Popular Science Monthly/Volume 20/February 1882/How Animals Breathe I
|HOW ANIMALS BREATHE.|
ANIMAL COMBUSTION.—Within every living organism there are two opposing forces. The "vital force," which produces all the phenomena of life, holds the material elements in unstable relations—against their will, so to speak—and it is antagonized by the natural chemical affinities of the elements, which tend to break down the organic compounds and rearrange the elements in more stable form. This decomposition takes place in some degree during the life of every organism, and when life ends, or when the vital force ceases to act, it rapidly destroys the structure.
The waste matter resulting from this disintegration must be immediately removed from the body of the living animal, otherwise it clogs and poisons the system. The method of its accomplishment is one of the most admirable functions of the animal economy. To remove the effete matter in the natural liquid or solid state would be very exhausting; consequently it is burned, and the gaseous products of its union with oxygen are then easily carried away. Literally speaking, this makes a furnace of the body of every animal; and the most pressing and ceaseless demand of the system is for oxygen to support its fires. Respiration is hence an absorbing and excreting process, whereby oxygen is received and carbonic acid and water removed. It thus becomes a measure of the amount of combustion.
In the "cold-blooded" animals respiration bears a direct proportion to the activity and the heat of the body, as the former causes a metamorphosis and waste of tissue, and the latter always aids decomposition. The fact is one of common observation. It is well illustrated in the quickened breathing of a tired animal, and in the almost entire suspension of respiration in the hibernating state. The respiration of a "cold-blooded" creature is increased by artificial heat. In
extremely hot weather frogs may have to leave the water entirely, and fishes come to the surface to procure air. Reversely, frogs can be kept for years in a state of suspended animation by a low temperature, and revived by warming. Some low animals can survive freezing or drying for an indefinite time; and, under such conditions, the waste of the tissues must be entirely suspended.
In "warm-blooded" animals—birds and mammals—a constant body-temperature, independent of the surrounding atmosphere, is maintained by the immediate use of the food as fuel. Consequently, in a warm atmosphere less internal combustion is required than in a cold one, and the respiration of birds and mammals is therefore inversely proportionate to the external temperature, although directly proportionate to the activity of the animal.
This constant body-temperature is the reason of the difference in the kind and quantity of food required according to season or latitude. While the people of the tropics subsist chiefly on vegetable food, which supplies little fuel, but on the contrary much fluid to cool the body by evaporation, the inhabitants of frigid regions use carbonaceous foods affording the greatest proportion of fuel.
Experiments tend to prove that human respiration is, as would theoretically be expected, less rapid in the tropics than in the cold regions. Every traveler knows that a less amount of food is required. But on the contrary, and for the reason above stated, respiration in the cold-blooded animals is more rapid in the tropics, and the quantity of food is greater.
Physiological Principle.—The process of respiration is in principle an interchange of gases between the fluids of the structure and the external medium, air or water. It should be unnecessary to say that aquatic animals do not breathe water, but the air which is absorbed by the water. This exchange is effected by the physical action known as osmosis. It is a question whether vital influence has any part in the process. The principle is identical in all creatures, air-breathers
and water-breathers—those which have special fluids, or blood, and those having none. It applies to plants also, as far as they have a true respiration.
In bringing the internal and the external fluids adjacent, Nature in this function, as in all others, is nicely economical of power. In "water-breathers" the blood, when existing, is commonly sent to the surface of the body, and a slight movement of the water produced—sufficient to renew that in contact with the respiratory organs. But "air-breathers" always draw the mobile atmosphere inward to the blood.
Respiration of Plants.—The leaf-respiration, so called, of plants is not an excretory process, but rather a nutritive one. In effect it is precisely the reverse of true respiration. It is a deoxidizing process—separating the components of carbonic acid under the influence of the sun's rays, and depositing the carbon; whereas respiration is an oxidizing process the production of carbonic acid. However, during germination and flowering, and in darkness, decomposition takes place within the plant, resulting in the production and elimination of carbonic acid a—true respiration.
