An introduction to biology for students in India

Front matter

22102077195

Med

PHYSIOLOGY, BIOLOGY, ZOOLOGY, Etc.

Preface

Owing to the new regulations, it has become necessary for all who wish to graduate in Medicine at the Calcutta University to obtain knowledge of the structure of animals by dissection. The anatomy of vertebrate types is described in many good books, but the common invertebrate animals of Bengal, which are peculiar to the country, have not been described hitherto in any text-book. It is necessary that the types for dissection should be of common animals, which may be obtained in large quantities. In England even Protozoa can be conveniently obtained by purchase. This is not the case in India; but the source of the material for our classes must be as certain as in other countries. The types selected are not in all cases those required by the University syllabus. Some of the types mentioned therein, such as Euglena, Sphenopus, Eunice, Bipalium, and certain Echinoderms, cannot be obtained in sufficient quantities for classes which may number as many as one hundred and fifty students. Because it was urgently required, the book has been written somewhat hurriedly, and is therefore likely to contain errors. I shall be grateful to other teachers in India who may point

VI

PREFACE them out. In drawing the figures, I have tried to express only what I have seen ; when possible, the outline was guided by the camera lucida.

The last chapters of the book deal with Evolution, Variation, and Heredity in a simple manner. It is difficult to decide what should be taught about these debatable subjects, hut as they are fast growing in importance at the present day, they should, I think, be introduced to the students’ notice.

I have treated these subjects here much as in the lecture room — that is to say, in the way that has seemed best to me as a result of observation and reading. My chief endeavour has been to awaken the interest of students in these subjects. However much of truth or untruth the teaching may contain, the interest shown in it hitherto by a number of my students has been encouraging.

I am much indebted to Messrs. Macmillan & Co. for their kindness in allowing me to make use of certain illustrations (viz. Plate XIV., in toto ; Plate IH., Fig. H ; Plate VI., Fig. C) ; and to Messrs. Smith, Elder & Co. for the kind permission to make a quotation from Marshall & Hurst’s “ Practical Zoology.”

R. E. L.

Contents

CHAPTER PAGE

Introduction ix

I. The Protozoa 1

Chapter I

Animals are composed of living material, which is called protoplasm. Except in the simplest animals the protoplasm is arranged in the form of separate units or cells, each a minute piece of living matter. A complex animal is composed of a vast number of these cells, which are of different kinds and are combined to form the fabric of the body, much in the same way as bricks, beams, and tiles are combined to form a house. It is the custom to regard the body of the simplest animals as though it were one cell ; that is to say, it may be compared, though not exactly, with one of the units which combine to form a complex annual. The simplest animals are, therefore, described as unicellular, while others are called multicellular. Unicellular animals are, as a group, named the Protozoa. Multicellular animals are named Metazoa.

Amoeba. — The Amoeba is among the simplest of the Protozoa. The body of an Amoeba consists of a minute portion of naked protoplasm, which is not constant in its form, but variable from moment to moment. Amoebae are very minute ; with the aid of a microscope they may

B be found in various situations — among vegetation in fresh-water ponds, in water which contains rotting organic material, and also as parasites in the intestines of many of the higher animals. In company with other kinds of Protozoa they may be obtained from what are called infusions. To make an infusion a quantity of hay or leaves must be placed in water. After about twenty-four hours or more, according to the temperature, the water looks yellowish, and the surface becomes covered with an opaque film. If some of this film be removed and examined under the microscope, it will be seen to contain numerous minute rod-like bodies, many of which move actively ; these are bacteria which have emerged from minute round bodies called spores, just ,as a chicken emerges from its egg. The spores of the bacteria are present everywhere, attached to the hay and floating in the air. They are not killed by ordinary heat and drought; they are reproduced in the bodies of parent bacteria. The appearance of the bacteria in the water marks the first stage of putrefaction. Soon after the appearance of the bacteria, various Protozoa may be found in the infusion ; these have emerged from bodies called cysts, which also are distributed everywhere. The Protozoa feed actively upon the bacilli and upon one another.

