The pearly nautilus (_Nautilus pompilius_) is a Cephalopod with four gills instead of two, as with the Decapoda and Octopoda, and is the only existing member of what was in the earlier times of the earth's history a large group of animals. The nautili live in rather shallow water usually creeping over the bottom feeding on small marine animals. They make a many-chambered spiral sh.e.l.l with its inner surface lined with beautiful pearly nacre.

CHAPTER XXIII

BRANCH CHORDATA: THE VERTEBRATES, ASCIDIANS, ETC.

The branch Chordata includes all the backboned animals or vertebrates, comprising the fishes, salamanders, frogs and toads, lizards, crocodiles, turtles and snakes, birds, and all the quadrupeds or mammals, and includes also a few small unfamiliar ocean animals which do not look at all like the backboned animals, but which agree with them in possessing a peculiar structure called the notochord. This notochord consists of a series or cord of cells extending longitudinally through the body from head to tail, above the alimentary ca.n.a.l and below the spinal nerve-cord. In all the vertebrates excepting a few low forms, the notochord while present in the young, is replaced in the adult by a segmented bony or cartilaginous axis, the spinal or vertebral column. But in the ascidians or sea-squirts (called also tunicates) it persists throughout life. In addition to this characteristic notochord, nearly all the Chordata are marked by the presence, either in embryonic or larval stages only, or else persisting throughout life, of a number of slits or clefts in the walls of the pharynx which serve for breathing, and which are called gill-slits.

=Structure of the vertebrates.=--As the backboned or vertebrate animals make up almost the whole of the branch Chordata, and as the few other chordates are animals the special structures of which we shall not undertake to study in this book, we may note here some of the other more obvious structural characteristics of the true vertebrates. The possession of a backbone or bony (sometimes cartilaginous) spinal column is the characteristic by which we distinguish them from the invertebrate or backboneless animals.

Furthermore, all of the vertebrates possess an internal skeleton which is in most cases composed of bone, and is firm and strong. In some of the lower fishes, as the sharks and sturgeons, the skeleton is made up of cartilage, tough but not hard. The vertebrate skeleton consists typically of an axial portion comprising the spinal column and head, and of two pairs of appendages or limbs, variously developed as fins, wings, legs and arms. In some vertebrates these limbs are represented by mere rudiments, and in the lowest fish-like forms, the lancelets and lampreys, there is not the slightest trace of limbs. A part of the central nervous system, the spinal cord, runs longitudinally through the body on the dorsal side of the alimentary ca.n.a.l; the circulatory system is closed, the blood being always confined in the heart and in vessels called arteries, veins, and capillaries, and the blood is red in color owing to the presence of numerous red corpuscles or blood-cells. The nervous system is highly developed, with a large brain in all the typical forms, and with complex and usually highly efficient special sense-organs. Respiration is carried on by means of external gills, or by internal lungs which communicate with the outside through the mouth and nostrils. To the lungs and gills the blood is brought to be "purified," i.e., to give up its carbonic-acid gas and to take up oxygen.

=Cla.s.sification.=--The Chordata are variously divided by zoologists into eight or ten cla.s.ses, of which (in the eight-cla.s.s system) the five cla.s.ses[15] Pisces (fishes), Batrachia (batrachians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals), belong to the true vertebrates. These cla.s.ses will be considered in the five following chapters.

The remaining three cla.s.ses include a number of strange marine forms which until recent years were considered as worms, but which are now known to be the nearest living allies of the earliest or primitive vertebrates. The relationship of these forms to early types is manifest, not in the appearance or structure of the adult stage, but only during embryonic or larval stages.

[Ill.u.s.tration: FIG. 111.--An ascidian or sea-squirt from the coast of California. (After Jordan and Kellogg.)]

=The ascidians.=--The sea-squirts, or Ascidians, common on the seash.o.r.e, compose one cla.s.s of these primitive chordate animals. They possess a simple, sac-like body (fig. 111), fastened to the rocks by one end, the other being provided with two openings, one for the ingress and the other for the exit of water, a strong current of which flows constantly through the body. By means of this current the ascidian obtains food.

