus both food and oxygen enter at the inhalent siphon. The foot is in the position of a lower lip, and if regarded as a greatly extended lower lip, the animal may be said to have what is to us the absurd habit of using its lower lip as a foot. The foot is sometimes said to be hatchet-shaped (Fig. [195]). Do you see any resemblance? Does the foot penetrate deep or shallow into the sand? (Fig. [190].) Why, or why not?

Fig. 195.—Mussel. From below. Level cut across both shells.
Se, palp; P, foot; O, mouth; G, liver; Gg, Vg, Pg, ganglia.

The food tube of the mussel is comparatively simple. Behind the mouth it enlarges into a swelling called the stomach (Fig. [193]). The bile ducts of the neighbouring liver empty into the stomach. The intestine makes several turns in the substance of the upper part of the foot and then passing upward, it runs approximately straight to the vent (or anus), which is in the wall of the exhalent siphon. The intestine not only runs through the pericardial cavity (celome) surrounding the heart, but through the ventricle of the heart itself (Fig. [196]).

Fig. 196.—Heart of Mussel, with intestine passing through it.

The kidneys consist of tubes which open into the pericardial chamber above and into the gill chamber below (Neph., Fig. [193]). The tubes are surrounded by numerous blood vessels (Fig. [198]) and carry off the waste matter from the blood.

Fig. 197.

The nervous system consists of three pairs of ganglia and nerves (Fig. [197]). The ganglia are distinguishable because of their orange colour. The pedal ganglia on the front of the foot are easily seen also; the visceral ganglia on the posterior adductor muscle may be seen without removing the mussel from the shell (Fig. [193]). The reproductive organs open into the rear portion of the gill cavity (Fig. [193]). The sperms, having been set free in the water, are drawn into the ova by the same current that brings the food. The eggs are hatched in the gills. After a while the young mussels go out through the siphon.

Summary.—In the gills (Fig. [198]) the blood gains what? Loses what? From the digestive tube the blood absorbs nourishment. In the kidneys the blood is partly purified by the loss of nitrogenous waste.

Fig. 198.—Diagram of Mussel cut across, showing mantle, ma; gills, kie; foot, f; heart, h; intestine, ed.

The cilia of the fringes on the inhalent, or lower, siphon, vibrate continually and drive water and food particles into the mouth cavity. Food particles that are brought near the labial palps are conveyed by them to the mouth. As the water passes along the perforated gills, its oxygen is absorbed; the mantle also absorbs oxygen from the water as it passes. The water, as stated before, goes next through a passage between the foot and the palp into the cavity above the gills and on out through the exhalent siphon. By stirring the water, or placing a drop of ink near the siphons of a mussel kept in a tub, the direction of its flow may be seen. The pulsations of the heart are plainly visible in a living mollusc.

Habits of the Mussel.—Is it abundant in clear or in muddy water; swift, still, or slightly moving water? Describe its track or furrow. What is its rate of travel? Can you distinguish the spots where the foot was attached to the ground? How long is one “step” compared to the length of the shell? The animal usually has the valves opened that it may breathe and eat. The hinge ligament acts like the case spring of a watch, and holds the valves open unless the adductor muscles draw them together (Fig. [189]).

Fig. 199.—Oyster.
C, mouth; a, vent; g, g′, ganglia; mt, mantle; b, gill.

When the mussel first hatches from the egg, it has a triangular shell. It soon attaches itself to some fish and thus travels about. After two months it drops to the bottom again.

Fig. 200.—Trochus.

Other Mollusca.—The oyster’s shells are not an exact pair, the shell which lies upon the bottom being hollowed out to contain the body, and the upper shell being flat. Can you tell by examining an oyster shell which was the lower valve? Does it show signs of having been attached to the bottom? The young oyster, like the young mussel, is free-swimming. Like the arthropoda, most molluscs undergo a metamorphosis to reach the adult stage (Fig. [199]).

Fig. 201.—Cypræa. (Univalve, with a long opening to shell.)

Examine the shells of clams, snails, scallops, and cockles. Make drawings of their shells. The slug is very similar to the snail except that it has no shell. If the shell of the snail shown in Fig. [202] were removed, there would be left a very good representation of a slug.

Economic Importance of Mollusca.—Several species of clams are eaten. One of them is the hardshell clam (quahog) found on the Atlantic coast from Cape Cod to Texas. Its shell is white. It often burrows slightly beneath the surface. The softshell clam is better liked as food. It lives along the shores of all northern seas. It burrows a foot beneath the surface and extends its siphons through the burrow to the surface when the tide is in, and draws into its shell the water containing animalcules and oxygen.

Oysters to the value of many millions of dollars are gathered and sold every year. The most valuable oyster fisheries of North America are in Chesapeake Bay. The young oysters, or “spat,” after they attach themselves to the bottom in shallow water, are transplanted. New oyster beds are formed in this way. The beds are sometimes strewn with pieces of rock, broken pottery, etc., to encourage the oysters to attach themselves. The dark spot in the fleshy body of the oyster is the digestive gland, or liver. The cut ends of the tough adductor muscles are noticeable in raw oysters. The starfish is very destructive in oyster beds.

Fig. 202.—A Snail.
l, mouth; vf, hf, feelers; e, opening of egg duct; f, foot; ma, mantle; lu, opening to lung; a, vent.

Pearls are deposited by bivalves around some irritating particle that gets between the shell and the mantle. The pearl oyster furnishes most of the pearls; sometimes pearls of great value are obtained from fresh-water mussels. Name articles that are made partly or wholly of mother-of-pearl.