As an example of an Amoeba, a large form which is sometimes found in infusions will be first described. To obtain it, place a piece of straw from a two or three days’ infusion upon a glass slide, and scrape it with a knife. Place a cover-slip over the dirty greyish-looking fluid which has exuded, and examine it under the low power of the microscope. Among the various organisms in the fluid may sometimes he seen an Amoeba large enough to be recognized without much difficulty. When found, examine it under the high power.

The Amoeba is a piece of living material or protoplasm ; it is translucent, but not equally so throughout ; it is surrounded by an outer layer, or ectoplasm, which is almost as clear and transparent as glass. The more central portion, or endoplasm, is opaque, and contains numerous granules. The shape of the Amoeba is not constant for any length of time, but changing, owing to the protrusion of blunt lobes which may be thrust out from any part of the surface. When such a lobe or pseudopodium is thrust out, a flowing movement of the granules of the endoplasm may be observed. The granules stream down the central axis of the pseudopodium until they reach the blunt end, where they are reflected outwards, so that in returning to the main body they take a superficial course. The movement within a pseudopodium is therefore somewhat like the movement within a fountain of water which is forced vertically into the air. When the animal wishes to move in a certain direction, a large pseudopodium is thrust out in that direction and the bulk of the endoplasm is poured into it, so that the whole animal shifts its position. Pseudopodia are also thrust outwards towards particles of food material, which become engulfed within tko ectoplasm to be passed into the endoplasm, where they are digested.

Within the endoplasm is a spherical body which is somewhat opaque, this is the nucleus ; sometimes two nuclei are present. (Every cell of a multicellular animal contains a nucleus ; it is because of this fact that the body of an Amoeba or other Protozoon appears to he a single cell.) There are many particles in the endoplasm which have the same appearance as the various particles in the surrounding water; they are ingested food material. Sometimes they can be seen to lie within minute spherical spaces in the endoplasm, which are called food vacuoles. Another kind of vacuole can always he seen in the endoplasm ; this is much larger, and is called the contractile vacuole. It should be watched while it expands slowly until it hursts like a bubble. When large it is near the surface ; and if then it he viewed from the side, the ectoplasm which covers it may be seen to bulge and become thinner and thinner, until it ruptures at a point and the contents of the vacuole are discharged. If it is viewed from above and not from the side, the ectoplasm around the point of discharge assumes a radiating appearance, which is perhaps due to minute folds of the surface, which must naturally form as the bulged ectoplasm sinks back into its normal position. As soon as the contractile vacuole has discharged its contents from the body, it again commences to fill and expand as before. The contractile vacuole is doubtless an excretory organ, the fluids that

THE PKOTOZOA are discharged must contain the waste products of metabolism (Chap. VIII.).

Other Kinds of Amoebae. — A smaller kind of Amoeba is common among the dark green slime which accumulates in wet gutters. With the help of a microscope we can see that the slime from a gutter consists of a tangled mass of green filaments, which are minute plants called Algae. A small piece of such material should be placed upon a slide in a drop of water, and broken up very finely with needles. By this means the Amoebae become dislodged from among the green filaments. The slide should be covered and examined. Amoebae found in this way, as well as those found among Algae in fresh-water ponds, are smaller than the kind just described, and have long, thin pseudopodia which are composed of clear ectoplasm throughout ; that is to say, a granular axis cannot be seen in them. Otherwise, these Amoebae are like the kind just described, but they can be obtained with much greater certainty. Another kind may generally be obtained from the intestine of the common cockroach. Amoebae which live in the intestines of multicellular animals are called Endamoebae ; one kind of Endamoeba causes dysentery and liver abscess in man. To obtain Amoebae from the cockroach, kill the insect with chloroform and pull off the end of the abdomen. The intestine will be seen as a black chord stretching between the torn-off portion and the rest of the body. Cut out a portion of this and place it upon a slide in a drop of salt solution. The intestine is a membranous tube with black contents ; it must be cut open with fine scissors, the contents scraped from the inner wall and mixed in the salt solution. The mixture must be covered and examined ; various parasites can usually be seen — threadworms (Oxyurus), large ciliated Protozoa (Nyctotherus), and numerous Endamoebae. The Endamoebae are small, and must be sought for under the high power of the microscope. They are peculiar in being very transparent, as though entirely composed of ectoplasm, in which are embedded a few large food particles. They are sometimes very active, and thrust out rounded pseudopodia with great rapidity ; at other times they assume a spherical form, as though resting. Although large Amoebae are not easily obtained in infusions, minute Amoebae may almost always be found in them. Such Amoebae are often very active ; but vacuoles and

even nuclei cannot easily be seen in them : they are

*

perhaps stages in the life-history of other Protozoa.