Usually sea-squirts live together in large colonies, and in some cases a number of individuals enclose themselves in a common gelatinous ma.s.s, forming what is called a compound ascidian.

The ascidian when born is a tiny, free-swimming, tadpole-like creature with a slender finned tail. It swims about freely for only a few hours, however, soon attaching itself to a rock, and in its further development becoming degenerate. It loses its tail and with it the short notochord possessed by the larva; the eye and the auditory organ are lost, and the nervous system and alimentary ca.n.a.l become much reduced and simplified. Sea-squirts in their adult stage are very simple degenerate animals, with low functional development, yet their embryonic and larval conditions show a considerable degree of structural specialization, and the presence of the notochord in these early stages reveals their affinity with the backboned animals.

FOOTNOTE:

[15] The animals included by some zoologists in the single cla.s.s Pisces, are held by other zoologists to const.i.tute three distinct cla.s.ses, thus making a subdivision of the branch into ten cla.s.ses.

CHAPTER XXIV

BRANCH CHORDATA (_Continued_): CLa.s.s PISCES (THE FISHES)

THE GOLDEN SUNFISH OR PUMPKIN SEED (_Apomotis_ sp.)

TECHNICAL NOTE.--The species of sunfish named, or some closely related species, can be obtained in any brook or stream in the United States. _Gibbosus_ lives in all streams north of Dubuque, Chicago, Pittsburg, and along the eastern coast north of Charleston. Closely allied species live in all the other parts of the country except in the higher Rocky Mountains west of Bismarck, Pueblo, and Santa Fe. One species is found in the streams of California, but none occurs in Washington or Oregon. In the few places where a sunfish cannot be had, any species of ba.s.s or perch may be used. Sunfish live in ponds and sluggish streams in deep holes under a log or at the foot of a stump. They take eagerly a hook baited with a worm, or they may be caught in nets. When sunfish cannot be kept fresh for study in cla.s.s, specimens may be preserved in alcohol or 4% formalin. But if possible to keep some alive for a time in a jar or tub with plenty of fresh water, the colors of the living fish, together with its manner of swimming and mode of breathing, can be observed.

=External structure=[16] (fig. 112).--Examine the general configuration and make-up of the body. Note the deep, laterally flattened _trunk_ and paddle-like _tail_. The _head_ is closely fitted to the trunk without any neck. Note that the body is thickly covered with firm, hard _scales_, arranged like the shingles on a roof. Remove one of these scales and examine it under a hand lens. What sort of an edge has it? Such a scale is said to be _ctenoid_.

The body of the sunfish terminates behind in the _caudal fin_, a series of cartilaginous rays connected by thin skin and attached to a bony plate at the end of the backbone. Along the median dorsal line will be noted another fin composed anteriorly of spines and posteriorly of soft rays jointed and branched. This is the _dorsal fin_. How many spines has it? Anterior to the caudal fin on the ventral surface is a median unpaired _a.n.a.l fin_. How many spines has it? Anterior to the a.n.a.l fin are the _ventral fins_, while on the sides of the body back of the head in a line with the mouth are found the _pectoral fins_. The ventral fins, attached to a rudimentary pelvis, correspond to the hind legs of the other vertebrates. The pectoral fins, attached to the shoulder girdle, correspond to the arms. In front of the a.n.a.l fin note a small pit-like opening, the opening from the kidneys and reproductive organs, and just anterior to this a large aperture, the _a.n.u.s_. At the anterior end of the head note the broad _mouth_, surrounded by a complicated system of bones.

Note the large _eyes_ surrounded by a series of small bones, the _orbital chain_. Just anterior to the eyes are two pairs of openings, one pair of each side opening into a closed sac. What are these openings? Note the presence of various bones on the side of the head, each covered with a thin layer of skin. These are _membrane bones_, characteristic of fishes. Are there any external ears in the fish?

Examine the inside of the mouth. Is there a _tongue_? If so, of what character? Are there _teeth_? If so, where are they situated?