Study of a Live Snail or Slug.—Is its body dry or moist? Do land snails and slugs have lungs or gills? Why? How many pairs of tentacles have they? What is their relative length and position? The eyes are dark spots at bases of tentacles of snail and at the tips of the rear tentacles of slug. Touch the tentacles. What happens? Do the tentacles simply stretch, or do they turn inside out as they are extended? Is the respiratory opening on the right or the left side of the body? On the mantle fold or on the body? (Figs. [202]–3–4.) How often does the aperture open and close?

Fig. 203.—A Slug.

Place the snail in a moist tumbler. Does the whole under surface seem to be used in creeping? Does the creeping surface change shape as the snail creeps? Do any folds or wrinkles seem to move either toward the front or the rear of its body? Is enough mucus left to mark the path travelled? The fold moves to the front, adheres, and smooths out as the slug or snail is pulled forward.

Fig. 204.—Circulation and Respiration in Snail.
a, mouth; b, b, foot; c, vent; d, d, lung; h, heart. Blood vessels are black. (Perrier.)

Cephalopods.—The highest and best developed molluscs are the cephalopods, or “head-footed” molluscs. Surrounding the mouth are eight or ten appendages which serve both as feet and as arms. These appendages have two rows of sucking disks by which the animal attaches itself to the sea bottom, or seizes fish or other prey with a firm grip. The commonest examples are the squid, with a long body and ten arms, and the octopus, or devilfish, with a short body and eight arms. Cephalopods have strong biting mouth parts and complex eyes somewhat resembling the eyes of backboned, or vertebrate, animals. The large and staring eyes add to the uncanny, terrifying appearance.

Fig. 205.—A Squid.

The sepia or “ink” discharged through the siphon of the squid makes a dark cloud in the water and favours its escape from enemies almost as much as does its swiftness (Fig. [205]). The squid sometimes approaches a fish with motion so slow as to be imperceptible, and then suddenly seizes it, and quickly kills it by biting it on the back behind the head.

Fig. 206.—Pearly Nautilus. (Shell sawed through to show chambers used when it was smaller, and siphuncle, S, connecting them. Tentacles, T.)

The octopus is more sluggish than the squid. Large species called devilfish sometimes have a spread of arms of twenty-five feet. The pearly nautilus (Fig. [206]) and the female of the paper argonaut (Fig. [207]) are examples of cephalopods that have shells. The cuttlefish is closely related to the squid.

Fig. 207.—Paper Argonaut (female). × ⅓ (i.e. the animal is three times as long and broad as figure).

Fig. 208.—Paper Argonaut (male). × ½.

General Questions.—The living parts of the mussel are very soft, the name mollusca being derived from the Latin word mollis, soft. Why is it that the softest animals, the molluscs, have the hardest coverings?

To which class of molluscs is the name acephala (headless) appropriate? Lamellibranchiata (platelike gills)?

Why is a smooth shell suited to a clam and a rough shell suited to an oyster? Why are the turns of a snail’s shell so small near the centre?

Why does the mussel have no use for head, eyes, or projecting feelers? In what position of the valves of a mussel is the hinge ligament in a stretched condition? How does the shape of the mussel’s gills insure that the water current and the blood current are brought in close contact?

The three main classes of molluscs are: the pelecypoda (hatchet-footed); gastropoda (stomach-footed); and cephalopoda (head-footed). Give an example of each class.

Comparison of Mollusks

	Mussel	Snail	Squid
Shell
Head
Body
Foot
Gills
Eyes

Comparative Review.—(To occupy an entire page in notebook.)

	Grasshopper	Spider	Crayfish	Centipede	Mussel
Bilateral or radiate
Appendages for locomotion
Names of divisions of body
Organs and method of breathing
Locomotion

CHAPTER X
FISHES

Suggestions.—The behaviour 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 other 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 the height? (Fig. [209].) Are the right and the 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, the trunk, and the 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 the 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.

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 colours 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 colour 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 the 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 its 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 on the sides of the head, or on 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 the 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. In what direction from the eyes are the nostrils (Fig. [211].) There are two pair of nostrils, but there is 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?

Fig. 210.—Blackboard Outline of Fish.

There are no external ears. The ear sacs are embedded in the bones of the skull. Is hearing acute or dull? When you are fishing, is it more necessary not to talk or to step lightly, so as not to jar the boat or bank?

Fig. 211.—Head of Carp.

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].)

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?

Fig. 212.—Skeleton of Perch.

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.

Fig. 213.

Fig. 214.—Soft-rayed and Spiny-rayed Fins.

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]).

Fig. 215.—Carp, with right gill cover removed to show gills.

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

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 colour 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] (k₁ to k₄). 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 that of 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.)

Fig. 217.—Circulation in Gills.

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 the 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?

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.

Fig. 220.—Anatomy of Carp. (See also coloured 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.

Is the air bladder in the perch simple or partly divided? In the carp? (Fig. [220].) Is it above or below the centre 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. (Coloured 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. (Coloured figure 1.)

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.

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.

The colour 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.

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. Behind the optic lobes (mid brain) is the cerebellum (hind brain) and behind it is the medulla oblongata or beginning of the spinal cord.

Fig. 223.—Brain of Perch, side view.

Fig. 224.—Brain of Perch, from above.

If you take the eyeball for comparison, is the whole brain as large as one eyeball? (Fig. [222].) If you judge 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]).

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?

Fig. 226.—Artificial Fecundation. The egg cells and sperm-cells are pressed out into a pan of water.

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

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.

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?