When the water in which Amoebae live is becoming dried up, they undergo encystment, assuming a spherical shape and surrounding themselves with an impervious coat or cyst. In this form they can resist drought and moderate heat ; they may be scattered by the wind until they find themselves in water again. Amoebae have been seen issuing from such cysts.

The reproduction of Amoeba is by simple division of the body, and is preceded by simple division of the nucleus. The round nucleus assumes the shape of a dumb-bell or hour-glass, and the two halves become separated. After division of the nucleus, the entire body undergoes a similar change ; on separation, each half of the body contains one-half of the old nucleus. This method of reproduction is essentially different from that of multicellular animals. In the reproduction of higher animals, portions of the body are thrown off from time to time to form offspring, but the parent body must finally undergo death. In Amoeba, the parent body divides and forms two offspring, so that it does not die, but becomes continuous in the offspring. Much remains to be discovered about the life-history of Amoebae.

Protozoa, such as Amoeba, which change their shape and thrust out pseudopodia are placed together in a group and named Lobosa. Some Lobosa form hard protecting shells for themselves. Of these the form known as Difflugia is sometimes found in Indian ponds. This Protozoon forms a balloon-shaped shell by cementing together particles of sand. The nucleus and most of the protoplasm remains within the shell, active pseudopodia are thrust out through an opening in the shell which represents the mouth of the balloon.

There are certain groups of Protozoa which must be passed over hurriedly, although each group is large enough to form the study of a lifetime.

THE FORAMENIFERA

These Protozoa live in the sea, floating near the surface or crawling on the bottom. Like Difflugia they form a hard shell/ sometimes of sand, more often of calcium carbonate, which they secrete from the seawater. The shells are always perforated by minute round apertures. The protoplasm lies partly within the shell and partly around it, and is continuous through the apertures. The mature shell is generally composed of several chambers. At the commencement of its life, the young Protozoon forms for itself a single chamber resembling that of Difflugia, except that it is perforated ; when it becomes too large for its shell it forms another, which is fixed to the first. Many others are formed in succession as the animal grows. The several chambers are arranged in different ways among the different kinds — in a straight line, in a spiral, or in other ways. The Foramenifera are of importance as builders of the earth’s surface. They are very numerous in the sea ; by thenunited power of secretion they convert vast quantities of dissolved calcium salts into solid calcium carbonate, which is added to the crust of the earth ; for when a Forameniferan dies, its shell sinks to the bottom of the ocean and remains there. In much of its area, the mud of the ocean is almost entirely composed of shells of Foramenifera, and it is also known that some of the rocks which constitute the earth’s surface are also largely composed of the minute shells of Foramenifera which lived in bygone ages (Chap. IN.). The Foramenifera differ from the Lobosa in their method of reproduction.

The Radiolaria are another large group of Protozoa which also are found in the sea ; they mostly secrete perforated shells of silica, which sink to the floor of the ocean and remain there.

THE SPOROZOA

These will be described in more detail, as they are common and sometimes cause disease. A mature Sporozoon is generally covered outwardly with a delicate and firm layer or cuticle, which prevents the protrusion of pseudopodia, but may not prevent all movement. Contractile and other vacuoles do not occur in Sporozoa, because the method of nutrition is different from that of other Protozoa. The Sporozoa live as parasites within the bodies of multicellular animals ; although they may often be found lying free in the various cavities of the body, they always pass some part of their life-history within a cell of their host, by the juices of which they are nourished.

Monocystis. — A large Sporozoon named Monocystis may be obtained from the organs known as the seminal vesicles of the earth-worm. It seems impossible to find an earth-worm which is not highly infected with this parasite. In India the degree of infection is much greater than in more temperate climates. If a small portion of one of the seminal vesicles of an earth-worm (Chap. III.) be pressed into a drop of salt solution upon a slide the drop will be rendered turbid. With the naked eye the drop may be seen to contain minute white specks, which are the Monocystis. The drop should be

PLATE I.

