TECHNICAL NOTE.--Remove the alimentary ca.n.a.l, detaching it from the a.n.a.l end first, and working forward.

Cut the stomach open. Note an anterior portion, the _cardiac chamber_, and a smaller posterior portion, the _pyloric chamber_. Examine its inner surface. What do you find here? This structure is called the _gastric mill_. Food, which for the most part consists of any dead organic matter, is chewed by the "stomach-teeth" into fine bits, and is then pa.s.sed into the pyloric chamber. It is here that the digestive glands empty their secretion into the food. These glands have the same office as have the liver and pancreas combined in the toad, and so they are often called the _hepato-pancreas_. When the stomach has been removed there will be noted in the anterior portion of the body paired, flattened bodies, already mentioned, which connect with openings at the base of each of the antennae by means of wide thin-walled sacs, the _ureters_. These organs are the _kidneys_, or _green glands_. Their office is similar to that of the kidneys in the toad, namely, the elimination of waste from the body.

TECHNICAL NOTE.--Carefully remove all of the alimentary ca.n.a.l, digestive glands, and reproductive organs. This process will expose the floor of the cephalothorax. Now cut away from either side the h.o.r.n.y floor or bridge at the bottom of the cephalothorax.

If the specimen has not already been immersed, place it in clear water for further dissection.

The foregoing dissection will expose the _central nervous system_. It extends as a series of paired _ganglia_ connected by a double nerve-cord along the ventral median line from the sophagus to the last segment of the abdomen. From what points do the lateral nerves arise? Anteriorly the double nerve-cord divides, the two parts pa.s.sing upward on each side of the sophagus, where they again meet to form the _supra-sophageal ganglion_ or _brain_. Where do the nerves run which rise from the brain? What is the difference between the position of the central nervous system in the crayfish and in the toad?

Make a drawing of the nervous system.

Just beneath the nerve-cord note a blood-vessel extending the length of the body. This is the _sternal artery_, which arises from the posterior end of the heart and pa.s.ses ventrally at one side of the alimentary ca.n.a.l and between the nerve-cords. Here the sternal artery divides into an anterior and a posterior branch, from which lesser branches are given off to each one of the appendages. The various arteries running to all parts of the body finally pour out the blood into the body-cavity, where it flows freely in the s.p.a.ces among the various tissues and organs. After the blood has bathed the body tissues it flows to the gills on either side, pa.s.sing up the outer side of the gill through delicate thin-walled vessels, where it is oxygenated as has already been described. From the gills the purified blood flows back on the inner side through a large chamber, _sinus_, into the pericardium, through the ostia of the heart, whence it is driven into the arteries once more. This sort of a circulatory system in which the blood in places is not enclosed in a definite vessel is known as an _open system_. In the toad we find the blood in a _closed system_, i.e., arteries leading into capillaries which in turn lead into veins, in no case allowing the blood to pa.s.s freely through the s.p.a.ces of the body.

CHAPTER V

THE MODIFICATION OF ORGANS AND FUNCTIONS

=Differences between crayfish and toad.=--In the dissection of the crayfish one of the most important things in the study of zoology has been learned. It is plain that the crayfish has a body composed, like the toad's, of parts or organs, and that most of these organs, although differing much in appearance and actual structure from those of the toad, correspond to similarly named organs of the toad, and perform the same functions or processes, although with many striking differences, essentially in the same way as in the toad. But the structure of the body is very different in the two animals. The toad has an internal body skeleton to which the muscles are attached, and a soft, yielding, outer body-covering or skin; the crayfish has no internal skeleton, but has its body covered by a h.o.r.n.y, firm body-wall to which the muscles are attached. The toad has its main nervous chain lying just beneath the dorsal wall of the body; the crayfish has its main nervous chain lying just above the ventral wall of the body. The toad has lungs and takes up oxygen from the air of the atmosphere; the crayfish has gills and takes up oxygen from the air which is mixed with the water. The toad has a single pair of jaws; the crayfish has several pairs of mouth-parts. The toad has four legs fitted for leaping; the crayfish has numerous legs fitted for crawling or swimming. The crayfish's body is composed of a series of body-rings or segments; the toad's body is a compact apparently unsegmented ma.s.s.

