CHAPTER XII

ADAPTATIONS OF FISHES

[Illustration: FIG. 133.--Mad-tom, _Schilbeodes furiosus_ Jordan and

Meek. Showing the poisoned pectoral spine. Family _Siluridæ_. Neuse

River.]

=Spines of the Catfishes.=--The catfishes or horned pouts (_Siluridæ_)

have a strong spine in the pectoral fin, one or both edges of this

being jagged or serrated. This spine fits into a peculiar joint and

by means of a slight downward or forward twist can be set immovably.

It can then be broken more easily than it can be depressed. A slight

turn in the opposite direction releases the joint, a fact known to the

fish and readily learned by the boy. The sharp spine inflicts a jagged

wound. Pelicans which have swallowed the catfish have been known to die

of the wounds inflicted by the fish's spine. When the catfish was first

introduced into the Sacramento, according to Mr. Will S. Green, it

caused the death of many of the native "Sacramento perch" (_Archoplites

interruptus_). This perch (or rather bass) fed on the young catfish,

and the latter erecting their pectoral spines in turn caused the death

of the perch by tearing the walls of its stomach. In like manner the

sharp dorsal and ventral spines of the sticklebacks have been known to

cause the death of fishes who swallow them, and even of ducks. In Puget

Sound the stickleback is often known as salmon-killer.

Certain small catfishes known as stone-cats and mad-toms (_Noturus_,

_Schilbeodes_), found in the rivers of the Southern and Middle Western

States, are provided with special organs of offense. At the base of the

pectoral spine, which is sometimes very jagged, is a structure supposed

by Professor Cope to be a poison gland the nature of which has not yet

been fully ascertained. The wounds made by these spines are exceedingly

painful like those made by the sting of a wasp. They are, however,

apparently not dangerous.

[Illustration: FIG. 134.--Black Nohu, or Poison-fish, _Emmydrichthys

vulcanus_ Jordan. A species with stinging spines, showing resemblance

to lumps of lava among which it lives. Family _Scorpænidæ_. From

Tahiti.]

=Venomous Spines.=--Many species of scorpion-fishes (_Scorpæna_,

_Synanceia_, _Pelor_, _Pterois_, etc.), found in warm seas, as well

as the European weavers (_Trachinus_), secrete poison from under the

skin of each dorsal spine. The wounds made by these spines are very

exasperating, but are not often dangerous. In some cases the glands

producing these poisons form an oblong bag excreting a milky juice, and

placed on the base of the spine.

In _Thalassophryne_, a genus of toad-fishes of tropical America, is

found the most perfect system of poison organs known among fishes.

The spinous armature of the opercle and the two spines of the first

dorsal fin constitute the weapons. The details are known from the

dissections of Dr. Günther. According to his[19] observations, the

opercle in _Thalassophryne_ "is very narrow, vertically styliform and

very mobile. It is armed behind with a spine eight lines long and of

the same form as the hollow venom-fang of a snake, being perforated at

its base and at its extremity. A sac covering the base of the spine

discharges its contents through the apertures and the canal in the

interior of the spine. The structure of the dorsal spines is similar.

There are no secretory glands imbedded in the membranes of the sacs

and the fluid must be secreted by their mucous membrane. The sacs are

without an external muscular layer and situated immediately below

the thick, loose skin which envelops the spines at their extremity.

The ejection of the poison into a living animal, therefore, can only

be effected as in _Synanceia_, by the pressure to which the sac is

subjected the moment the spine enters another body."

[Illustration: FIG. 135.--Brown Tang, _Teuthis bahianus_ (Ranzani).

Tortugas, Florida.]

=The Lancet of the Surgeon-fish.=--Some fishes defend themselves by

lashing their enemies with their tails. In the tangs, or surgeon-fishes

(_Teuthis_), the tail is provided with a formidable weapon, a

knife-like spine, with the sharp edge directed forward. This spine when

not in use slips forward into a sheath. The fish, when alive, cannot be

handled without danger of a severe cut.

In the related genera, this lancet is very much more blunt and

immovable, degenerating at last into the rough spines of _Balistapus_

or the hair-like prickles of _Monacanthus_.

=Spines of the Sting-ray.=--In all the large group of sting-rays the

tail is provided with one or more large, stiff, barbed spines, which

are used with great force by the animal, and are capable of piercing

the leathery skin of the sting-ray itself. There is no evidence that

these spines bear any specific poison, but the ragged wounds they make

are always dangerous and often end in gangrene. It is possible that the

mucus on the surface of the spine acts as a poison on the lacerated

tissues, rendering the wound something very different from a simple cut.

[Illustration: FIG. 136.--Common Filefish, _Stephanolepis hispidus_

(Linnæus). Virginia.]

=Protection Through Poisonous Flesh of Fishes.=--In certain groups of

fishes a strange form of self-protection is acquired by the presence in

the body of poisonous alkaloids, by means of which the enemies of the

species are destroyed in the death of the individual devoured.

Such alkaloids are present in the globefishes (_Tetraodontidæ_), the

filefishes (_Monacanthus_), and in some related forms, while members

of other groups (_Batrachoididæ_) are under suspicion in this regard.

The alkaloids produce a disease known as ciguatera, characterized by

paralysis and gastric derangements. Severe cases of ciguatera with men,

as well as with lower animals, may end fatally in a short time.

The flesh of the filefishes (_Stephanolepis tomentosus_), which the

writer has tested, is very meager and bitter, having a decidedly

offensive taste. It is suspected, probably justly, of being poisonous.

In the globefishes the flesh is always more or less poisonous, that

of _Tetraodon hispidus_, called muki-muki, or death-fish, in Hawaii,

is reputed as excessively so. The poisonous fishes have been lately

studied in detail by Dr. Jacques Pellegrin, of the Museum d'Histoire

Naturelle at Paris. He shows that any species of fish may be poisonous

under certain circumstances, that under certain conditions certain

species are poisonous, and that certain kinds are poisonous more

or less at all times. The following account is condensed from Dr.

Pellegrin's observations.

[Illustration: FIG. 137.--_Tetraodon meleagris_ (Lacépède). Riu Kiu

Islands.]

The flesh of fishes soon undergoes decomposition in hot climates. The

consumption of decayed fish may produce serious disorders, usually

with symptoms of diarrhoea or eruption of the skin. There is in this

case no specific poison, but the formation of leucomaines through

the influence of bacteria. This may take place with other kinds of

flesh, and is known as botulism, or allantiasis. For this disease, as

produced by the flesh of fishes, Dr. Pellegrin suggests the name of

ichthyosism It is especially severe in certain very oily fishes, as the

tunny, the anchovy, or the salmon. The flesh of these and other fishes

occasionally produces similar disorders through mere indigestion. In

this case the flesh undergoes decay in the stomach.

In certain groups (wrasse-fishes, parrot-fishes, etc.) in the tropics,

individual fishes are sometimes rendered poisonous by feeding on

poisonous mussels, holothurians, or possibly polyps, species which

at certain times, and especially in their spawning season, develops

alkaloids which themselves may cause ciguatera. In this case it is

usually the very old or large fishes which are liable to be infected.

In some markets numerous species are excluded as suspicious for this

reason. Such a list is in use in the fish-market of Havana, where the

sale of certain species, elsewhere healthful, or at the most suspected,

was rigidly prohibited under the Spanish régime. A list of these

suspicious fishes has been given by Prof. Poey.

[Illustration: FIG. 138.--The Trigger-fish, _Balistes carolinensis_

Gmelin. New York.]

In many of the eels the serum of the blood is poisonous, but its venom

is destroyed by the gastric juice, so that the flesh may be eaten with

impunity, unless decay has set in. To eat too much of the tropical

morays is to invite gastric troubles, but no true ciguatera. The true

ciguatera is produced by a specific poisonous alkaloid. This is most

developed in the globefishes or puffers (_Tetraodon_, _Spheroides_,

_Tropidichthys_, etc.). It is present in the filefishes (_Monacanthus_,

_Alutera_, etc.), probably in some toad-fishes (_Batrachoides_, etc.),

and similar compounds are found in the flesh of sharks and especially

in sharks' livers.

These alkaloids are most developed in the ovaries and testes, and in

the spawning season. They are also found in the liver and sometimes

elsewhere in the body. In many species otherwise innocuous, purgative

alkaloids are developed in or about the eggs. Serious illness has been

caused by eating the roe of the pike and the barbel. The poison is

less virulent in the species which ascend the rivers. It is also much

less developed in cooler waters. For this reason ciguatera is almost

confined to the tropics. In Havana, Manila, and other tropical ports it

is of frequent occurrence, while northward it is practically unknown

as a disease requiring a special name or treatment. On the coast of

Alaska, about Prince William Sound and Cook Inlet, a fatal disease

resembling ciguatera has been occasionally produced by the eating of

clams.

[Illustration: FIG. 139.--Numbfish, _Narcine brasiliensis_ Henle,

showing _electric cells_. Pensacola, Florida.]

The purpose of the alkaloids producing ciguatera is considered by Dr.