Various Protozoa.

A. An active Amoeba from fresh water. 1. Nucleus ; 2. Contractile

vacuole ; 3. Pseudopodium.

B. The same in a state of contraction.

C. An Amoeba from the intestine of a man who was suffering from

dysentery. 1. the nucleus ; 2. remains of ingested corpuscles from the man’s blood.

D. Amoeba from the intestine of a cockroach.

E. Amoebulae from an infusion.

F. Vorticella (side view). 1. macronucleus ; 2. micronucleus ; 3. con-

tractile vacuole; 4. undulating membrane; 5. food vacuole; 6. paroral cilia ; 7. adoral cilia ; 8. contractile fibre.

G. Vorticella (from above) ; 1 and 8 indicate the first and last cilia of

the adoral row ; 2. vestibule ; 5. disc. (For convenience, only the bases of the adoral cilia are shown.)

H. Nyctotherus from the intestine of the cockroach ; 1. macronucleus;

2. micronucleus ; 3. anus ; 4. contractile vacuole ; 5. vestibule.

J. Herpctomonas from the intestine of a house-fly. 1. kinetonucleus ;

2. trophonucleus ; 3. flagella.

J'. Herpetomonas undergoing fission.

(N.B. — Figures A — H arc drawn approximately to the same scale X 450. J— J' x 2000.)

covered and examined under the microscope. The many parasites which are visible are not all alike, for they have reached different stages in their careers. The stages have received different names.

First Stage : The Trophozoite. — A trophozoite feeds and grows, and stores up reserve food material ; many other animals and plants, besides nourishing themselves for the moment, store up food material for future use. The body of the trophozoite is a single elongated cell surrounded by a thin cuticle, beneath which is a thin layer of clear ectoplasm. The endoplasm forms the mass of the body ; it contains many oval granules of reserve nutriment. The ectoplasm contains a layer of fibrils which, like the muscle cells of multicellular animals, are capable of contraction, and perform such movements as the animal exhibits ; these fibrils cannot easily be seen. The nucleus is like a vacuole, in which lies an opaque body called the karyosome. When the trophozoite is young, it lies embedded in the protoplasm of a special cell of the earth-worm which is called a spermatoblast ; but as it grows it absorbs more and more of the spermatoblast until it becomes too large to be contained within that cell. The mature trophozoite is found free in the cavity of the seminal vesicle.

Second Stage : The Gamctocyte. — It has been mentioned that most organisms reproduce their kind by throwing off minute portions of their body, which grow and develop until they resemble the parent. Such portions always contain a nucleus, and are called germ cells, or gametes. Gametes are of two kinds, male and female, which are usually, hut not always, produced by separate individuals. The two kinds of gametes are usually unlike one another in outward appearance, in which case the female gamete is called an ovum, and the male gamete a spermatozoon ; contrary to the rule, the gametes of Monocystis are all alike in outward appearance. Whether the two kinds of gametes be outwardly alike or unlike, their behaviour is the same, for they seek one another in pairs, and unite to form a single nucleated cell, which is called a zygote. The single nucleus of the zygote is formed by the fusion of two nuclei, one from each gamete. The formation of a zygote is the first stage in the reproduction of all organisms (except those few which reproduce their kind in the manner called parthenogenetic) .

