Elementary Zoology Part 3

The most important thing known about the chemical const.i.tution of protoplasm is that there are always present in it certain complex alb.u.minous substances which are never found in inorganic bodies. And it is certain that it is on the presence of these substances that the power possessed by protoplasm of performing the fundamental life-processes depends. Protoplasm is the primitive physical basis of life, but it is the presence of the complex alb.u.minous substances in it that makes it so.

The physical const.i.tution of protoplasm seems to be that of a viscous liquid containing many fine globules of a liquid of different density and numerous larger globules of a liquid of still other density. Some naturalists believe the fine globules to be solid grains, while still others believe that numerous fine threads of dense protoplasm lie coiled and tangled in the clearer, viscous protoplasm. But the little we know of the physical structure of protoplasm throws almost no light on the remarkable properties of this fundamental life-substance.

CHAPTER VIII

CELLULAR STRUCTURE OF THE TOAD (OR FROG)

LABORATORY EXERCISE

=The blood.=--TECHNICAL NOTE.--The blood of a frog can be studied as it flows through the small vessels in the membranes between the toes while the animal is alive. Place a frog on a small flat board which has had a hole cut near one end, and with a piece of cloth bind it to the board. Spread the web between two toes over the hole in the board and keep it in place with pins. This done, examine the distended web under the compound microscope first with low then with higher power, and observe the blood-vessels and the blood circulating in them. For a further study of the blood kill a toad or frog and place a drop of the blood on a slide with a cover-gla.s.s over it.

Put the prepared slide under the microscope and note that the blood, which as seen with the unaided eye appears to be a red fluid, is made up of a great many yellowish elliptical disks or _cells_, the _blood-corpuscles_, floating in a liquid, the _blood-plasma_. Here and there you may notice _amboid blood-corpuscles_. These are irregular-shaped cells which move about by thrusting out pseudopodia.

They look like some of the unicellular animals, as the _Amba_. Can you distinguish a nucleus and cell-wall in the blood-cells?

Make drawings of these blood-cells.

=The skin.=--TECHNICAL NOTE.--Keep a live toad or frog in water for some time and note if its skin becomes loose or begins to slip away. If the outer skin, epidermis, comes off, take some of the shed skin and wash it in water, then stain for three or four minutes in a solution of methyl-green and acetic acid (see p.

451). Cut the pieces of stained skin into small bits and examine one of these under the microscope.

With the low power of the microscope you will note that the skin is made up of a great many flat _cells_ placed edge to edge. Each one has its cell-wall and a central darkly stained nucleus.

Make a drawing of a portion of the toad's skin.

=The liver.=--TECHNICAL NOTE.--Cut through the fresh liver of a toad, and with a knife-blade sc.r.a.pe from the cut surface some of the liver-cells and place them on a slide with cover-gla.s.s.

Examine under the microscope and observe many polygonal _cells_. Place some of the methyl-green acetic stain under the cover-gla.s.s and note, after the cells are stained, that they have definite boundaries and a central nucleus.

Draw some of these scattered liver-cells.

=The muscles.=--TECHNICAL NOTE.--Take a piece of intestine from a freshly killed toad, wash it thoroughly and place it in a concentrated solution of salicylic acid in 70% alcohol for 24 hours, then gradually heat until about the boiling-point, when the muscles will fall to pieces. Transfer the preparation to a watch-crystal and tease small bits of isolated muscle with dissecting-needles. Place some of the teased muscle-fibres on a slide, cover with cover-gla.s.s, and add a drop of the methyl-green acetic acid.

Note the small spindle-shaped _muscle-fibres_. Each one of these fibres is a _cell_ possessing all of the structures common to cells, namely, cell-wall, nucleus, etc.

Make a drawing of a few isolated fibres of muscle.

From this study of some of the tissues in a toad it will be noted that in the first case we had in the blood separate cells which moved about freely in the plasma. In the second case, that of the epidermis, the cells are fixed edge to edge, thus forming a thin tissue; while in the third and fourth cases, that of the liver and muscle, the cells are not only placed edge to edge, but aggregated into vast ma.s.ses or bundles, in one case to form the liver and in the other case a muscle. The entire body of the toad is built up of a colony of simple units (cells) combined in various forms to make all the various tissues and organs.