Pellegrin as protective, saving the species by the poisoning of its

enemies. The sickness caused by the specific poison must be separated

from that produced by ptomaines and leucomaines in decaying flesh or

in the oil diffused through it. Poisonous bacteria may be destroyed by

cooking, but the alkaloids which cause ciguatera are unaltered by heat.

It is claimed in tropical regions that the germs of the bubonic plague

may be carried through the mediation of fishes which feed on sewage.

It is suggested by Dr. Charles B. Ashmead that leprosy may be so

carried. It is further suggested that the custom of eating the flesh of

fishes raw almost universal in Japan, Hawaii, and other regions may be

responsible for the spread of certain contagious diseases, in which the

fish acts as an intermediate host, much as certain mosquitoes spread

the germ of malaria and yellow fever.

=Electric Fishes.=--Several species of fishes possess the power to

inflict electric shocks not unlike those of the Leyden jar. This is

useful in stunning their prey and especially in confounding their

enemies. In most cases these electric organs are evidently developed

from muscular substance. Their action, which is largely voluntary,

is in its nature like muscular action. The power is soon exhausted

and must be restored by rest and food. The effects of artificial

stimulation and of poisons are parallel with the effect of similar

agents on muscles.

[Illustration: FIG. 140.--Electric Catfish, _Torpedo electricus_

(Gmelin). Congo River. (Alter Boulenger.)]

In the electric rays or torpedos (_Narcobatidæ_) the electric organs

are large honeycomb-like structures, "vertical hexagonal prisms,"

upwards of 400 of them, at the base of the pectoral fins. Each prism

is filled "with a clear trembling jelly-like substance." These fishes

give a shock which is communicable through a metallic conductor, as

an iron spear or the handle of a knife. It produces a peculiar and

disagreeable sensation not at all dangerous. It is said that this

living battery shows all the known qualities of magnetism, rendering

the needle magnetic, decomposing chemical compounds, etc. In the Nile

is an electric catfish (_Torpedo electricus_) having similar powers.

Its electric organ extends over the whole body, being thickest below.

It consists of rhomboidal cells of a firm gelatinous substance.

The electric eel (_Electrophorus electricus_), the most powerful of

electric fishes, is not an eel, but allied rather to the sucker or

carp. It is, however, eel-like in form and lives in rivers of Brazil

and Guiana. The electric organs are in two pairs, one on the back of

the tail, the other on the anal fin. These are made up of an enormous

number of minute cells. In the electric eel, as in the other electric

fishes, the nerves supplying these organs are much larger than those

passing from the spinal cord for any other purpose. In all these cases

closely related species show a no trace of the electric powers.

[Illustration: FIG. 141.--Star-gazer (_Astroscopus guttatus_) settling

in the sand. (From life by R. W. Shufeldt.)]

Dr. Gilbert has described the electric powers of species of star-gazer

(_Astroscopus y-græcum_ and _A. zephyreus_), the electric cells lying

under the naked skin of the top of the head. Electric power is ascribed

to a species of cusk (_Urophycis regius_), but this perhaps needs

verification.

=Photophores or Luminous Organs.=--Many fishes, chiefly of the

deep seas, develop organs for producing light. These are known as

luminous organs, phosphorescent organs, or photophores. These are

independently developed in four entirely unrelated groups of fishes.

This difference in origin is accompanied by corresponding difference in

structure. The best-known type is found in the Iniomi, including the

lantern-fishes and their many relatives. These may have luminous spots,

differentiated areas round or oblong which shine star-like in the

dark. These are usually symmetrically placed on the sides of the body.

They may have also luminous glands or diffuse areas which are luminous,

but which do not show the specialized structure of the phosphorescent

spots. These glands of similar nature to the spots are mostly on the

head or tail. In one genus, _Æthoprora_, the luminous snout is compared

to the headlight of an engine.

[Illustration: FIG. 142.--Headlight Fish, _Æthoprora lucida_ Goode and

Bean. Gulf Stream.]

[Illustration: FIG. 143.--_Corynolophus reinhardti_ (Lütken), showing

luminous bulb (modified after Lütken). Family _Ceratiidæ_. Deep sea off

Greenland.]

Entirely different are the photophores in the midshipman or

singing-fish (_Porichthys_), a genus of toad-fishes or _Batrachoididæ_.

This species lives near the shore and the luminous spots are outgrowths

from pores of the lateral line.

In one of the anglers (_Corynolophus reinhardti_) the complex bait

is said to be luminous, and luminous areas are said to occur on the

belly of a very small shark of the deep seas of Japan (_Etmopterus

lucifer_). This phenomenon is now the subject of study by one of the

numerous pupils of Dr. Mitsukuri. The structures in _Corynolophus_ are

practically unknown.

[Illustration: FIG. 144.--_Etmopterus lucifer_ Jordan and Snyder.

Misaki, Japan.]

=Photophores in Iniomous Fishes.=--In the _Iniomi_ the luminous organs

have been the subject of an elaborate paper by Dr. R. von Lendenfeld

(Deep-sea Fishes of the Challenger. Appendix B). These he divides

into ocellar organs of regular form or luminous spots, and irregular

glandular organs or luminous areas. The ocellar spots may be on the

scales of the lateral line or on other definite areas. They may be

raised above the surface or sunk below it. They may be simple, with

or without black pigment, or they may have within them a reflecting

surface. They are best shown in the _Myctophidæ_ and _Stomiatidæ_, but

are found in numerous other families in nearly all soft-rayed fishes of

the deep sea.

The glandular areas may be placed on the lower jaw, on the barbels,

under the gill cover, on the suborbital or preorbital, on the tail, or

they may be irregularly scattered. Those about the eye have usually the

reflecting membrane.

In all these structures, according to Dr. von Lendenfeld, the whole

or part of the organ is glandular. The glandular part is at the base

and the other structures are added distally. The primitive organ was

a gland which produced luminous slime. To this in the process of

specialization greater complexity has been added.

[Illustration: FIG. 145.--_Argyropelecus olfersi_ Cuvier. Gulf Stream.]

The luminous organs of some fishes resemble the supposed original

structure of the primitive photophore, though of course these cannot

actually represent it. The simplest type of photophore now found is in

_Astronesthes_, in the form of irregular glandular luminous patches

on the surface of the skin. There is no homology between the luminous

organs of any insect and those of any fish.

=Photophores of Porichthys.=--Entirely distinct in their origin are

the luminous spots in the midshipman (_Porichthys notatus_), a shore

fish of California. These have been described in detail by Dr. Charles

Wilson Greene (late of Stanford University, now of the University of

Missouri) in the _Journal of Morphology_, xv., p. 667. These are found

on various parts of the body in connection with the mucous pores of

the lateral lines and about the mucous pores of the head. The skin in

_Porichthys_ is naked, and the photophores arise from a modification of

its epidermis. Each is spherical, shining white, and consists of four

parts--the lens, the gland, the reflector, and the pigment. As to its

function Prof. Greene observes:

"I have kept specimens of _Porichthys_ in aquaria at the Hopkins

Seaside Laboratory, and have made numerous observations on them with

an effort to secure ocular proof of the phosphorescence of the living

active fish. The fish was observed in the dark when quiet and when

violently excited, but, with a single exception, only negative results

were obtained. Once a phosphorescent glow of scarcely perceptible

intensity was observed when the fish was pressed against the side of

the aquarium. Then, this is a shore fish and quite common, and one

might suppose that so striking a phenomenon as it would present if

these organs were phosphorescent in a small degree would be observed

by ichthyologists in the field, or by fishermen, but diligent inquiry

reveals no such evidence.

"Notwithstanding the fact that _Porichthys_ has been observed to

voluntarily exhibit only the trace of phosphorescence mentioned

above, still the organs which it possesses in such numbers are beyond

doubt true phosphorescent organs, as the following observations will

demonstrate. A live fish put into an aquarium of sea-water made

alkaline with ammonia water exhibited a most brilliant glow along the

location of the well-developed organs. Not only did the lines of organs

shine forth, but the individual organs themselves were distinguishable.

The glow appeared after about five minutes, remained prominent for a

few minutes, and then for twenty minutes gradually became weaker until

it was scarcely perceptible. Rubbing the hand over the organs was

followed always by a distinct increase in the phosphorescence. Pieces

of the fish containing the organs taken five and six hours after the

death of the animal became luminous upon treatment with ammonia water.

"Electrical stimulation of the live fish was also tried with good

success. The interrupted current from an induction coil was used, one

electrode being fixed on the head over the brain or on the exposed

spinal cord near the brain, and the other moved around on different

parts of the body. No results followed relatively weak stimulation

of the fish, although such currents produced violent contractions of

the muscular system of the body. But when a current strong enough to

be quite painful to the hands while handling the electrodes was used

then stimulation of the fish called forth a brilliant glow of light

apparently from every well-developed photophore. All the lines on

the ventral and lateral surfaces of the body glowed with a beautiful

light, and continued to do so while the stimulation lasted. The single

well-developed organ just back of and below the eye was especially

prominent. No luminosity was observed in the region of the dorsal

organs previously described as rudimentary in structure. I was also

able to produce the same effect by galvanic stimulation, rapidly making

and breaking the current by hand.