The toad has eyes each with a single large lens and capable of moving in the head and of changing their shape and hence their focus; the crayfish's eyes are immovable and have a fixed focus, and are composed of hundreds of tiny eyes each with lens and special retina of its own.

And so a long list of differences might be gone through with.

=Resemblances between toad and crayfish.=--But on the other hand there are many resemblances--resemblances both in structure and life-processes or physiology. Both toad and crayfish have organs for the prehension of food, its digestion and its a.s.similation. And these organs, the organs of the digestive system, while differing in details are alike in being composed princ.i.p.ally of a long tube, the alimentary ca.n.a.l, running through the body, open anteriorly for the taking in of food, and open posteriorly for the discharge of indigestible useless matter. Both alimentary ca.n.a.ls are divided into various special regions for the performance of the various special processes connected with the digestion and a.s.similation of food. Each is adapted for the special kind of food which it is the habit of the particular animal to take. The two sets of organs are essentially alike and have the same essential function or life-process to perform. But this process differs in the details of its performance, and the organs which perform this function and which const.i.tute the digestive system of each are modified to suit the special habits or kind of life of the animal.

Both toad and crayfish have a heart with blood-vessels leading from it. In the case of the toad the heart is more complex than in the crayfish, and the system of blood-vessels is far more extensive and elaborate. But the heart and blood-vessels in both animals subserve the same purpose; their function is the circulation of the blood, this being the means by which oxygen and food are carried to all growing or working parts of the body, and by which carbonic acid gas and other poisonous waste products are brought away from these parts.

But this function differs somewhat in its performance in the two animals, and the organs which perform the function are correspondingly modified in structural condition.

Both toad and crayfish have organs for respiration, that is, for breathing in oxygen and breathing out carbonic acid gas. But the toad takes its oxygen from the atmosphere about it; its respiratory organs are the lungs, the sac-like tube leading to the mouth, and the external openings for the ingress and exit of the gases. The crayfish, living mostly in the water, takes its oxygen from the air which is mechanically mixed with the water. Its respiratory organs are its gills. There is a great difference, apparently, in the structural conditions of the organs of respiration in the two animals. As a matter of fact the difference is less great than, at first sight, appears to be the case. The lungs of the toad are composed primarily of a thin membrane, in the form of a sac, richly supplied with blood-vessels. Air is brought to this thin respiratory membrane and by osmosis the oxygen pa.s.ses through the membrane and through the thin walls of the fine blood-vessels, and is taken up by the blood. At the same time the carbonic acid gas brought by the blood to the lungs from all parts of the body is given up by it and pa.s.ses through the membranes in order to leave the body. The air comes in contact with the respiratory membrane (which is situated inside the body) by means of a system of external openings and a conducting chamber, and by these same openings and chamber the carbonic acid gas leaves the body.

In the crayfish the gills are nothing else than a large number of small flattened sacs each composed of a thin membrane richly supplied with blood-vessels. This respiratory membrane is not, in the crayfish, situated inside the body, but on the outer surface, although protected by being in a sort of pocket with a covering flap, and it comes into immediate contact with the air held in the water which freely bathes the gills. By osmosis the oxygen of this air pa.s.ses in through the gill-membranes, while the carbonic acid gas brought by the blood pa.s.ses out through them. Exactly the same exchange of gases is accomplished as in the toad. But because of the great difference in the conditions of life of the toad and crayfish, one living in water, the other living out of water, the character of the performance of the function of respiration, and correspondingly the structural condition of the organs performing this function, are strikingly different.

=Modification of functions and structure to fit the animal to special conditions of its life.=--As has been done with the organs of digestion, circulation, and respiration, so we might compare the other organs of the crayfish and the toad. There would be found not only many very marked differences between organs which have the same general function in the two animals, but we should find also numerous organs in the toad which are not present at all in the crayfish, and conversely; and this means, of course, that the toad can do numerous things, perform numerous functions, which the crayfish cannot, and, conversely, that the crayfish does some things which the toad cannot.

But both of these animals agree in possessing in common the capability of performing those processes such as taking food, breathing, reproducing, etc., to which attention has been called as being indispensable to all animal life. These processes, however, are performed by the two animals in different ways and the organs for the performance of these processes, although at very bottom essentially alike, are in outer and superficial details of position, appearance and general structure markedly different. Animals are fitted to live in different places amid different surroundings by having their bodies modified and the performance of their life-processes modified to suit the special conditions of their life.