Many biologists now hold that there is a constant decomposition of crude nutriment in the interior of plants, and therefore a slight respiration, but that it is masked by the more prominent nutritive process. The leaves are commonly, but wrongly, called the lungs of the plant, for their chief function, as we observe, is not respiration, but nutrition. It were more correct to regard them as the stomach of the plant.
Organs of Respiration.—As the function of respiration is so simple in principle, being a single physical action, it can be conducted almost anywhere in the body, or wherever the blood can be conveniently exposed to the surrounding medium. The nature of an animal, as regards other less easily modified functions, and its peculiar circumstances, may therefore greatly determine the character of its breathing organs. So it results that these organs, in the various groups of animals, are exceedingly diverse in form, structure, and position. They are much more diverse, indeed, than are the organs of any other function, and any classification now possible is quite arbitrary.
In purpose, however, or physiologically, all breathing-organs are simply an expansion of surface for the more rapid aeration of the circulating fluids. And in origin, or morphologically, they are fundamentally a modification or development of the skin; being developed either primarily, i. e., directly from the skin itself, or secondarily, i. e., from the alimentary canal, which is itself so derived.
The following tabular classification shows the general morphological character of the respiratory organs in the primary groups of the animal kingdom. While correct in the main, it does not cover all exceptional cases, but fairly exhibits the development of the organs and the "differentiation" of the function from the lowest to-the highest animals. Except in the vertebrates no regard in this table is paid to any distinction between air and water breathing.
In explanation: A stands opposite the mode of respiration which is characteristic of the group beneath which the letter occurs; and the diminishing degrees in importance of the various methods are indicated by the small letters according to the alphabetical order: but of course the same letter is not of equal value for different groups of animals.
The reader will observe the common use throughout the whole animal kingdom of the skin unmodified. Also that the special organs of the invertebrates, which number more than nine tenths of the whole, are quite limited to modifications of the skin; while those of vertebrates belong only to the alimentary canal. The description to follow will adhere to the order of the table:
Respiration by the Unmodified Skin and Food-Tract.—The general surface of the body has in all soft-skinned animals an aërating or respiratory action; and the simplest expression of organs is found in those animals where the body-surface is the only breathing-organ. Fig. 4.—Tubicola. a, Serpula contortuplicata, showing the branchiæ and operculum; b, Spirorbis communis. But a simpler expression of the function is shown in an amœba, for example, where by the amœboid movements every part of the structureless body eventually becomes surface, and is brought into immediate contact with the water. The amœba has no permanent skin, and no organs of any kind; consequently it breathes without special organs, and the other nutritive functions are equally destitute of them.
Many animals of higher groups breathe mainly or entirely by the skin. Among articulates, the leech and earth-worm are examples. In these a network of minute blood-vessels is spread beneath the delicate skin, thus bringing the blood into proximity with the water or air. The lowest crustaceans, and some sea-snails, and sea-spiders also, respire by the skin alone.
Most amphibians use the skin largely in respiration. In cold water, frogs will breathe entirely by the skin, and can not be killed by forced immersion, so long as they are provided with food. Even at ordinary
temperature, it is easier to drown some fishes by preventing them from reaching air than it is to suffocate frogs under the same conditions; yet the former have only gills, and the latter lungs alone. The hell-bender, found in our Western rivers—the largest American salamander—breathes entirely by its skin under ordinary circumstances. To expose more surface, the skin is greatly expanded, lying in ugly folds; while, to renew the contiguous water, the body is frequently arched and rolled from side to side, which motion, together with the aspect of the creature, has given it the above expressive name.
As the outer surface of the animal body may without any special modification have a respiratory function, so we find the same to be true of the food-canal. The body-cavity of the polyp and jelly-fish, with its tubes and branches, has in previous articles been described as the organ of digestion, circulation, and respiration in common. Many
annelids breathe partly by currents of water passing through the intestine. A slight modification of the lining of the food-tract is found in the larvæ of the dragon-flies, which, living in water, have respiratory villi in the rectum; and the sea-cucumber has foliated processes in its stomach, bathed by water-currents, which doubtless have a respiratory function.