[Illustration: FIG. 146.--Luminous organs and lateral line of

Midshipman, _Porichthys notatus_ Girard. Family _Batrachoididæ_.

Monterey, California. (After Greene.)]

"The light produced in _Porichthys_ was, as near as could be determined

by direct observation, a white light. When produced by electric

stimulation it did not suddenly reach its maximal intensity, but came

in quite gradually and disappeared in the same way when the stimulation

ceased. The light was not a strong one, only strong enough to enable

one to quite easily distinguish the apparatus used in the experiment.

"An important fact brought out by the above experiment is that an

electrical stimulation strong enough to most violently stimulate the

nervous system, as shown by the violent contractions of the muscular

system, may still be too weak to produce phosphorescence. This fact

gives a physiological confirmation of the morphological result stated

above that no specific nerves are distributed to the phosphorescent

organs.

"I can explain the action of the electrical current in these

experiments only on the supposition that it produces its effect by

direct action on the gland.

[Illustration: FIG. 147.--Cross-section of a ventral phosphorescent

organ of the Midshipman, _Porichthys notatus_ Girard. _l_, lens; _gl_,

gland; _r_, reflector; _bl_, blood; _p_, pigment. (After Greene.)]

"The experiments just related were all tried on specimens of the

fish taken from under the rocks where they were guarding the young

brood. Two specimens, however, taken by hooks from the deeper water

of Monterey Bay, could not be made to show phosphorescence either by

electrical stimulation or by treatment with ammonia. These specimens

did net have the high development of the system of mucous cells of the

skin exhibited by the nesting fish. My observations were, however, not

numerous enough to more than suggest the possibility of a seasonal high

development of the phosphorescent organs.

[Illustration: FIG. 148.--Section of the deeper portion of

phosphorescent organ of _Porichthys notatus_, highly magnified. (After

Greene.)]

"Two of the most important parts of the organ have to do with

the physical manipulation of light--the reflector and the lens,

respectively. The property of the reflector needs no discussion other

than to call attention to its enormous development. The lens cells are

composed of a highly refractive substance, and the part as a whole

gives every evidence of light refraction and condensation. The form

of the lens gives a theoretical condensation of light at a very short

focus. That such is in reality the case, I have proved conclusively

by examination of fresh material. If the fresh fish be exposed to

direct sunlight, there is a reflected spot of intense light from each

phosphorescent organ. This spot is constant in position with reference

to the sun in whatever position the fish be turned and is lost if the

lens be dissected away and only the reflector left. With needles and a

simple microscope it is comparatively easy to free the lens from the

surrounding tissue and to examine it directly. When thus freed and

examined in normal saline, I have found by rough estimates that it

condenses sunlight to a bright point a distance back of the lens of

from one-fourth to one-half its diameter. I regret that I have been

unable to make precise physical developments.

"The literature on the histological structure of known phosphorescent

organs of fishes is rather meager and unsatisfactory. Von Lendenfeld

describes twelve classes of phosphorescent organs from deep-sea

fishes collected by the _Challenger_ expedition. All of these,

however, are greater or less modifications of one type. This type

includes, according to von Lendenfeld's views, three essential parts,

_i.e._, a gland, phosphorescent cells, and a local ganglion. These

parts may have added a reflector, a pigment layer, or both; and all

these may be simple or compounded in various ways, giving rise to

the twelve classes. Blood-vessels and nerves are distributed to the

glandular portion. Of the twelve classes direct ocular proof is given

for one, i.e., ocellar organs of _Myctophum_ which were observed

by Willemoes-Suhm at night to shine 'like a star in the net.' Von

Lendenfeld says that the gland produces a secretion, and he supposes

the light or phosphorescence to be produced either by the 'burning or

consuming' of this secretion by the phosphorescent cells, or else by

some substance produced by the phosphorescent cells. Furthermore, he

says that the phosphorescent cells act at the 'will of the fish' and

are excited to action by the local ganglion.

"Some of these statements and conclusions seem insufficiently grounded,

as, for example, the supposed action of the phosphorescent cells,

and especially the control of the ganglion over them. In the first

place, the relation between the ganglion and the central nervous

system in the forms described by von Lendenfeld is very obscure, and

the structure described as a ganglion, to judge from the figures

and the text descriptions, may be wrongly identified. At least it

is scarcely safe to ascribe ganglionic function to a group of adult

cells so poorly preserved that only nuclei are to be distinguished.

In the second place, no structural character is shown to belong to

the 'phosphorescent cells' by which they may take part in the process

ascribed to them.[20]

"The action of the organs described by him may be explained on other

grounds, and entirely independent of the so-called 'ganglion cells' and

of the 'phosphorescent cells.'

"Phosphorescence as applied to the production of light by a living

animal is, according to our present ideas, a chemical action, _an

oxidation process_. The necessary conditions for producing it are

two--an oxidizable substance that is luminous on oxidation, i.e., a

photogenic substance on the one hand, and the presence of free oxygen

on the other. Every phosphorescent organ must have a mechanism for

producing these two conditions; all other factors are only secondary

and accessory. If the gland of a firefly can produce a substance that

is oxidizable and luminous on oxidation, as shown as far back as

1828 by Faraday and confirmed and extended recently by Watasé, it is

conceivable, indeed probable, that phosphorescence in _Myctophum_ and

other deep-sea forms is produced in the same direct way, that is, by

direct oxidation of the secretion of the gland found in each of at

least ten of the twelve groups of organs described by von Lendenfeld.

Free oxygen may be supplied directly from the blood in the capillaries

distributed to the gland which he describes. The possibility of the

regulation of the supply of blood carrying oxygen is analogous to what

takes place in the firefly and is wholly adequate to account for any

'flashes of light' 'at the will of the fish.'

"In the phosphorescent organs of _Porichthys_ the only part the

function of which cannot be explained on physical grounds is the

group of cells called the gland. If the large granular cells of this

portion of the structure produce a secretion, as seems probable from

the character of the cells and their behavior toward reagents, and

this substance be oxidizable and luminous in the presence of free

oxygen, i.e., photogenic, then we have the conditions necessary for

a light-producing organ. The numerous capillaries distributed to the

gland will supply free oxygen sufficient to meet the needs of the

case. Light produced in the gland is ultimately all projected to the

exterior, either directly from the luminous points in the gland or

reflected outward by the reflector, the lens condensing all the rays

into a definite pencil or slightly diverging cone. This explanation

of the light-producing process rests on the assumption of a secretion

product with certain specific characters. But comparing the organ

with structures known to produce such a substance, i.e., the glands

of the firefly or the photospheres of Euphausia, it seems to me

the assumption is not less certain than the assumption that twelve

structures resembling each other in certain particulars have a common

function to that proved for one only of the twelve.

"I am inclined to the belief that whatever regulation of the action of

the phosphorescent organ occurs is controlled by the regulation of the

supply of free oxygen by the blood-stream flowing through the organ;

but, however this may be, the essential fact remains that the organs in

_Porichthys_ are true phosphorescent organs." (GREENE.)

Other species of _Porichthys_ with similar photophores occur in Texas,

Guiana, Panama, and Chile. The name midshipman alludes to these shining

spots, compared to buttons.

[Illustration: FIG. 149.--Sucking-fish, or Pegador, _Leptecheneis

naucrates_ (Linnæus). Virginia.]

=Globefishes.=--The globefishes (_Tetraodon_, etc.) and the

porcupine-fishes have the surface defended by spines. These fishes have

an additional safeguard through the instinct to swallow air. When one

of these fishes is seriously disturbed it rises to the surface, gulps

air into a capacious sac, and then floats belly upward on the surface.

It is thus protected from other fishes, although easily taken by man.

The same habit appears in some of the frog-fishes (_Antennarius_) and

in the Swell sharks (_Cephaloscyllium_).

The writer once hauled out a netful of globefishes (_Tetraodon

hispidus_) from a Hawaiian lagoon. As they lay on the bank a dog

came up and sniffed at them. As his nose touched them they swelled

themselves up with air, becoming visibly two or three times as large as

before. It is not often that the lower animals show surprise at natural

phenomena, but the attitude of the dog left no question as to his

feeling.

=Remoras.=--The different species of Remora, or shark-suckers, fasten

themselves to the surface of sharks or other fishes and are carried

about by them often to great distances. These fishes attach themselves

by a large sucking-disk on the top of the head, which is a modified

spinous dorsal fin. They do not harm the shark, except possibly to

retard its motion. If the shark is caught and drawn out of the water,

these fishes often instantly let go and plunge into the sea, swimming

away with great celerity.

=Sucking-disks of Clingfishes.=--Other fishes have sucking-disks

differently made, by which they cling to rocks. In the gobies the

united ventrals have some adhesive power. The blind goby (_Typhlogobius

californiensis_) is said to adhere to rocks in dark holes by the

ventral fins. In most gobies the adhesive power is slight. In the

sea-snails (_Liparididæ_) and lumpfishes (_Cyclopteridæ_) the united

ventral fins are modified into an elaborate circular sucking-disk.