=Vertebrate and invertebrate.=--In selecting the toad and the crayfish as the first animals to study and to compare with each other, we have chosen representatives of the two great groups into which the complexly organized animals are divided, viz., the group of backboned or vertebrate animals, and the group of backboneless or invertebrate animals. To the vertebrates belong all those which have an internal bony skeleton (and a few without such a skeleton) and which have also an arrangement of body-organs on the general plan of the toad's body. A conspicuous feature of this arrangement is the situation of the spinal cord or main great nerve-trunk along the back or dorsal wall of the animal, and inside of a backbone. All the fishes, batrachians (frogs, toads, salamanders, etc.), reptiles (snakes, lizards, alligators, etc.), birds, and mammals (quadrupeds, whales, seals, etc.) belong to the vertebrates. The backboneless or invertebrate animals have no internal bony skeleton and have their main nerve-trunk usually along the ventral wall of the body, sometimes in a circle around the mouth, but never in a backbone. To the invertebrates belong all insects, lobsters, crabs, clams, squids, snails, worms, starfishes and sea-urchins, corals and sponges, altogether a great host of animals, mostly small.

CHAPTER VI

AMBA AND PARAMCIUM

LABORATORY EXERCISE

=Amba.=--TECHNICAL NOTE.--_Ambae_ are found in stagnant pools of water on the dead leaves, sticks and slime at the bottom. To obtain them, collect slime and water from various puddles in separate bottles and take them to the laboratory. Place a small drop of slime on a slide under a cover-gla.s.s. Examine under the low power first and note any small transparent or opalescent objects in the field. Examine these objects with the higher power and note that some are mere granular jelly-like specks, which slowly (but constantly) change their form. These are _Ambae_.

A teacher of zoology recommends the following method of obtaining a large supply of _Ambae_: "For rearing _Ambae_ place two or three inches of sand in a common tub, which is then filled with water and placed some feet from a north window; three or four opened mussels, with merest trace of the mud from the stream in which they are taken, are partially buried in the sand and a handful of _Nitella_ and a couple of crayfish cut in two are added; as decomposition goes on a very gentle stream is allowed to flow into the tub, and after from two to four weeks abundant _Ambae_ are to be found on the surface of the sand and in the sc.u.m on the sides of the tub; small _Ambae_ appear at first, and later the large ones."

Having found an _Amba_ (fig. 5) note its irregular shape, and if it moves actively observe its method of moving. How is this accomplished?

The viscous, jelly-like substance which composes the whole body of an _Amba_ is called _protoplasm_. The little processes which stick out in various directions are the "false feet" (_pseudopodia_). Note that the outer portion, the _ectosarc_, of the protoplasmic body is clear, while the inner, the _endosarc_, is more or less granular in structure. Has _Amba_ a definite body-wall? Do the pseudopodia protrude only from certain parts of the body? Within the endosarc note a clear globular spot which contracts and expands, or pulsates, more or less regularly. This is the _contractile vacuole_. Note the small granules which move about within the endosarc. These are food-particles which have been taken in through the body-wall. Note how pseudopodia flow about food-particles in the water and how these are digested by the protoplasm. If an _Amba_ comes into contact with a particle of sand, note how it at once retreats. Note within the endosarc an oval transparent body which shows no pulsations. This is the _nucleus_, a very complex little structure of great importance in the make-up of _Amba_.

[Ill.u.s.tration: FIG. 5.--_Amba_ sp.; showing the forms a.s.sumed by a single individual in four successive changes. (From life.)]

Note that _Amba_ has no mouth or alimentary ca.n.a.l; no nostrils or lungs, no heart or blood-vessels, no muscles, no glands. It is an animal body not made up of distinct organs and diverse tissues. Its whole body is a simple minute speck of protoplasm, a single animal cell. But it takes in food, it moves, it excretes waste matter from the body, is sensitive to the touch of surrounding objects, and, as we may be able to see, it can reproduce itself, i.e., produce new _Ambae_. _Amba_ is the simplest living animal.

It is only rarely that we can find an _Amba_ actually reproducing.