CHAPTER IX

THE MANY-CELLED ANIMAL BODY.--DIFFERENTIATION OF THE CELL

=The many-celled animal body.=--In the study of certain of the tissues and organs of the toad we have learned that the body of this animal is composed of many cells, thousands and thousands of these microscopic structural units being combined to form the whole toad. This many-celled or multicellular condition of the body is true of all the animals except the simplest, the unicellular Protozoa. Corals, starfishes, worms, clams, crabs, insects, fishes, frogs, reptiles, birds, and mammals, all the various kinds of animals in which the body is composed of organs and tissues, agree in the multicellular character of the body, and may be grouped together and called the many-celled animals in contrast to the one-celled animals. This division is one which is recognized by many systematic zoologists as being more truly primary or fundamental than the division of animals into Vertebrates and Invertebrates. The one-celled animals are called Protozoa, and the many-celled animals Metazoa.

=Differentiation of the cell.=--It is apparent at first glance that the cells which compose the body of a many-celled animal are not like the simple primitive cell which makes up the body of the _Amba_, nor are they like the more complexly arranged cell of the _Paramcium_.

Nor are they all like each other. The cells in the toad's blood are of two kinds, the white blood-cells, which are very like the body of _Amba_, and the elliptical disk-like red blood-cells. The cells composing the muscles are, moreover, like neither kind of blood-cells, and the cells of which the liver is composed are not like the cells of the muscles. That is, there are many different kinds of cells in the body of a many-celled animal. While the single cell which composes the whole body of the _Amba_ is able to do all the things necessary to maintain life, the various cells in the body of a complex animal are differentiated or specialized, certain cells devoting themselves to a certain function or special work, and others to other special functions. For example, the cells which compose the organs of the nervous system, the brain, ganglia, and nerves, devote themselves almost exclusively to the function of sensation, and they are especially modified for this purpose. The highly specialized nerve-cells resemble very little the primitive generalized body-cell of _Amba_. The muscle-cells of the complex animal body have developed to a high degree that power of contraction which is possessed, though in but slight degree, by _Amba_. These muscle-cells have for their special function this one of contraction, and ma.s.sed together in great numbers they form the strongly contractile muscular tissue and muscles of the body on which the animal's power of motion depends. The cells which line certain parts of the alimentary ca.n.a.l are the ones on which the function of digestion chiefly rests. And so we might continue our survey of the whole complex body. The point of it all is that the thousands of cells which compose the many-celled animal body are differentiated and specialized; that is, have become changed or modified from the generalized primitive amboid condition, so that each kind of cell is devoted to some special work or function and has a special structural character fitting it for its special function. In the Protozoan body the single cell can perform and does perform all the functions or processes necessary to the life of the animal. In the Metazoan body each cell performs, in co-operation with many other similar cells, some one special function or process. The total work of all the cells is the living of the animal.

CHAPTER X

HYDRA

LABORATORY EXERCISE

TECHNICAL NOTE.--_Hydra_ lives in fresh water, attached to stones, sticks, or decayed leaves. It can be found in most open fresh-water ponds not too stagnant, often attached to _Chara_.

There are two species occurring commonly, _H. viridis_, the green _Hydra_, and _H. fuscus_, the brown or flesh-colored _Hydra_. Both are very small forms and have to be looked for carefully.

Specimens should be brought to the laboratory, put into a large dish of water and left in the light. _Hydra_ is best studied alive. Place a living specimen attached to a bit of weed in a watch-crystal filled with water or on a slide with plenty of water and examine with the low power of the microscope.

Note the cylindrical body (fig. 7, _A_, _B_) with its flat basal attachment and _radial tentacles_ (varying in number) which crown the upper end and surround the centrally located _mouth_. Note the movements of _Hydra_, its powers of contraction, and method of taking in food.

TECHNICAL NOTE.--To feed _Hydra_, place very small "water-fleas"

(_Daphnia_ sp.) in the water with it.

Observe the method by which "water-fleas" are taken into the mouth.

Food is caught on stinging cells (to be studied later) and conveyed to the mouth by the tentacles. Note that the cylindrical body encloses a cavity, the _digestive cavity_. How is this connected with the exterior? If _Hydra_ captures prey too large or is no longer hungry, the prey is released.