In the clingfishes (_Gobiesocidæ_) the sucking-disk lies between

the ventral fins and is made in part of modified folds of the naked

skin. Some fishes creep over the bottom, exploring it with their

sensitive barbels, as the gurnard, surmullet, and goatfish. The suckers

(_Catostomus_) test the bottom with their thick, sensitive lips, either

puckered or papillose, feeding by suction.

[Illustration: FIG. 150.--Clingfish, _Caularchus mæandricus_ (Girard).

Monterey, California.]

=Lampreys and Hagfishes.=--The lampreys suck the blood of other fishes

to which they fasten themselves by their disk-like mouth armed with

rasping teeth.

The hagfishes (_Myxine_, _Eptatretus_) alone among fishes are truly

parasitic. These fishes, worm-like in form, have round mouths, armed

with strong hooked teeth. They fasten themselves at the throats of

large fishes, work their way into the muscle without tearing the skin,

and finally once inside devour all the muscles of the fish, leaving the

skin unbroken and the viscera undisturbed. These fishes become living

hulks before they die. If lifted out of the water, the slimy hagfish

at once slips out and swims quickly away. In gill-nets in Monterey

Bay great mischief is done by hagfish (_Polistotrema stouti_). It is

a curious fact that large numbers of hagfish eggs are taken from the

stomachs of the male hagfish, which seems to be almost the only enemy

of his own species, keeping the numbers in check.

[Illustration: FIG. 151.--Hagfish, _Polistotrema stouti_ (Lockington).]

=The Swordfishes.=--In the swordfish and its relatives, the sailfish

and the spearfish, the bones of the anterior part of the head are

grown together, making an efficient organ of attack. The sword of the

swordfish, the most powerful of these fishes, has been known to pierce

the long planks of boats, and it is supposed that the animal sometimes

attacks the whale. But stories of this sort lack verification.

=The Paddle-fishes.=--In the paddle-fishes (_Polyodon spatula_ and

_Psephurus gladius_) the snout is spread out forming a broad paddle

or spatula. This the animal uses to stir up the mud on the bottoms

of rivers, the small organisms contained in mud constituting food.

Similar paddle-like projections are developed in certain deep-water

Chimæras (_Harriottia_, _Rhinochimæra_), and in the deep-sea shark,

_Mitsukurina_.

[Illustration: FIG. 152.--Indian Sawfish, _Pristis zysron_ Latham.

River mouths of Hindustan. (After Day.)]

=The Sawfishes.=--A certain genus of rays (_Pristis_, the sawfish) and

a genus of sharks (_Pristiophorus_, the saw-shark), possess a similar

spatula-shaped snout. But in these fishes the snout is provided on

either side with enamelled teeth set in sockets and standing at right

angles with the snout. The animal swims through schools of sardines

and anchovies, strikes right and left with this saw, destroying the

small fishes, who thus become an easy prey. These fishes live in

estuaries and river mouths, _Pristis_ in tropical America and Guinea,

_Pristiophorus_ in Japan and Australia. In the mythology of science,

the sawfish attacks the whale, but in fact the two animals never come

within miles of each other, and the sawfish is an object of danger only

to the tender fishes, the small fry of the sea.

[Illustration: FIG. 153.--Saw-shark, _Pristiophorus japonicus_ Günther.

Specimen from Nagasaki.]

=Peculiarities of Jaws and Teeth.=--The jaws of fishes are subject

to a great variety of modifications. In some the bones are joined by

distensible ligaments and the fish can swallow other fishes larger than

itself. In other cases the jaws are excessively small and toothless,

at the end of a long tube, so ineffective in appearance that it is a

marvel that the fish can swallow anything at all.

In the thread-eels (_Nemichthys_) the jaws are so recurved that they

cannot possibly meet, and in their great length seem worse than useless.

In some species the knife-like canines of the lower jaw pierce through

the substance of the upper.

In four different and wholly unrelated groups of fishes the teeth are

grown fast together, forming a horny beak like that of the parrot.

These are the Chimæras, the globefishes (_Tetraodon_), and their

relatives, the parrot-fishes (_Scarus_, etc.), and the stone-wall perch

(_Oplegnathus_). The structure of the beak varies considerably in

these four cases, in accord with the difference in the origin of its

structures. In the globefishes the jaw-bones are fused together, and

in the Chimæras they are solidly joined to the cranium itself.

=The Angler-fishes.=--In the large group of angler-fishes the first

spine of the dorsal fin is modified into a sort of bait to attract

smaller fishes into the capacious mouth below. This structure is

typical in the fishing-frog (_Lophius_), where the fleshy tip of this

spine hangs over the great mouth, the huge fish lying on the bottom

apparently inanimate as a stone. In other related fishes this spine

has different forms, being often reduced to a vestige, of little value

as a lure, but retained in accordance with the law of heredity. In a

deep-sea angler the bait is enlarged, provided with fleshy streamers

and a luminous body which serves to attract small fishes in the depths.

The forms and uses of this spine in this group constitute a very

suggestive chapter in the study of specialization and ultimate

degradation, when the special function is not needed or becomes

ineffective.

Similar phases of excessive development and final degradation may be

found in almost every group in which abnormal stress has been laid on a

particular organ. Thus the ventral fins, made into a large sucking-disk

in _Liparis_, are lost altogether in _Paraliparis_. The very large

poisoned spines of _Pterois_ become very short in _Aploactis_, the high

dorsal spines of _Citula_ are lost in _Alectis_, and sometimes a very

large organ dwindles to a very small one within the limits of the same

genus. An example of this is seen in the poisoned pectoral spines of

_Schilbeodes_.

=Relation of Number of Vertebræ to Temperature and the Struggle for

Existence.=--One of the most remarkable modifications of the skeleton

of fishes is the progressive increase of the number of vertebræ as

the forms become less specialized, and that this particular form of

specialization is greatest at the equator.[21]

It has been known for some years that in several groups of fishes

(wrasse-fishes, flounders, and "rock-cod," for example) those species

which inhabit northern waters have more vertebræ than those living

in the tropics. Certain arctic flounders, for example, have sixty

vertebræ; tropical flounders have, on the average, thirty. The

significance of this fact is the problem at issue. In science it is

assumed that all facts have significance, else they would not exist. It

becomes necessary, then, to find out first just what the facts are in

this regard.

[Illustration: FIG. 154.--Skeleton of Pike, _Esox lucius_ Linnæus, a

river fish with many vertebræ.]

Going through the various groups of non-migratory marine fishes we

find that such relations are common. In almost every group the number

of vertebræ grows smaller as we approach the equator, and grows larger

again as we pass into southern latitudes. Taking an average netful of

fishes of different kinds at different places along the coast, the

variation would be evident. At Point Barrow or Cape Farewell or North

Cape a seineful of fishes would perhaps average eighty vertebræ each,

the body lengthened to make room for them; at Sitka or St. Johns or

Bergen, perhaps sixty vertebræ; at San Francisco or New York or St.

Malo, thirty-five; at Mazatlan or Pensacola or Naples, twenty-eight;

and at Panama or Havana or Sierra Leone, twenty-five. Under the equator

the usual number of vertebræ in shore fishes is twenty-four. Outside

tropical and semi-tropical waters this number is the exception. North

of Cape Cod it is virtually unknown.

=Number of Vertebræ.=--The numbers of vertebræ in different groups may

be summarized as follows:

_Lancelets._--Among the lancelets the numbers of segments range from 50

to 80, there being no vertebræ.

_Lampreys._--In this group the number of segments ranges from 100 to

150.

_Elasmobranchs._--Among sharks and skates the usual number of segments

is from 100 to 150 and upwards. In the extinct species as far as known

the numbers are not materially different. The Carboniferous genus,

_Pleuracanthus_, has about 115 vertebræ. The _Chimæras_ have similar

numbers; _Chimæra monstrosa_ has about 100 in the body and more than as

many more in the filamentous tail.

_Cycliæ._--_Palæospondylus_ has about 85 vertebræ.

_Arthrodires._--There are about 100 vertebræ in _Coccosteus_.

_Dipnoans._--In Protopterus there are upwards of 100 vertebræ, the last

much reduced in size. Figures of _Neoceratodus_ show about 80.

_Crossopterygians._--_Polypterus_ has 67 vertebræ; _Erpetichthys_, 110;

_Undina_, about 85.

_Ganoids._--In this group the numbers are also large--95 in _Amia_,

about 55 in the short-bodied _Microdon_. The Sturgeons all have more

than 100 vertebræ.

=Soft-rayed Fishes.=--Among the _Teleostei_, or bony fishes, those

which first appear in geological history are the _Isospondyli_, the

allies of the salmon and herring. These have all numerous vertebræ,

small in size, and none of them in any notable degree modified or

specialized. They abound in the depths of the ocean, but there are

comparatively few of them in the tropics. The _Salmonidæ_ which inhabit

the rivers and lakes of the northern zones have from 60 to 65 vertebræ.

The _Myctophidæ, Stomiatidæ_, and other deep-sea forms have from 40

upwards in the few species in which the number has been counted. The

group of _Clupeidæ_ is nearer the primitive stock of _Isospondyli_ than

the salmon are. This group is essentially northern in its distribution,

but a considerable number of its members are found within the tropics.