The process, in its gross features, is very simple. First the _Amba_ draws in all of its pseudopodia and remains dormant for a time. Next, certain changes take place in the nucleus, which divides into equal portions, one part withdrawing to one end of the protoplasmic body, the other to the opposite end. Soon the body protoplasm itself begins to divide into two parts, each part collecting about its own half of the nucleus. Finally the two halves pull entirely away from each other and form two new _Ambae_, each like the original, but only half as large. This is the simplest kind of reproduction found among animals.

_Ambae_ continue to live and multiply as long as the conditions surrounding them are favorable. But when the pond dries up the _Ambae_ in it would be exterminated were it not for a careful provision of nature. When the pond begins to dry up each _Amba_ contracts its pseudopodia and the protoplasm secretes a h.o.r.n.y capsule about itself.

It is now protected from dry weather and can be blown by the winds from place to place until the rains begin, when it expands, throws off the capsule and commences active life again in some new pond.

=The Slipper Animalcule= (_Paramcium_ sp.)--_Technical Note_.--_Paramcia_ can be secured in most pond water where leaves or other vegetation are decaying. However, if specimens are not readily secured place some hay or finely cut dry clover in a gla.s.s dish, cover with water and leave in the sun for several days. In this mixture specimens will develop by thousands. Place a drop of water containing _Paramcia_ on a slide with cover-gla.s.s over it.

Using a low power, note the many small animals darting hither and thither in the field. Run a thin mixture of cherry gum in water under the cover-gla.s.s. In this mixture they can be kept more quiet and be better studied.

How does _Paramcium_ (fig. 6) differ from _Amba_ in form and movement? Has the body an anterior and a posterior end? The delicate, short, thread-like processes, on the surface of the body, which beat about very rapidly in the water are called _cilia_, and they are simply fine prolongations of the body protoplasm. What is their function? Note a fine _cuticle_ covering the body. Note also many minute oval sacs lying side by side in the ectosarc. These are called _trichocysts_ and from each a fine thread can be thrust out.

Note on one side, beginning at the anterior end, the _buccal groove_ leading into the interior through the _gullet_. Observe also that by the action of the cilia in the buccal groove food-particles are swept into the gullet. Rejected or waste particles are ejected from the body occasionally. Where? Note about midway of the _Paramcium_ an ovoid body with a smaller oval one attached to its side, the former being the _macronucleus_, the latter the _micronucleus_. Note that there are two contractile vacuoles in the _Paramcium;_ also that the food-vacuoles have a definite course in their movement inside the endosarc.

Make a drawing of a _Paramcium_.

In comparing _Paramcium_ with _Amba_ it is apparent that the body of the first is less simple than that of the second. The definite opening for the ingress of food, the two nuclei, the fixed cilia, and the definite cell-wall giving a fixed shape to the body, are all specializations which make _Paramcium_ more complex than _Amba_. But the whole body is still composed of a single cell, and there is, as in _Amba_, no differentiation of the body-substance into different tissues, and no arrangement of body-parts as systems of organs.

_Paramcium_ may occasionally be found reproducing. This process takes place very much as in _Amba_. The animal remains dormant for a while, the micronucleus then divides, the macronucleus elongates and finally divides in two, the protoplasm of the body becomes constricted into two parts, each part ma.s.sing itself about the withdrawn halves of the macro- and micro-nuclei, and lastly the whole breaks into two smaller organisms which grow to be like the original. After multiplication or reproduction has gone on in this way for numerous generations (about one hundred), a fusion of two _Paramcia_ seems necessary before further divisions take place. This process of fusion, called _conjugation_, may be noted at some seasons. Two _Paramcia_ unite with their buccal grooves together, part of the macronucleus and micronucleus of each pa.s.ses over to the other, and the mixed elements fuse together to form a new macro- and micronucleus in each half. The conjugating _Paramcia_ now separate, and each divides to form two new individuals.

[Ill.u.s.tration: FIG. 6.--_Paramcium_ sp.; buccal groove at right.

(From life.)]