Note along each side extending to the base of the tail a line of modified scales, on each scale a little mucous tube, the whole series const.i.tuting the _lateral line_. These scales are intimately a.s.sociated with a large nerve (the _vagus_), and probably serve an important part, not yet clearly understood, in the life of the fish.

Lift up the flap in front of one of the pectoral fins. This is the _opercular flap_ which covers the gills that lie beneath. Bend this forward and find four _gill-arches_, each with its double fringe of _gills_. Note the _gill-rakers_, short and blunt, on the first gill-arch. Note also on the under side of the flaps turned back, delicate red gill-like structures covered by a membrane. These are the _false gills_ or _pseudo-branchiae_, larger in most fishes than in the sunfish. The gills in the fish subserve the same function as the gills of the crayfish, that of purifying the blood by eliminating carbonic-acid gas from it and taking up oxygen from the air mixed with or dissolved in the water. Organs subserving the same purposes in different kinds of animals as, for example, the gills in fish and in crayfish, are called _a.n.a.logous structures_. But there is an important morphological difference between the fish's gills and the gills of the crayfish. In the latter animal they are outgrowths of the basal segments of the walking-legs; in the fish they are outgrowths from the alimentary ca.n.a.l. The internal gills of the young toad (tadpole) arise in the same way as those of a fish. Structures which are identical in their origin, like the gills of tadpole and fish, are called _h.o.m.ologous structures_.

Make a drawing of the sunfish from a lateral aspect, showing the external parts named.

=Internal Structure.=--TECHNICAL NOTE.--Insert one point of the scissors a little to one side of the a.n.u.s and cut dorsally on the left side of the body to the backbone. Now cut anteriorly from the a.n.u.s along the ventral wall to where the jaws unite, and cut, also anteriorly, along the dorsal wall until the left side of the body can be removed. Bend the opercular flap backward over the eye and pin the entire fish, uncut side down, to the bottom of the dissecting-pan, covering it with water.

The above operation will have severed the large powerful _muscles_ forming the body-wall and extending along the sides. Note a membranous sac completely filling a large dorsal cavity. This is the _swim-bladder_, a float filled with air which tends to give the fish the same weight as the water it displaces. It arises as a diverticulum from the alimentary ca.n.a.l, but soon becomes permanently shut off from it. Beneath the swim-bladder is a large cavity filled with various organs, collectively known as the _viscera_. In vertebrate animals the cavity which contains the viscera is generally called the _peritoneal cavity_. It is lined by the _peritoneum_, a delicate membrane, part of which is deflected as the _mesentery_ over the alimentary ca.n.a.l and the other organs, thus suspending them all from the dorsal wall. Note in the anterior end of the peritoneal cavity a large bi-lobed gland, the _liver_, red in fresh, yellowish in alcoholic specimens. Its function, like that of the liver of the toad, is to store up nutriment for the blood and to secrete a digestive fluid called _bile_. Behind the liver note a long, convoluted tube. What is this tube? Unfold this tube, separating it from its enveloping membrane, the mesentery.

Thrust a probe down the throat and note that it pa.s.ses into a thick-walled sac, the _stomach_. The mouth and gill-slits open into the front part of the alimentary ca.n.a.l called the _pharynx_, which leads by a short tube, the _sophagus_, into the stomach. Note the large, thickened portion of the alimentary ca.n.a.l leading from the stomach. This is the _pylorus_, and to its walls are attached a number of finger-like projections, the _pyloric caeca_. The pyloric caeca secrete a fluid which is poured into the alimentary ca.n.a.l and which a.s.sists in the process of digestion somewhat as does the secretion from the pancreas of the toad. From the pylorus, pa.s.sing backwards in one or two loops, is the _small intestine_. Trace this to its exit.

Lying within the mesentery near the posterior end of the body-cavity note a small red glandular ma.s.s, the _spleen_.