The common herring (_Clupea harangus_) ranges farther into the arctic

regions than any other. Its vertebræ are 56 in number. In the shad

(_Alosa sapidissima_), a northern species which ascends the rivers,

the same number is recorded. The sprat (_Clupea sprattus_) and sardine

(_Sardinia pilchardus_), ranging farther south, have from 48 to 50,

while in certain small herrings (_Sardinella_) which are strictly

confined to tropical shores the number is but 40. Allied to the herring

are the anchovies, mostly tropical. The northernmost species, the

common anchovy of Europe (_Engraulis enchrasicolus_), has 46 vertebræ.

A tropical species (_Anchovia browni_) has 41.

There are, however, a few soft-rayed fishes confined to the tropical

seas in which the numbers of vertebræ are still large, an exception

to the general rule. Among these are _Albula vulpes_, the bonefish,

with 70 vertebræ, _Elops saurus_, the ten-pounder, with 72, the tarpon

(_Tarpon atlanticus_), with about 50, and the milkfish, _Chanos

chanos_, with 72.

In a fossil Eocene herring from the Green River shales (_Diplomystus_)

I count 40 vertebræ; in a bass-like fish (_Mioplosus_) from the same

locality 24--these being the usual numbers in the present tropical

members of these groups.

The great family of _Siluridæ_, or catfishes, is represented in all

the fresh waters of temperate and tropical America, as well as in the

warmer parts of the Old World. One division of the family, containing

numerous species, abounds on the sandy shores of the tropical seas.

The others are all fresh-water fishes. So far as the vertebræ in

the _Siluridæ_ have been examined, no conclusions can be drawn. The

vertebræ in the marine species range from 35 to 50; in the North

American forms, from 37 to 45; and in the South American fresh-water

species, where there is almost every imaginable variation in form and

structure, the numbers range from 28 to 50 or more. The _Cyprinidæ_

(carp and minnows), confined to the fresh waters of the northern

hemisphere, and their analogues, the _Characinidæ_ of the rivers of

South America and Africa, have also numerous vertebræ, 36 to 50 in most

cases.

In general we may say of the soft-rayed fishes that very few of them

are inhabitants of tropical shores. Of these few, some which are

closely related to northern forms have fewer vertebræ than their

cold-water analogues. In the northern species, the fresh-water species,

and the species found in the deep sea the number of vertebræ is always

large, but the same is true of some of the tropical species also.

=The Flounders.=--In the flounders, the halibut and its relatives,

arctic genera (_Hippoglossus_ and _Atheresthes_), have from 49 to

50 vertebræ. The northern genera (_Hippoglossoides, Lyopsetta_, and

_Eopsetta_) have from 43 to 45; the members of a large semi-tropical

genus (_Paralichthys_) of wide range have from 35 to 41; while the

tropical forms have from 35 to 37.

In the group of turbots and whiffs none of the species really belong

to the northern fauna, and the range in numbers is from 35 to 43. The

highest number, 43, is found in a deep-water species (_Monolene_),

and the next, 40, in species (_Lepidorhombus, Orthopsetta_) which

extend their range well toward the north. Among the plaices, which are

all northern, the numbers range from 35 to 65, the higher numbers,

52, 58, 65, being found in species (_Glyptocephalus_) which inhabit

considerable depths in the arctic seas. The lowest numbers (35) belong

to shore species (_Pleuronichthys_) which range well toward the south.

=Spiny-rayed Fishes.=--Among the spiny-rayed fishes the facts are more

striking. Of these, numerous families are chiefly or wholly confined to

the tropics, and in the great majority of all the species the number

of vertebræ is constantly 24,--10 in the body and 14 in the tail

(10+14). This is true of all or nearly all the _Berycidæ_, _Serranidæ_,

_Sparidæ_, _Sciænidæ_, _Chætodontidæ_, _Hæmulidæ_, _Gerridæ_,

_Gobiidæ_, _Acanthuridæ_, _Mugilidæ_, _Sphyrænidæ_, _Mullidæ_,

_Pomacentridæ_, etc.

In some families in which the process of reduction has gone on to an

extreme degree, as in certain _Plectognath_ fishes, there has been a

still further reduction, the lowest number, 14, existing in the short

inflexible body of the trunkfish (_Ostracion_), in which the vertebral

joints are movable only in the base of the tail. In all these forms the

process of reduction of vertebræ has been accompanied by specialization

in other respects. The range of distribution of these fishes is chiefly

though not quite wholly confined to the tropics.

Thus _Balistes_, the trigger-fish, has 17 vertebræ; _Monacanthus_ and

_Alutera_, foolfishes, about 20; the trunkfish, _Ostracion_, 14; the

puffers, _Tetraodon_ and _Spheroides_, 18; _Canthigaster_, 17; and the

headfish, _Mola_, 17. Among the _Pediculates, Malthe_ and _Antennarius_

have 17 to 19 vertebræ, while in their near relatives, the anglers,

_Lophiidæ_, the number varies with the latitude. Thus, in the northern

angler, _Lophius piscatorius_, which is never found south of Cape

Hatteras, there are 30 vertebræ. In a similar species, inhabiting the

north of Japan (_Lophius litulon_), there are 27. In another Japanese

species, ranging farther south, _Lophiomus setigerus_, the vertebræ

are but 19. Yet in external appearance these two fishes are almost

identical. It is, however, a notable fact that some of the deep-water

_Pediculates_, or angling fishes, have the body very short and the

number of vertebræ correspondingly reduced. _Dibranchus atlanticus_,

from a depth of 3600 fathoms, or more than 4 miles, has but 18

vertebræ, and others of its relatives in deep waters show also small

numbers. These soft-bodied fishes are simply animated mouths, with a

feeble osseous structure, and they are perhaps recent offshoots from

some stock which has extended its range from muddy bottom or from

floating seaweed to the depths of the sea.

A very few spiny-rayed families are wholly confined to the northern

seas. One of the most notable of these is the family of viviparous

surf-fishes (_Embiotocidæ_), of which numerous species abound on the

coasts of California and Japan, but which enter neither the waters of

the frigid nor of the torrid zone. The surf-fishes have from 32 to

42 vertebræ, numbers which are never found among tropical fishes of

similar appearance or relationship.

The facts of variation with latitude were first noticed among the

_Labridæ_. In the northern genera (_Labrus_, _Tautoga_, etc.) there

are 38 to 41 vertebræ; in the semi-tropical genera (_Crenilabrus_,

_Bodianus_, etc.), 30 to 33; in the tropical genera (_Halichoeres_,

_Xyrichthys_, _Thalassoma_, etc.), usually 24.

Equally striking are the facts in the great group of _Pareioplitæ_, or

mailed-cheek fishes, composed of numerous families, diverging from each

other in various respects, but agreeing in certain peculiarities of the

skeleton.

Among these fishes the family most nearly related to ordinary fishes is

that of the _Scorpænidæ_ (scorpion-fishes, etc.).

This is a large family containing many species, fishes of local habits,

swarming about the rocks at moderate depths in all zones. The species

of the tropical genera have all 24 vertebræ. Those genera chiefly found

in cooler waters, as in California, Japan, Chile, and the Cape of Good

Hope, have in all their species 27 vertebræ, while in the arctic genera

there are 31.

Allied to the _Scorpænidæ_, but confined to the tropical or

semi-tropical seas, are the _Platycephalidæ_, with 27 vertebræ, and

the _Cephalacanthidæ_ (flying gurnards), with but 22. In the deeper

waters of the tropics are the _Peristediidæ_, with 33 vertebræ, and

extending farther north, belonging as much to the temperate as to the

torrid zone, is the large family of the _Triglidæ_ (gurnards) in which

the vertebræ range from 25 to 38.

The family of _Agonidæ_ (sea-poachers), with 36 to 40 vertebræ, is

still more decidedly northern in its distribution. Wholly confined

to northern waters is the great family of the _Cottidæ_ (sculpins),

in which the vertebræ ascend from 30 to 50. Entirely polar and often

in deep waters are the _Liparididæ_ (sea-snails), an offshoot from

the _Cottidæ_, with soft, limp bodies, and the vertebræ 35 to 65. In

these northern forms there are no scales, the spines in the fins have

practically disappeared, and only the anatomy shows that they belong to

the group of spiny-rayed fishes. In the _Cyclopteridæ_ (lumpfishes),

likewise largely arctic, the body becomes short and thick, the

back-bone inflexible, and the vertebræ are again reduced to 28. In

most cases, as the number of vertebræ increases, the body becomes

proportionally elongate. As a result of this, the fishes of arctic

waters are, for the most part, long and slender, and not a few of them

approach the form of eels. In the tropics, however, while elongate

fishes are common enough, most of them (always excepting the eels)

have the normal number of vertebræ, the greater length being due to

the elongation of their individual vertebræ and not to their increase

in number. Thus the very slender goby, _Gobionellus oceanicus_, has

the same number (25) of vertebræ as its thick-set relative _Gobius

soporator_ or the chubby _Lophogobius cyprinoides_. In the great

group of blenny-like fishes the facts are equally striking. The

arctic species are very slender in form as compared with the tropical

blennies, and this fact, caused by a great increase in the number of

their vertebræ, has led to the separation of the group into several

families. The tropical forms composing the family of _Blenniidæ_ have

from 28 to 49 vertebræ, while in the arctic genera the numbers range

from 75 to 100.