CHAPTER VII

THE SINGLE-CELLED ANIMAL BODY.--PROTOPLASM AND THE CELL

=The single-celled body.=--The study of _Amba_ and _Paramcium_ has made us acquainted with an animal body very different from that of the toad or the crayfish. These extraordinarily minute animals have a body so simple in its composition, compared with the toad's, that if the toad's body be taken for the type of the animal body, _Amba_ might readily be thought not to be an animal at all. The body of _Amba_ is not composed of organs, each with a particular function or work to perform. Whatever an _Amba_ does is done, we may say, with its whole body. But as we learn the things that this formless viscid speck of matter does, we see that it is truly an animal; that it really does those things which we have learned are the necessary life-processes of an animal. _Amba_ takes up and digests food composed of organic particles; it has the power of motion; it knows when its body comes in contact with some external object, that is, it can feel or has the power of sensation. _Amba_ takes in oxygen and gives out carbonic acid gas, and it can produce new individuals like itself, that is, it has the power of reproduction. But for the performance of these various life-processes or functions it has no special parts or organs, no mouth or alimentary ca.n.a.l, no lungs or gills, no legs, no special reproductive organs. We have here to do with one of the "simplest animals." With a minute, organless, soft speck of viscous matter called protoplasm for a body, the simplest structural condition to be found among living beings, _Amba_ nevertheless is capable of performing, in the simplest way in which they may be performed, those processes which are essential to animal life.

_Paramcium_ has a body a little less simple than _Amba_. The food-particles are taken into the body always at a certain spot; this might be spoken of as a mouth. And the body has some special locomotory organs, if they may be so called, in the presence of the cilia. The body, too, has a definite shape or form. But, as in _Amba_ there is no alimentary ca.n.a.l, nor nervous system, nor respiratory system, nor reproductive system. The whole body feels and breathes and takes part in reproduction.

A long jump has been made from the toad and crayfish to _Amba_ and _Paramcium_; from the complex to the simplest animals. But, as will later be seen, the great difference between the bodies of these simplest animals and those of the highly complex ones is only a difference of degree; there are animals of all grades and stages of structural condition connecting the simplest with the most complex.

When animals are studied systematically, as it is called, we begin with the simplest and proceed from them to the slightly complex, from these to the more complex, and finally to the most complex. There are hundreds of thousands of different kinds of animals, and they represent all the degrees of complexity which lie between the extremes we have so far studied.

=The cell.=--The characteristic thing about the body of _Amba_ and _Paramcium_ and the other "simplest animals"--for there are many members of the group of "simplest animals," or Protozoa--is that it is composed, for the animal's whole lifetime, of a single cell. A cell is the structural unit of the animal body. As will be learned in the next exercise, the bodies of all other animals except the Protozoa, the simplest animals, are composed of many cells. These cells are of many kinds, but the simplest kind of animal cell is that shown by the body of an _Amba_, a tiny speck of viscous, nearly colorless protoplasm without fixed form. The protoplasm composing the cell is differentiated to form two parts or regions of the cell, an inner denser part, called the nucleus, and an outer clearer part, called the cytoplasm. Sometimes, as in the _Paramcium_, the cell is enclosed by a cell-wall which may be simply a denser outer layer of the cytoplasm, or may be a thin membrane secreted by the protoplasm. Thus the cell is not what its name might lead us to expect, typically cellular in character; that is, it is not (or only rarely is) a tiny sac or box of symmetrical shape. While the cell is composed essentially of protoplasm, yet it may contain certain so-called cell-products, small quant.i.ties of various substances produced by the life-processes of the protoplasm. These cell-products are held in the protoplasmic body-ma.s.s of the cell, and may consist of droplets of water or oil or resin, or tiny particles of starch or pigment, etc. The cell cannot be said to be composed of organs, because the word organ, as it is commonly used in the study of an animal, is understood to mean a part of the animal body which is composed of many cells. But the single cell can be somewhat differentiated into parts or special regions, each part or special region being especially a.s.sociated with some one of the life-processes. In _Paramcium_, for example, the food is always taken in through the so-called mouth-opening; the fine protoplasmic cilia enable the cell to swim freely in the water, the waste products of the body are always cast out through a certain part, and so on. But this is a very simple sort of differentiation, and the whole body is only one of those structural units, the cells, of which so many are included in the body of any one of the complex animals.

=Protoplasm.=--The protoplasm, which is the essential substance of the typical animal cell and hence of the whole animal body, is a substance of very complex chemical and physical make-up. No chemist has yet been able to determine its exact chemical const.i.tution, and the microscope has so far been unable to reveal certainly its physical characters.