At the anterior end of the body in front of the liver and between the sets of gills note the small _pericardial cavity_ within which is contained the _heart_. The pericardial cavity is separated from the peritoneal cavity by a thick muscular wall against which the liver abuts. The heart consists of four parts. The posterior part is a thin-walled reservoir, the _sinus venosus_, into which blood enters through the _jugular vein_ from the head and through the _cardinal vein_ from the kidney. From the sinus venosus it pa.s.ses forward into a large chamber, the _auricle_. Next it flows into the _ventricle_, where, by the contraction of the walls, rhythmical pulsations force it into the _conus arteriosus_, thence into the _ventral aorta_, and lastly into the gills, where it is purified. After pa.s.sing through the capillaries in the fine gill-filaments it is again collected, now pure, by paired arteries from each pair of gills, which arteries unite to form the _dorsal aorta_ extending backward just below the backbone to the end of the tail. From the dorsal aorta a pair of arteries, the _subclavian_, are given off to the pectoral fins. At this point two other arteries branch off ventrally, the first being the _cardiac artery_, which distributes blood to the stomach and pyloric caeca. The second divides into several long _mesenteric arteries_ supplying blood to all parts of the intestine and spleen. In the caudal region blood is taken up through the _caudal vein_ and carried forward to the kidneys. These strain out the impurities arising from waste of tissues, after which the blood is carried back to the sinus venosus through the cardinal vein. From the intestine it is gathered into the large _portal vein_ as in the toad.

The portal vein carries blood to the liver, where nutriment may be stored up, and from thence it flows back to the sinus venosus through a very short thin-walled vessel, the _hepatic sinus_.

The _kidneys_, more or less united in one ma.s.s, lie in the posterior part of the body-cavity along the dorsal wall. Note running from each side of the kidney a _ureter_ which unites with its fellow and opens into a small _urinary bladder_ which discharges through a small opening immediately back of the a.n.u.s.

The reproductive organs lie below the swim-bladder near the posterior end of the body-cavity. If the fish are caught in the spring, the greater part of the body-cavity of the female is found to be filled with small eggs. When mature, these eggs are deposited by the mother fish in the gravel of the stream-bed where they are fertilized by the sperm-cells poured over them by the male and left floating in the water.

The nervous system of fishes is best studied in a specimen treated with nitric acid. Carefully remove the roof of the skull, thereby exposing the brain. Most anteriorly make out, as in the toad, the paired _olfactory lobes_. These are attached by long stalks to the _cerebrum_ or _forebrain_, which is followed by two large hollow lobes, the _midbrain_ or _optic lobes_. Behind the midbrain is the _cerebellum_. Following the cerebellum is the elongate _medulla oblongata_, which tapers backward into the _spinal cord_. How far backward does the spinal cord extend? On each side of the brain-case about opposite the cerebellum are located the _auditory organs_, each consisting of three _semicircular ca.n.a.ls_ which lie in different planes, and of the _vestibule_. These parts are filled with liquid, and suspended in the liquid in the vestibule are small calcareous bodies called _otoliths_ or _ear-stones_. Running out beneath from the midbrain are the _optic nerves_, which cross, the left one connected with the right eye, the right one with the left eye. From each side of the medulla oblongata there is given off a large nerve, the _vagus_, which sends branches to the lateral line organs on either side, and extends backward to the stomach and viscera.

For further study of the nervous system see Parker's "Zootomy," pp.

122-130.

Make a drawing of the nervous system as worked out.

TECHNICAL NOTE.--To make a good skeleton immerse a fresh or preserved specimen for some time in a hot soap solution. When the muscles have commenced to soften remove the body from the solution, pick the flesh away, and leave to dry.

Note that the main axis of the _skeleton_ is composed of _vertebrae_ placed end to end. How many vertebrae are there? What vertebrae bear _ribs_? The ribless ones beyond the body-cavity are called _caudal vertebrae_. Note the _interspinal bones_ which support the fins, with large muscles on either side to control their action. Note that the group of bones supporting the pectoral fin is attached to the back of the brain-case and makes up the _shoulder girdle_. The ventral fins are attached to a rudimentary _pelvic girdle_, attached in front to the shoulder girdle, as the shoulder girdle is in turn attached to the skull. It will be seen that the sunfish has no neck and we may say, also, no back. Its skeleton consists only of a tail attached to the skull. The brain-case is made up of a number of bones closely joined together. From it is suspended the lower jaw, which comprises a number of bones but loosely attached to each other. Overlying these is the system of membrane bones already mentioned, including the opercle or gill-cover.