Of the true _Blennidæ_, which are all tropical or semi-tropical,

_Blennius_ has 28 to 35 vertebræ; _Salarias_, 35 to 38; Lepisoma, 34;

_Clinus_, 49; _Cristiceps_, 40. A fresh-water species of _Cristiceps_

found in Australia has 46. Blennioid fishes in the arctic seas are

_Anarrhichas_, with 76 vertebræ; _Anarrhichthys_, with 100 or more;

_Lumpenus_, 79; _Pholis_, 85; _Lycodes_, 112; _Gymnelis_, 93. _Lycodes_

and _Gymnelis_ have lost all the dorsal spines.

In the cod family (_Gadidæ_) the number of vertebræ is usually about

50. The number is 51 in the codfish (_Gadus callarias_), 58 in the

Siberian cod (_Eleginus navaga_), 54 in the haddock (_Melanogrammus

æglifinus_), 54 in the whiting (_Merlangus merlangus_), 54 in the

coalfish (_Pollachius virens_), 52 in the Alaskan coalfish (_Theragra

chalcogramma_), 51 in the hake (_Merluccius merluccius_). In the burbot

(_Lota lota_), the only fresh-water codfish, 59; in the deep-water ling

(_Molva molva_), 64; in the rocklings (_Gaidropsarus_), 47 to 49. Those

few species found in the Mediterranean and the Gulf of Mexico have

fewer fin-rays and probably fewer vertebræ than the others, but none

of the family enter warm water, the southern species living at greater

depths.

In the deep-sea allies of the codfishes, the grenadiers or rat-tails

(_Macrouridæ_), the numbers range from 65 to 80.

=Fresh-water Fishes.=--Of the families confined strictly to the fresh

waters the great majority are among the soft-rayed or physostomous

fishes, the allies of the salmon, pike, carp, and catfish. In all of

these the vertebræ are numerous. A few fresh-water families have their

affinities entirely with the more specialized forms of the tropical

seas. Of these the _Centrarchidæ_ (comprising the American fresh-water

sunfish and black bass) have on the average about 30 vertebræ, the

pirate perch 29, and the _Percidæ_, perch and darters, etc., 35 to

45, while the _Serranidæ_ or sea-bass, the nearest marine relatives

of all these, have constantly 24. The marine family of damsel-fishes

(_Pomacentridæ_) have 26 vertebræ, while 30 to 40 vertebræ usually

exist in their fresh-water analogues (or possibly descendants), the

_Cichlidæ_, of the rivers of South America and Africa. The sticklebacks

(_Gasterosteidæ_), a family of spiny fishes, confined to the rivers and

seas of the north, have from 31 to 41 vertebræ.

=Pelagic Fishes.=--Among the free-swimming or migratory pelagic fishes,

the number of vertebræ is usually greater than among their relatives of

local habits. This fact is most evident among the scombriform fishes,

the allies of the mackerel and tunny. All of these belong properly to

the warm seas, and the reduction of the vertebræ in certain forms has

no evident relation to the temperature, though it seems to be related

in some degree to the habits of the species. Perhaps the retention of

many segments is connected with that strength and swiftness in the

water for which the mackerels are preeminent.

The variations in the number of vertebræ in this group led Dr. Günther

to divide it into two families, the _Carangidæ_ and _Scombridæ_.

The _Carangidæ_ or _Pampanos_ are tropical shore fishes, local or

migratory to a slight degree. All these have from 24 to 26 vertebræ.

In their pelagic relatives, the dolphins (_Coryphæna_), there are from

30 to 33; in the opah (_Lampris_), 45; in Brama, 42; while the great

mackerel family (_Scombridæ_), all of whose members are more or less

pelagic, have from 31 to 50.

The mackerel (_Scomber scombrus_) has 31 vertebræ; the chub mackerel

(_Scomber japonicus_), 31; the tunny (_Thunnus thynnus_), 39; the

long-finned albacore (_Germo alalonga_), 40; the bonito (_Sarda

sarda_), 50; the Spanish mackerel (_Scomberomorus maculatus_), 45.

Other mackerel-like fishes are the cutlass-fishes (_Trichiuridæ_),

which approach the eels in form and in the reduction of the fins. In

these the vertebræ are correspondingly numerous, the numbers ranging

from 100 to 160. _Aphanopus_ has 101 vertebræ; _Lepidopus_, 112;

_Trichurus_, 159.

In apparent contradiction to this rule, however, the pelagic family of

swordfishes (_Xiphias_), remotely allied to the mackerels, and with

even greater powers of swimming, has the vertebræ in normal number, the

common swordfish having but 24.

=The Eels.=--The eels constitute a peculiar group of soft-rayed

ancestry, in which everything else has been subordinated to muscularity

and flexibility of body. The fins, girdles, gill-arches, scales, and

membrane bones are all imperfectly developed or wanting. The eel is

perhaps as far from the primitive stock as the most highly "ichthyized"

fishes, but its progress has been of another character. The eel would

be regarded in the ordinary sense as a degenerate type, for its bony

structure is greatly simplified as compared with its ancestral forms,

but in its eel-like qualities it is, however, greatly specialized. All

the eels have vertebræ in great numbers. As the great majority of the

species are tropical, and as the vertebræ in very few of the deep-sea

forms have been counted, no conclusions can be drawn as to the relation

of their vertebræ to the temperature.

It is evident that the two families most decidedly tropical in

their distribution, the morays (_Murænidæ_) and the snake-eels

(_Ophichthyidæ_), have diverged farthest from the primitive stock. They

are most "degenerate," as shown by the reduction of their skeleton.

At the same time they are also most decidedly "eel-like," and in

some respects, as in coloration, dentition, muscular development,

most highly specialized. It is evident that the presence of numerous

vertebral joints is essential to the suppleness of body which is the

eel's chief source of power.

So far as known the numbers of vertebræ in eels range from 115 to 160,

some of the deep-sea eels (_Nemichthys_, _Nettastoma_, _Gordiichthys_)

having much higher numbers, in accord with their slender or whip-like

forms.

Among the morays, _Muræna helena_ has 140; _Gymnothorax meleagris_,

120; _G. undulatus_, 130; _G. moringa_, 145; _G. concolor_, 136;

_Echidna catenata_, 116; _E. nebulosa_, 142; _E. zebra_, 135. In other

families the true eel, _Anguilla anguilla_, has 115; the conger-eel,

_Leptocephalus conger_, 156; and _Murænesox cinereus_, 154.

=Variations in Fin-rays.=--In some families the number of rays in

the dorsal and anal fins is dependent on the number of vertebræ. It

is therefore subject to the same fluctuations. This relation is not

strictly proportionate, for often a variable number of rays with their

interspinal processes will be interposed between a pair of vertebræ.

The myotomes or muscular bands on the sides are usually coincident

with the number of vertebræ. As, however, these and other characters

are dependent on differences in vertebral segmentation, they bear the

same relations to temperature or latitude that the vertebræ themselves

sustain.

Thus in the _Scorpænidæ_, _Sebastes_, and _Sebastolobus_ arctic genera

have the dorsal rays xv, 13, the vertebræ 12+19. The tropical genus

_Scorpæna_ has the dorsal rays xii, 10, the vertebræ 10+14, while the

genus _Sebastodes_ of temperate waters has the intermediate numbers of

dorsal rays xii, 12, and vertebræ 12+15.

=Relation of Numbers to Conditions of Life.=--Fresh-water fishes have

in general more vertebræ than marine fishes of shallow waters. Pelagic

fishes and deep-sea fishes have more than those which live along the

shores, and more than localized or non-migratory forms. To each of

these generalizations there are occasional partial exceptions, but not

such as to invalidate the rule.

The presence of large numbers of vertebræ is noteworthy among those

fishes which swim for long distances, as, for example, many of the

mackerel family. Among such there is often found a high grade of

muscular power, or even of activity, associated with a large number

of vertebræ, these vertebræ being individually small and little

differentiated. For long-continued muscular action of a uniform kind

there would be perhaps an advantage in the low development of the

vertebral column. For muscular alertness, moving short distances with

great speed, the action of a fish constantly on its guard against

enemies or watching for its prey, the advantage would be on the side of

a few vertebræ. There is often a correlation between the free-swimming

habit and slenderness and suppleness of the body, which again is often

dependent on an increase in numbers of the vertebral segments. These

correlations appear as a disturbing element in the problem rather than

as furnishing a clew to its solution. In some groups of fresh-water

fishes there is a reduction in number of vertebræ, not associated with

any degree of specialization of the individual bone, but correlated

with simple reduction in size of body. This is apparently a phenomenon

of degeneration, a survival of dwarfs, where conditions are unfavorable

in full growth.

All these effects should be referable to the same group of causes.