Many fishes swallow air as an aid to respiration. In the loach (Cobitis fossilis), and numerous others, it is certain that this air traverses the stomach and intestines.
Special Organs of the Skin.—Of the special breathing-organs we will first consider those which are developed directly from the skin. They are especially characteristic of the invertebrates, as they include all the special respiratory apparatus of that division of the animal kingdom, whether adapted for water or air, and they are not found beyond this group.
Aquatic Organs of the Skin.—The higher radiates and the lower articulates possess an arrangement of tubes and vessels known as the "water-vascular" or "aquiferous" system. Although the relation of these vessels to the circulation is not fully determined, it is probable that they are chiefly or entirely for the purpose of conveying water inward, and thus constitute respiratory organs. They are rendered necessary because of the undeveloped condition of the blood-circulating system. Like other cavities, they are only an inflection of the surface-tissue.
This aquiferous system is of very great complexity and variety, and full description is impracticable. It is at once greatly developed in the sea-urchin and star-fish; and is especially complex in the cucumber, where it is connected with gill-like appendages about the mouth. The intestinal worms and other annelids possess this style of Fig. 7.—Anatomy of a Bivalve Mollusk (Mya arenaria). The left, valve and mantle-lobe and half the siphons are removed, ss, respiratory siphons, the currents; a a', adductor muscles; b, gills; h, heart; o, mouth, surrounded by (p) labial palpi; f, foot; v, anus; m, cut edge of the mantle. (After Woodward.) breathing-organs. Later we shall find in the insects a similar and perhaps homologous system for conveying air.
Most aquatic invertebrate animals which have a true circulating fluid send this to expansions of the skin to be there oxygenated by the surrounding water. This is termed. "branchial" respiration. The branchiæ or gills may be wholly exposed and external to the body, or may be contained in suitable cavities. They may develop on any part of the body; and they exhibit a great variety in many respects. With this system is frequently found some special accessory apparatus for producing and regulating a flow of water over the gills.
The sea-worms have a great variety of external gills developed immediately from the skin. Renewal of water is secured by cilia which cover the body or even the gills. These branchiæ are finely shown in the common Serpula, where they form a crown of scarlet plume-like about the mouth. The Arenicola, or sand-worm, has two rows of crimson rosettes along the sides of the body. The Eunice, Nereis, and sea-mouse, are other worms possessing beautiful arborescent fringes of branchiæ.
As the crustaceans, excepting a few of the arrows indicating the direction the lowest, are covered with a hard shell of dead matter, they can not respire through the skin, and hence of necessity require special organs. These are commonly leaf-like expansions, usually attached to the locomotive or other appendages; the purpose of such attachment being to secure rapid change of the surrounding water. In some groups, the entire limb becomes foliaceous and respiratory, so that it is literally correct to say that the fairy shrimp breathes with its legs. Other forms, including the sand-hopper, whale-louse, and fresh-water shrimp, breathe by vesicles or bladders attached to the limbs. The water-fleas, as Cypris and Cyclops, so common in our fresh waters, have their branchiæ attached to the jaws. Many forms, among which are the king-crab, or Limulus, and the group of isopods, respire by foliaceous or feathery appendages beneath the abdomen.
The higher crustaceans—crabs, lobsters, and shrimps—have arborescent gills inclosed in cavities on the side of the thorax. They are attached
to the bases of the legs, but the motion of the latter would here have little effect to renew the water. This is accomplished by a curious valve placed in the excurrent orifice of each chamber, which resembles in principle the screw-propeller of a steamship. The observer will find the excurrent orifice at the side of the mouth, and the incurrent
orifice between the edge of the shell and the bases of the legs. Curious combs and brushes are provided to keep the gills clean and the filaments separate.