For a detailed study of the fish-skeleton see Parker's "Zootomy," pp.

86-101, or Parker and Haswell's "Zoology," vol. II. pp. 183-195.

=Life-history and habits.=--The sunfish or "pumpkin-seed" lives in quiet corners of the brooks and rivers, preferably under a log or at the root of an old stump. It is a beautiful fish, shining "like a coin fresh from the mint." Its body is mottled golden, orange and blue, with metallic l.u.s.tre, darker above, pale or yellowish below. Its fins are of the same color. The tip of its opercle is prolonged like an ear and jet black in color, with a dash of bright scarlet along its lower edge. Nearly all the thirty species of sunfish found in the United States have this black ear, but some have it long, some short, and in some it is trimmed with yellow or blue instead of scarlet.

The sunfish lays its eggs in the spring in a rude nest it scoops in the gravel, over which it stands guard with its bright fins spread, looking as big and dangerous as possible. When thus employed it takes the hook savagely, perhaps regarding the worm as a dangerous enemy. The young fishes soon hatch, looking very much like their parents, although more transparent and not so brightly colored. They grow rapidly, feeding on insects and other small creatures, and reach their growth in two or three years. They do not wander far and never willingly migrate.

Students should verify this account on the different species. A more exact study of the nests of the different species and the fishes'

defence of them would be a valuable addition to our knowledge. The most striking traits of the habits of this fish are its vivacity and courage; it reveals its great muscular strength when captured. The s.e.xes are similar in appearance and both defend the nest alike.

OTHER FISHES.

[Ill.u.s.tration: FIG. 112.--Dissection of the sunfish, _Apomotis_ sp.]

Fishes const.i.tute the largest cla.s.s of vertebrate animals and are to be found everywhere in ponds, streams, or ocean. About 15,000 species of fish are known, of which 3,000 live in North America. The largest of all fishes is the basking shark (_Cetorhinus_), which reaches a length of thirty-six feet. The smallest is the dwarf goby (_Mistichthys_), less than half an inch long, found in Luzon, one of the Philippine Islands. Between these extremes is every variety in size, form, and relative proportions. The body, for example, may be greatly elongated and almost cylindrical as in the eels; or long and flattened from side to side as in the ribbon-fishes; or the head may be very large, wider and higher than the rest of the body as in the anglers, or may have a great beak as in the sword-fish.

=Body form and structure.=--When we consider the fish as a whole, we find first a body formed for progression in the water, the typical fish being pointed at each end (the shorter point in front), and having the sides flattened, the back and belly rather narrow, and the motive power located in the fin on the tail. From this typical form diverge all conceivable variations, adaptations to every sort of fish life.

Most fishes have the body covered with scales, although many have the skin naked or covered with small scales so hidden in the skin as to be hardly visible. The scales are small h.o.r.n.y or bony plates which fit into small pockets or folds of the skin, and are usually arranged shingle-fashion, overlapping each other. They are of various shapes, mostly cla.s.sified as of three kinds, namely, squarish enamelled scales called ganoid, roundish smooth-edged called cycloid, and roundish tooth-edged called ctenoid.

The skeleton of the fish is relatively complex. Its bones are comparatively soft, having little lime in them, indeed in many cases they are mere cartilage. The vertebral column is made of twenty-four vertebrae in the typical fishes, the number in the others being variously increased, or sometimes diminished. These vertebrae are of two cla.s.ses, abdominal or body, and caudal or tail vertebrae. The former have a neural arch which encloses the spinal cord and from which projects a spine.

Below, the processes spread apart, surrounding the kidneys and partly enclosing the air-bladder. To these processes ribs are loosely attached.