The trophozoite of Monocystis when full grown begins to form gametes, and is therefore referred to as a gametocyte. Just before the process commences, the trophozoites become associated in pairs : they lie side by side and surround themselves with a thin layer of transparent material. They are thus associated for purposes of reproduction within a spherical chamber which is called a cyst. The nucleus of each divides by mitosis (Chap. VIII.) first into 2, then into 4, 8, 16, etc., until a large number of nuclei are formed. The process of division is not easy to see : it proceeds within the endoplasm, and is obscured by the granules. The result of the process however, can he seen without difficulty, for as soon as the many small nuclei are formed, they move towards the surface of the individual gametocyte which produced them, and lie each within a small portion of clear protoplasm. A gametocyte therefore becomes entirely surrounded by small units of protoplasm, each of which contains a nucleus. These units are the gametes. Changes of this kind occur similarly and simultaneously in both of the associated gametocytes. On their completion, the gametes become detached from their fixed position, and can be seen lying free in the cyst. They then become united in couples, and it is justly assumed that those formed by one gametocyte seek and unite with those formed by the other ; the fact, however, is difficult to observe, because the gametes produced by each individual are exactly alike in appearance. As a result of their union a number of bodies called zygotes are formed. Each, zygote becomes surrounded by a transparent coat of material called chitin. In this condition the zygote is referred to as a spore, because it leaves the body of the worm and lies in the earth until it is swallowed by another worm. The contents of the spore are saved from the effects of heat and drought by its resistent coat. The spores are oval in shape, with pointed ends like the outline of a boat. The contents of a spore become divided into eight minute elongated bodies which are called sporozoites. The formation of the coat and the division of the contents of the spores takes place within the cyst. Cysts containing ripe spores and others containing ripening spores can be found without difficulty. Those cysts which contain ripening spores also contain opaque granular material, the remnant of the endoplasm of the parent gametoeytes which is expended in nourishing the spores and in providing them with their hard coats.

Third Stage : The Sporozoite— There are eight sporozoites in every spore, each with a nucleus. When a spore is swallowed by a worm the sporozoites emerge and find their way into a special cell of the worm. They grow and become trophozoites, i.e. the first stage of the cycle.

Coccidia. — The Coccidia are another important group of Sporozoa which are parasitic within the cells of higher animals; they are not found in body cavities, nor do they enter blood cells ; they are oval in shape and motionless. Their method of reproduction is completely known, and is the same in principle as that of the Sporozoon which causes malaria in man ; it will therefore he described here, although it cannot be studied practically.

The Development of Coccidium schuhergi. — The method of reproduction differs from that of Monocystis in two respects. In Monocystis reproduction occurs only by the formation and union of gametes ; reproduction of this kind is called sexual. In Coccidium, however, there are two kinds of reproduction, sexual and asexual. Gametes play no part in asexual reproduction. The second respect in which Monocystis and Coccidium differ refers to the outward appearance of the gametes,

Iu Monocystis they appear alike, but in Cocci clium they are outwardly of two kinds, as in the vast majority of animals. Coccidium schubergi lives in the intestinal cells of the centipede Lithobius forjicatus.

The Trophozoite. — The trophozoite is a motionless oval body which lies wholly within the cell of the intestine of the host. It absorbs nourishment and grows ; having reached maturity it undergoes changes for the sake of reproduction. As we have already mentioned, these changes are of two kinds.

Asexual Reproduction {Schizogony) . — By this method the parasites become multiplied within the body of the centipede, so that more and more of the cells of the intestine become infected until five days after the entrance of the first parasite, when most of the intestine is destroyed and the host begins to die. When the trophozoite is mature its nucleus divides many times ; subsequently the protoplasm also divides, so that each division contains a portion of nucleus. The dividing trophozoite is called a schizont, and becomes resolved into a large number of elongated bodies called rnerozoites. The merozoites part company, fall into the gut of the host, and, by means of their pointed ends, penetrate the other healthy cells which line the gut; after penetrating a cell, they grow rapidly, and become mature trophozoites, which again become schizonts. The events therefore succeed one another in a cycle which is repeated again and again until few healthy cells remain in the centipede’s intestine ; other events then occur.

THE PEOTOZOA

Sexual Reproduction ( Sporogony ). — In this process resistent spores are formed which pass out of the centipede and into the earth, where they may be eaten by other centipedes. Spores are formed, as in Monocystis, by the union of gametes, which are of two kinds, large macrogametes and small microgametes. The gametes are produced in the following way. When the centipede is becoming exhausted by the repeated asexual cycles, the trophozoites become altered in appearance : some of them become macrogametes, others produce a number of microgametes. A macrogamete is a large bean-shaped cell ; it is merely an altered trophozoite. A microgamete is a minute filamentous body which moves actively, being impelled by two lashing threads or flagella, which are attached at either end of it. The microgametes are formed much in the same way as merozoites, by repeated divisions of a trophozoite, which is in this case called a gametocyte. An active microgamete moves towards the nearest macrogamete, which is usually lying passively in the gut of the host, and becomes fused with it. Thus the body called a zygote is formed, from which the sporozoites are derived. The zygote becomes covered with a tough, chitinous membrane, in which condition it can survive outside the body of the centipede. The zygote is a single cell with a single nucleus formed by the union of two nuclei, which may be called male and female : the nucleus of the macro - gamete is called the female nucleus, the other which was contributed by the microgamete is the male nucleus.