Bivalve mollusks, as clams, mussels, scallops, etc., have gills made of foldings of the "mantle" or skin. (See Fig. 10, page 467, of the August "Monthly.") There are generally two for each side of the
fluid and feeding the helpless inhabitants of these shells. Every filament of the gill-fringe, examined under a powerful microscope, is found to be covered with countless cilia, in constant vibration, causing, by their united efforts, powerful and rapid currents. ... So energetic,
indeed, is this ciliary movement over the entire extent of the branchial organs that, if any portion of the gills be cut off with a pair of scissors, it immediately swims away, and continues to row itself in a given direction as long as the cilia upon its surface continue their mysterious activity."
In the oysters and scallops, the mantle is open, exposing the gills; while the mussels and giant Tridacna have the mantle closed but pierced with two apertures for the reception and expulsion of water. The remaining lamellibranchs, which include the common hard clam, razor-shell, pholades, and teredo, have long inhalent and exhalent tubes, called siphons, which enable the creature to burrow in the sand and still obtain water. The two siphons may be separate or united, and are retractile. The presence of such tubes can be determined by examination of a dead shell; as the muscles which retract the siphons produce an indentation or "sinus" in the "pallial line."
Perhaps the most conspicuous and beautiful gills are those possessed by the naked sea-snails or "sea-slugs." They are situated on the back and sides of the body, and are frequently retractile. In the Doris they have a flower-like or star-shaped arrangement. In the Eolis they are papilliform and tufted, along the sides of the animal, or they may be tree-like as in the Triton, or feathery in other forms. The Aphysia, or sea-hare, has the gills placed on the back and protected by a fold of the mantle. The Phyllidia have them as a fringe along each side of the body, and covered by a projecting fold of the skin; while the Limpet and Chiton have leafy gills forming quite an entire circle about the body, and also covered by the mantle.
The sea-snails having shells carry their gills in cavities in the side of the neck, and may or may not have siphons. The possession of the latter is shown by a notch in the aperture of the shell.
All the mollusks mentioned thus far are sluggish animals, with little need of rapid respiratory changes of the blood. But the highest of mollusks, the cephalopods, or devil-fishes, are active and muscular creatures—the most powerful of invertebrates. They have accessory hearts to accelerate the flow of blood through the gills, which are large and plumose, and contained in a cavity. Water is freely supplied to the gill-chamber by the open mantle; but, by the contraction of the latter, the water is forcibly expelled through a tube termed the "funnel." This expulsion of water from the branchial chamber is their chief means of locomotion.
Aërial Organs of the Skin.—Special organs developed from the outer surface of the body and adapted to air-breathing are confined to
|Fig. 14.—Spiracle of Cockchafer-Grub.||Fig. 15.—Spiracle or Fly.|
the articulates and mollusks. None are found in the radiates, this great division being wholly aquatic.
Although the crustaceans always possess gills, and are theoretically classed as water-breathers, yet certain species live entirely on land, and some are even killed by long immersion in water. The gill-chambers of the land-crabs are proportionately very large, exposing more surface to the air, and the openings are small to prevent evaporation. Fig. 16.—Spiracle Leather-Coat. Some species inhabit the highest ground of West India islands, but they seek damp or sheltered places, and possess some means of keeping the gills moist, either by carrying a small quantity of water, or by a secretion of the sponge-lining of the chamber. Structurally these organs are gills, but functionally they are true lungs.
Another crustacean, the wood-louse (Oniscus), lives in damp places, and breathes air by foliaceous gills beneath the abdomen.
The only groups of articulates wholly air-breathing are the myriapods, insects, and arachnids. The myriapods show a transition from the skin-respiration of the leech and earth-worm to the air-tubes of the insect. The latter possesses the most peculiar and effective system for aeration. As in the annelids a complex system of tubes conveys water to the tissues, so in the insects all parts of the body, and the nutritive fluid, are bathed by air admitted through a complex tube-system. So complete is the aeration of the whole structure, that practically the blood of an
insect is wholly arterial. The necessity for this lies in the slow and imperfect circulation, coupled with an unparalleled activity.