They may, in fact, be combined in one statement. All other fishes now

extant, as well as all fishes existing prior to Cretaceous times, have

a larger number of vertebræ than the marine shore fishes of the tropics

of the present period. There is good reason to believe that in most

groups of spiny-rayed fishes, those with the smaller number of segments

are at once the most highly organized and the most primitive. This is

true among the blennies, the sculpins, the flounders, the perches, and

probably the labroid fishes as well. The present writer once held the

contrary view, that the forms with the higher numbers were primitive,

but the evidence both from comparative anatomy and from palæontology

seems to indicate that among spiny-rayed fishes the forms most ancient,

most generalized, and most synthetic are those with about 24 vertebræ.

The soft-rayed fishes without exception show larger numbers, and these

are still more primitive. This apparent contradiction is perhaps

explained by Dr. Boulenger's suggestion that the prevalence of the

same number, 24, in the vertebræ of various families of spiny-rayed

fishes is due to common descent, probably from Cretaceous berycoids

having this number. In this theory, perches, sparoids, carangoids,

chætodonts, labroids, parrot-fishes, gobies, flounders, and sculpins

must be regarded as having a common origin from which all have diverged

since Jurassic times. This view is not at all unlikely and is not

inconsistent with the facts of palæontology. If this be the case, the

members of these and related families which have larger numbers of

vertebræ must have diverged from the primitive stock. The change has

been one of degeneration, the individual vertebræ being reduced in size

and complexity, with a vegetative increase in their number. At the

same time, the body having the greater number of segments is the more

flexible though the segments themselves are less specialized.

The primitive forms live chiefly along tropical shores, while forms

with increased numbers of vertebræ are found in all other localities.

This fact must be considered in any hypothesis as to the causes

producing such changes. If the development of large numbers be a phase

of degeneration the causes of such degeneration must be sought in the

colder seas, in the rivers, and in the oceanic abysses. What have these

waters in common that the coral reefs, the lava crags, and tide-pools

of the tropics have not?

It is certain that the possession of fewer vertebræ indicates the

higher rank, the greater specialization of parts, even though the many

vertebræ be a feature less primitive. The evolution of fishes is rarely

a movement of progress toward complexity. The time movement in some

groups is accompanied by degradation and loss of parts, by vegetative

repetition of structures, and often by a movement from the fish-form

toward the eel-form. Water life is less exacting than land life,

having less variation of conditions. It is, therefore, less effective

in pushing forward the differentiation of parts. When vertebræ are

few in number each one is relatively larger, its structure is more

complicated, its appendages larger and more useful, and the fins

with which it is connected are better developed. In other words, the

tropical fish is more intensely and compactly a fish, with a better

fish equipment, and in all ways better fitted for the business of a

fish, especially for that of a fish that stays at home.

[Illustration: FIG. 155.--Skeleton of Red Rockfish, _Sebastodes

miniatus_ Jordan and Gilbert. California.]

[Illustration: FIG. 156.--Skeleton of a spiny-rayed fish of the

tropics, _Holacanthus ciliaris_ (Linnæus).]

In the center of competition no species can afford to be handicapped by

a weak back-bone and redundant vertebræ. Those who are thus weighted

cannot hold their own. They must change or perish.

The conditions most favorable to fish life are among the rocks and

reefs of the tropical seas. About the coral reefs is the center of

fish competition. A coral archipelago is the Paris of fishes. In such

regions is found the greatest variety of surroundings, and therefore

the greatest number of possible adjustments. The struggle is between

fish and fish, not between fishes and hard conditions of life. No form

is excluded from the competition. Cold, darkness, and foul water do not

shut out competitors, nor does any evil influence sap the strength. The

heat of the tropics does not make the sea-water hot. It is never sultry

or laden with malaria.

[Illustration: FIG. 157.--Skeleton of the Cowfish, _Lactophrys

tricornis_ (Linnæus).]

From conditions otherwise favorable in arctic regions the majority

of competitors are excluded by their inability to bear the cold.

River life is life in isolation. To aquatic animals river life has

the same limitations that island life has to the animals of the land.

The oceanic islands are far behind the continents in the process of

evolution in so far as evolution implies specialization of parts. In a

like manner the rivers are ages behind the seas, so far as progress is

concerned, though through lack of competition the animals in isolation

may be farthest from the original stock.

Therefore the influences which serve as a whole to intensify fish life,

to keep it up to its highest effectiveness, and which tend to rid the

fish of every character or structure it cannot "use in its business,"

are most effective along the shores of the tropics. One phase of this

is the retention of low numbers of vertebræ, or, more accurately, the

increase of stress on each individual bone.

Conversely, as the causes of these changes are still in operation,

we should find that in cold waters, deep waters, dark waters, fresh

waters, and inclosed waters the strain would be less, the relapses to

less complex organization more frequent, the numbers of vertebræ would

be larger, while the individual vertebræ would become smaller, less

complete, and less perfectly ossified.

This in a general way is precisely what we do find in examining the

skeletons of a large variety of fishes.

The cause of the increased numbers of vertebræ in cold waters or

extratropical waters is as yet unknown. Several guesses have been made,

but these can scarcely rise to the level of theories. To ascribe it to

natural selection, as the present writer has done, is to do little more

than to restate the problem.

As a possible tentative hypothesis we may say that the retention of the

higher primitive traits in the tropics is due to continuous selection,

the testing of individuals by the greater variety of external

conditions. The degeneration of extratropical fishes may be due to

isolation and cessation or reversal of selection. Thus fresh waters,

the arctic waters, the oceanic abysses are the "back woods" of fish

life, localities favorable to the retention of primitive simplicity,

equally favorable to subsequent degeneration. Practically all deep-sea

fishes are degenerate descendants of shore fishes of various groups.

Monotony and isolation permit or encourage degeneration of type. Where

the struggle for existence is most intense the higher structures will

be retained or developed. Among such facts as these derived from

natural selection the cause of the relation of temperature to number of

vertebræ must be sought. How the Cretaceous berycoids first acquired

their few vertebræ and the high degree of individual specialization of

these structures we may not know. The character came with the thoracic

ventrals with reduced number of rays, the ctenoid scales, the toothless

maxillary, and other characters which have long persisted in their

subsequent descendants.

An exception to the general rule in regard to the number of vertebræ

is found in the case of the eel. Eels inhabit nearly all seas, and

everywhere they have many vertebræ. The eels of the tropics are at once

more specialized and more degraded. They are better eels than those of

northern regions, but, as the eel is a degraded type, they have gone

farther in the loss of structures in which this degradation consists.

It is not well to push this analogy too far, but perhaps we can find in

the comparison of the tropics and the cities some suggestion as to the

development of the eel.

In the city there is always a class which follows in no degree the

general line of development. Its members are specialized in a wholly

different way. By this means they take to themselves a field which

others have neglected, making up in low cunning what they lack in

humanity or intelligence.

Thus, among fishes, we have in the regions of closest competition this

degenerate and non-fish-like type, lurking in holes among the rocks,

or creeping in the sand; thieves and scavengers among fishes. The

eels thus fill a place otherwise left unfilled. In their way they are

perfectly adapted to the lives they lead. A multiplicity of vertebral

joints is useless to the tropical fish, but to the eel strength and

suppleness are everything. No armature of fin or scale or bone is so

desirable as its power of escaping through the smallest opening. With

the elongation of the body and its increase in flexibility there is a

tendency toward the loss of the paired fins, the ventrals going first,

and afterwards the pectorals. This tendency may be seen in many groups.

Among recent fishes, the blennies, the eel-pouts, and the sea-snails

furnish illustrative examples.

=Degeneration of Structures.=--In the lancelet, which is a primitively

simple organism, the various structures of the body are formed of

simple tissues and in a very simple fashion. It is probable from the

structure of each of these that it has never been very much more

complex. As the individual develops in the process of growth each

organ goes as it were straight to its final form and structure without

metamorphosis or especial alterations by the way. When this type of

development occurs, the organism belongs to a type which is primitively

simple. But there are other forms which in their adult state appear

feeble or simple, in which are found elements of organs of high

complexity. Thus in the sea-snail (_Liparis_), small, weak, with feeble

fins and flabby skin, we find the essential anatomy of the sculpin

or the rosefish. The organs of the latter are there, but each one is

reduced or degenerate, the bones as soft as membranes, the spines

obsolete or buried in the skin. Such a type is said to be degenerate.

It is very different from one primitively simple, and it is likely in

its earlier stages of development to be more complex than when it is

fully grown.

[Illustration: FIG. 158.--Liparid, _Crystallias matsushimæ_ (Jordan and

Snyder). Family _Liparididæ_. Matsushima Bay, Japan.]

[Illustration: FIG. 159.--Yellow-backed Rockfish, _Sebastichthys

maliger_ Jordan and Gilbert. Sitka, Alaska.]

In the evolution of groups of fishes it is a common feature that

some one organ will be the center of a special stress, in view of

some temporary importance of its function. By the process of natural

selection it will become highly developed and highly specialized.

Some later changes in conditions will render this specialization

useless or even harmful for at least a part of the species possessing

it. The structure then undergoes degeneration, and in many cases it

is brought to a lower estate than before the original changes. An

example of this may be taken from the loricate or mailed-cheek fishes.