c

The nucleus of the zygote divides into four pieces, each of which becomes surrounded by a cyst wall, and further divides into two sporozoites. The zygote therefore resembles that of Monocystis in that it is covered with a tough, resistant wall, and breaks up into eight sporozoites, but in Coccidium the sporozoites are formed in couples. When a centipede swallows a zygote along with its food, the sporozoites issue from the cyst, enter the cells of the intestine and become mature trophozoites, which undergo schizogony. The foregoing description is of the main facts of the reproduction, but many other important details are known, which have not been mentioned.

A union of two unlike gametes, the one small and motile, the other large and passive, occurs in the reproduction of all the higher animals ; this sexual method of reproduction has long been known in them, but that a similar process occurs among the Protozoa is a more recent discovery.

Haemospokidia. — In this group the trophozoite is a parasite within the red blood cells of warm-blooded animals. The most important of them are Plasmodium and Laverania, which cause malarial fever in man. The life-history of these parasites is essentially like that of Coccidium. In it there are two separate cycles. The asexual cycle (schizogony) occurs within the blood of man. The malarial parasites use the red corpuscles of man in exactly the same way as Coccidium uses the intestinal cells of the centipede ; in other words, as a sporozoite they enter a red blood cell and feed upon it as a trophozoite ; when full grown they divide into a number of merozoites, which disperse and infect other healthy blood cells, just as the merozoites of Coccidium infect other healthy intestinal cells. This cycle of schizogony is repeated several times ; at each dispersal of merozoites the infected man gets an attack of ague. The asexual cycle of Plasmodium malariae occupies seventy-two hours, and causes quartan ague ; that of P. vivax occupies forty-eight hours, and therefore causes tertian ague. The trophozoites of these parasites are amoeboid; in this respect they differ from Coccidium. The method and meaning of the sexual cycle is essentially the same as in Coccidium. By schizogony, Coccidium cannot pass from centipede to centipede, neither can the malarial parasite pass from man to man. Only by the sexual cycle (sporogony) can a fresh host be infected. In both cases the sexual process results in the formation of sporozoites ; in Coccidium these lie in the earth for a time, and are therefore confined within a resistent cyst. In the malarial parasite the whole sexual cycle is passed in the body of a mosquito. Some of the trophozoites in a man’s blood become gametocytes. When the mosquito sucks the blood of an infected man, the gametocytes pass with the blood into the stomach or mid-gut of the mosquito, where unequal gametes are formed and unite to produce zygotes, which are soon after found projecting from the outer surface of the organ. Within each zygote a large number of sporozoites are formed, which leave the zygote cyst and find their way into the salivary glands of the mosquito and lie ready to infect a healthy man. The details of this process are usually treated as part of Pathology.

CILIATA

The Ciliata are Protozoa which possess cilia. Cilia are delicate threads of protoplasm which are attached to the surface of a living body, as our own hair is attached. Cilia are outgrowths of the protoplasm of living cells, and are in motion throughout the life of the cells which bear them. The movement of a cilium is like that of a man who bows rapidly, profoundly and repeatedly, always in the same direction, and becomes more slowly erect between each bow. A cilium is not independent in its movements. Cilia are placed in rows, those in a row move one after another, so that the movement in a ciliated surface looks like the waves which pass over a field of corn when the wind blows upon it. Cilia only occur on surfaces which are in contact with water, or covered by a film of moisture. The result of a vast number of cilia moving in the manner described must be either a flowing movement in the water, which is compelled to pass over the ciliated surface, or a movement of the ciliated body through the water, like a boat which is rowed by oars. The production of currents in water by means of cilia is frequent in the animal kingdom. Nearly all animals, except Arthropods, possess cilia on