The air-tubes, "tracheæ," traverse every part of an insect's body, even the brain and eye, and form the nervures or framework of the wings, where they are sheathed by another tube conveying blood. Fig. 18.—Grub of Chameleon-Fly. They are prevented from collapsing by a mechanism, one of the most admirable and exquisite found in nature. A delicate, elastic thread is wound in a close spiral between the two membranes of the walls of the tubes a contrivance which we have imitated in the flexible gas-tube of a drop-light.
The openings to the tracheæ, termed "spiracles" or "stigmata," are generally located on the sides, of the abdomen and thorax, a pair for each segment, and exhibit great variety and adaptation to their purpose. So constructed as to admit air, they exclude water and dust, and can be opened and closed at the will of the creature. Sometimes they are a mere slit, like a button-hole. In the soft-skinned larvæ they are kept open by a horny ring. The aperture is sometimes protected by interlacing hairs; sometimes it is grated, or in other species closed by a thin membrane pierced by innumerable small pores a real sieve. A drop of oil placed upon the abdomen immediately fills the spiracles and smothers the creature.
The change of air is produced in the tracheal system by the enlargement and contraction, or bellows-action, of the abdomen, the segments sliding in and out with a telescopic movement, very perceptible in the larger insects.
Many insect larvæ live in water, but are still air-breathers. The mosquito is an example. Here the eighth joint of the abdomen sends off a long tube, crowned at the extremity with feathery bristles, which are brought to the surface of the water to seize a. drop of air. With this the larva descends and the repellent action of the setæ prevents the water from dissolving it. Some water-larvæ have tubes several inches long. Other larvæ, destitute of tubes, have the spiracles at the posterior end of the abdomen. In the pupa state the mosquito has two respiratory tubes on the back of the thorax.
The aquatic larvæ of the dragon-flies (Libellulidœ) have, as already mentioned, hair-like processes, "villi," of the lining of the rectum, which absorb air from the water and convey it into the tracheal system. It is difficult to determine if such larvæ should be regarded as aquatic or aërial. Certainly in their habits, and in their method of procuring oxygen, they are aquatic and possess gills. But the oxygen is not immediately given to the blood in the villi, as in true gills, but is conveyed through tracheæ just as in adult insects. On the latter account these larvæ are generally considered aërial.
Adult water-insects carry air into the depths of the water by holding it under the wings or legs, or by the minute hairs which cover the body. This frequently gives them the silvery appearance of a globule of mercury, and sometimes renders the creature so buoyant that it has to descend by muscular effort—swimming downward, or crawling down a plant-stem.
Insects of powerful flight have sacs developed on the trachea?, which doubtless serve to store air, and also to render the body lighter.
Spiders lack the tracheae, but have the spiracles opening directly into sacs. These are various in structure, sometimes folded like gills, or in other cases cellular, like a rudimental lung in structure.
Among mollusks only the true snails are air-breathing, and some of these inhabit water. They have organs resembling those of the spider, concentrated into a single sac. This is lined with blood-capillaries and constitutes a true external lung. The orifice opens on the right side of the neck, and can be closed by a circular muscle.
In the pond-snails we have a clear illustration of the identity in principle of air and water respiration. For, in the Lymneæ, the sac, which in the early stages of the animal breathes water alone, can suddenly change to a lung, and thereafter breathe only air. And in specimens from deep water the change may occur in the adult animal. The Ampullariæ have the breathing-sac partially divided into an upper or air chamber and a lower or gill cavity; using first one and then the other, or breathing air and water alternately at intervals of a few minutes.
- Other articles in comparative physiology are contained in the "Monthly" of April, June, and September, 1880; and August and September, 1881.
- T. Rymer Jones, "The Animal Creation," page 194.