One of the primitive members of this group is the rockfish known as

priestfish (_Sebastodes mystinus_). In this fish the head is weakly

armed, covered with ordinary scales. A slight suggestion of cranial

ridges and a slight prolongation of the third suborbital constitute the

chief suggestions of its close affinity with the mailed-cheek fishes.

In other rockfishes the cranial ridges grow higher and sharper. The

third suborbital extends itself farther and wider. It becomes itself

spinous in still others. Finally it covers the whole cheek in a coat of

mail. The head above becomes rough and horny and at last the whole body

also is enclosed in a bony box. But while this specialization reaches

an extraordinary degree in forms like _Agonus_ and _Peristedion_,

it begins to abate with _Cottus_, and thence through _Cottunculus_,

_Psychrolutes_, _Liparis_, and the like, and the mailed cheek finds

its final degradation in _Parliparis_. In this type no spines are

present anywhere, no hard bone, no trace of scales, of first dorsal,

or of ventral fins, and in the soft, limp structure covered with a

fragile, scarf-like skin we find little suggestion of affinity with

the strong rockfish or the rough-mailed _Agonus_. Yet a study of the

skeleton shows that all these loricate forms constitute a continuous

divergent series. The forms figured constitute only a few of the stages

of specialization and degradation which the members of this group

represent.

[Illustration: FIG. 160.--European Sculpin, _Myoxocephalus scorpius_

(Linnæus). Cumberland Gulf, Arctic America]

[Illustration: FIG. 161.--Sea-raven, _Hemitripterus americanus_

(Gmelin). Halifax, Nova Scotia.]

Some of the features of the habits and development of certain

fresh-water fishes are mentioned in the following chapter.

[Illustration: FIG. 162.--Lumpfish, _Cyclopterus lumpus_ (Linnæus).

Eastport, Maine.]

The degeneration of the eye of the blind fishes of the caves of the

Mississippi Valley, _Amblyopsis_, _Typhlichthys_, and _Troglichthys_,

have been very fully studied by Dr. Carl H. Eigenmann.

According to his observations

"The history of the eye of _Amblyopsis spelæus_ may be divided into

four periods:

[Illustration: FIG. 163.--Sleek Sculpin, _Psychrolutes paradoxus_

(Günther). Puget Sound.]

"(_a_) The first extends from the appearance of the eye till the embryo

is 4-5 mm. long. This period is characterized by a normal palingenic

development, except that the cell division is retarded and there is

very little growth.

[Illustration: FIG. 164.--Agonoid-fish, _Pallasina barbata_

(Steindachner). Port Mulgrave, Alaska.]

"(_b_) The second period extends till the fish is 10 mm. long. It is

characterized by the direct development of the eye from the normal

embryonic stage reached in the first period to the highest stage

reached by the _Amblyopsis_ eye.

[Illustration: FIG. 165.--Blindfish of the Mammoth Cave, _Amblyopsis

spelæus_ (De Kay). Mammoth Cave, Kentucky.]

"(_c_) The third, from 10 mm. to about 80 or 100 mm. It is

characterized by a number of changes which are positive as contrasted

with degenerative. There are also distinct degenerative processes

taking place during this period.

"(_d_) The fourth, 80-100 mm. to death. It is characterized by

degenerative processes only.

"The eye of _Amblyopsis_ appears at the same stage of growth as in

normal fishes developing normal eyes. The eye grows but little after

its appearance.

"All the developmental processes are retarded and some of them give out

prematurely. The most important, if the last, is the cell division and

the accompanying growth that provide material for the eye.

"The lens appears at the normal time and in the normal way, but its

cells never divide and never lose their embryonic character.

"The lens is first to show degenerative steps and disappears entirely

before the fish is 10 mm. long.

[Illustration: FIG. 166.--Blind Brotula, _Lucifuga subterranea_

(Poey), showing viviparous habit. Joignan Cave, Pinar del Rio, Cuba.

Photographed by Dr. Eigenmann.]

"The optic nerve appears shortly before the fish reaches 5 mm. It does

not increase in size with the growth of the fish and disappears in old

age.

"The scleral cartilages appear when the fish is 10 mm. long; they grow

very slowly, possibly till old age.

"There is no constant ratio between the extent and degree of ontogenic

and phylogenic degeneration.

"The eye is approaching the vanishing point through the route indicated

by the eye of _Troglichthys rosæ_.

"There being no causes operative or inhibitive, either within the fish

or in the environment, that are not also operative or inhibitive in

_Chologaster agassizii_, which lives in caves and develops well-formed

eyes, it is evident that the causes controlling the development

are hereditarily established in the egg by an accumulation of such

degenerative changes as are still notable in the later history of the

eye of the adult.

"The foundations of the eye are normally laid, but the superstructure,

instead of continuing the plan with additional material, completes

it out of the material provided for the foundations. The development

of the foundation of the eye is phylogenic; the stages beyond the

foundations are direct."

=Conditions of Evolution among Fishes.=--Dr. Bashford Dean ("Fishes,

Living and Fossil") has the following observations on the processes of

adaptation among fishes:

"The evolution of groups of fishes must accordingly have taken place

during only the longest periods of time. Their aquatic life has

evidently been unfavorable to deep-seated structural changes, or

at least has not permitted these to be perpetuated. Recent fishes

have diverged in but minor regards from their ancestors of the Coal

Measures. Within the same duration of time, on the other hand,

terrestrial vertebrates have not only arisen, but have been widely

differentiated. Among land-living forms the amphibians, reptiles,

birds, and mammals have been evolved, and have given rise to more than

sixty orders.

"The evolution of fishes has been confined to a noteworthy degree

within rigid and unshifting bounds; their living medium, with its

mechanical effects upon fish-like forms and structures, has for ages

been almost constant in its conditions; its changes of temperature and

density and currents have rarely been more than of local importance,

and have influenced but little the survival of genera and species

widely distributed; its changes, moreover, in the normal supply of food

organisms cannot be looked upon as noteworthy. Aquatic life has built

few of the direct barriers to survival, within which the terrestrial

forms appear to have been evolved by the keenest competition.

"It is not, accordingly, remarkable that in their descent fishes are

known to have retained their tribal features, and to have varied from

each other only in details of structure. Their evolution is to be

traced in diverging characters that prove rarely more than of family

value; one form, as an example, may have become adapted for an active

and predatory life, evolving stronger organs of progression, stouter

armoring, and more trenchant teeth; another, closely akin in general

structures, may have acquired more sluggish habits, largely or greatly

diminished size, and degenerate characters in its dermal investiture,

teeth and organs of sense or progression. The flowering out of a

series of fish families seems to have characterized every geological

age, leaving its clearest imprint on the forms which were then most

abundant. The variety that to-day maintains among the families of

bony fishes is thus known to be paralleled among the carboniferous

sharks, the Mesozoic Chimæroids, and the Palæozoic lung-fishes and

Teleostomes. Their environment has retained their general characters,

while modelling them anew into forms armored or scaleless, predatory

or defenseless, great, small, heavy, stout, sluggish, light, slender,

blunt, tapering, depressed.

"When members of any group of fishes became extinct, those appear to

have been the first to perish which were the possessors of the greatest

number of widely modified or _specialized_ structures. Those, for

example, whose teeth were adapted for a particular kind of food, or

whose motions were hampered by ponderous size or weighty armoring,

were the first to perish in the struggle for existence; on the other

hand, the forms that most nearly retained the ancestral or tribal

characters--that is, those whose structures were in every way least

extreme--were naturally the best fitted to survive. Thus _generalized_

fishes should be considered those of medium size, medium defenses,

medium powers of progression, omnivorous feeding habits, and wide

distribution, and these might be regarded as having provided the

staples of survival in every branch of descent.

"Aquatic living has not demanded wide divergence from the ancestral

stem, and the divergent forms which may culminate in a profusion of

families, genera, and species do not appear to be again productive of

more generalized groups. In all lines of descent specialized forms

do not appear to regain by regression or degeneration the potential

characters of their ancestral condition. A generalized form is like

potter's clay, plastic in the hands of nature, readily to be converted

into a needed kind of cup or vase; but when thus specialized may never

resume unaltered its ancestral condition: the clay survives; the cup

perishes." (DEAN.)

FOOTNOTES:

[19] Günther, Introd. to the Study of Fishes, p. 192.

[20] The cells which von Lendenfeld designates 'phosphorescent cells'

have as their peculiar characteristic a large, oval, highly refracting

body imbedded in the protoplasm of the larger end of the clavate cells.

These cells have nothing in common with the structure of the cells of

the firefly known to be phosphorescent in nature. In fact the true

phosphorescent cells are more probably the 'gland-cells' found in ten

of the twelve classes of organs which he describes.

[21] See a more technical paper on this subject entitled "Relations of

Temperature to Vertebræ among Fishes," published in the Proceedings of

the United States National Museum for 1891, pp. 107-120. Still fuller

details are given in a paper contained in the Wilder Quarter-Century

Book, 1893. The substance is also included in Chapter VIII of

foot-notes to Evolution: D. Appleton & Co.