[Ill.u.s.tration: FIG. 7.--A, _Hydra fusca_, with expanded body and a budding individual; B, _H. fusca_, contracted; C, _H. fusca_, part of outer surface of a tentacle, greatly magnified. (A and B drawn from live specimens, C, from a preparation) D, _Grantia_ sp. (a sponge), three individuals; E, _Grantia_ sp., longitudinal section; F, _Grantia_ sp., spicules. (D, E, and F drawn from preserved specimens.)]

TECHNICAL NOTE.--Place small slips of paper on the slide near the _Hydra_, put cover-gla.s.s over the whole, and examine with the low power of the microscope.

Note that the whole animal is made up of cells closely joined. Are the cells in the tentacles all alike? Note nodule-like projections above some of the cells; these are _stinging cells_, or _cnidoblasts_. In some cases a small hair-like process, the trigger hair or _cnidocil_, may be seen projecting above the surface of the cell. Note in some of the tentacles dark-colored particles. These are food-particles which have been taken through the mouth into the digestive cavity and have pa.s.sed thence into the tentacles. The central digestive cavity communicates freely with the cavities in the tentacles, for the tentacles are merely ev.a.g.i.n.ations of the body-wall.

Make drawings of the _Hydra_ expanded and of the same individual contracted.

TECHNICAL NOTE.--From the preparation which you have under the microscope pull out the slips of paper, thus letting the cover-gla.s.s drop down on the specimen. With a small pipette put a drop of anilin-acetic stain (see p. 451) on the slide at one side of the cover-gla.s.s and with a piece of filter-paper draw the water through from the other side of the cover-gla.s.s. When the stain is diffused press down the cover-gla.s.s gently and examine the tentacles first under a low power of the microscope, then under a high one.

Note the distortion that the animal has undergone through the action of the reagent. Observe the cnidoblasts of the tentacles and note that many of them have thrown out long whip-like processes (fig. 7, _C_).

On what parts of the body do the cnidoblasts occur? Carefully examine one of the cnidoblasts which has been discharged and note a clear transparent bag-like structure within, the _nematocyst_, to which is attached the long whip-like process. In another cnidoblast cell which has not been discharged note that the whip-like process is coiled about inside of the bag-like structure. The whole apparatus is like the inturned finger of a glove which can be blown out by pressure from the inside. The mechanism is simple. The cnidocil or trigger-hair is touched by some animal, an impulse is conveyed to the delicate fibres interspersed among the cells (nerve-cells) which stimulate the cnidoblast cell, whereupon there is a contraction of the contents and, the cnidoblast being compressed, the inverted whip-like process turns wrong side out and impales the animal on its points or barbs.

TECHNICAL NOTE.--The teacher should be provided with microscopical sections, both transverse and longitudinal, of the _Hydra_ stained in some good general stain (haematoxylin or borax carmine). If the teacher has no means of making such preparations, they may be procured from dispensers of microscopical supplies.

From the cross-section of the _Hydra_ make out the general structure of the body. Note that it is a hollow cylinder consisting of two well-defined layers of cells, an outside _ectoderm_ layer and an inner _endoderm_ layer. Between these two is yet another thin non-cellular layer called the _mesogla_.

Thus it will be seen that _Hydra_ is made up of two layers of cells, the outer ectoderm or skin, which is specialized to perform the office of capturing prey as well as that of protection, and the inner endoderm, which surrounds the digestive cavity and performs the function of digestion. The endoderm lines the body-cavity, particles taken in as food being digested by certain digestive cells which thrust out amboid processes and ingest particles of food. Other cells in the endoderm have long flagellate processes which vibrate back and forth in the digestive cavity, thereby creating currents in the water containing food-particles.

Note, in a cross-section, that there are small ovoid or cuboid cells at the bases of the large ectoderm cells. These are the _interst.i.tial cells_. Some of the interst.i.tial cells become modified and pushed up between the ectoderm cells to form cnidoblast cells. Many of the endoderm as well as ectoderm cells have muscle-processes which spread out from the base of the cell and which serve to contract and expand the body.

TECHNICAL NOTE.--In the specimens which have been collected perhaps two methods of reproduction will be observed. Place healthy _Hydrae_ in a wide-mouthed jar in the sunlight with plenty of water and food. In a few days active budding will take place.