The Slipper Animalcule or Paramecium
Suggestions.—Stagnant water often contains the paramecium as well as the amœba; or they may be found in a dish of water containing hay or finely cut clover, after the dish has been allowed to stand in the sun for several days. A white film forming on the surface is a sign of their presence. They may even be seen with the unaided eye as tiny white particles by looking through the side of the dish or jar. Use at first a ⅓ or ¼ in. objective. Restrict their movements by placing cotton fibres beneath the cover glass; then examine with ⅕ or ⅙ objective. Otherwise, study figures.
Shape and Structure.—The paramecium’s whole body, like the amœba’s, is only one cell. It resembles a slipper in shape, but the pointed end is the hind end, the front end being rounded (Fig. [14]). The paramecium is propelled by the rapid beating of numerous fine, threadlike appendages on its surface, called cilia (Latin, eyelashes) (Figs.). The cilia, like the pseudopods of the amœba, are merely prolongations of the cell protoplasm, but they are permanent. The separation between the outer ectoplasm and the interior granular endoplasm is more marked than in the amœba (Fig. [14]).
Fig. 14.—Paramecium, showing cilia, c.
Two contractile vacuoles, cv; the macronucleus, mg; two micronuclei, mi; the gullet (Œ), a food ball forming and ten food balls in their course from gullet to vent, a.
Fig. 15.
Nucleus and Vacuoles.—There is a large nucleus called the macronucleus, and beside it a smaller one called the micronucleus. They are hard to see. About one third of the way from each end is a clear, pulsating space (bb. Fig. [15]) called the pulsating vacuole. These spaces contract until they disappear, and then reappear, gradually expanding. Tubes lead from the vacuoles which probably serve to keep the contents of the cell in circulation.
Fig. 16.—Two Paramecia exchanging parts of their nuclei.
Feeding.—A depression, or groove, is seen on one side; this serves as a mouth (Figs.). A tube which serves as a gullet leads from the mouth-groove to the interior of the cell. The mouth-groove is lined with cilia which sweep food particles inward. The particles accumulate in a mass at the inner end of the gullet, become separated from it as a food ball (Fig. [14]), and sink into the soft protoplasm of the body. The food balls follow a circular course through the endoplasm, keeping near the ectoplasm.
Fig. 17.—Vorticella (or bell animalcule), two extended, one withdrawn.
Reproduction.—This, as in the amœba, is by division, the constriction being in the middle, and part of the nucleus going to each half. Sometimes two individuals come together with their mouth-grooves touching and exchange parts of their nuclei (Fig. [16]). They then separate and each divides to form two new individuals.
Fig. 18.—Euglena.
We thus see that the paramecium, though of only one cell, is a much more complex and advanced animal than the amœba. The tiny paddles, or cilia, the mouth-groove, etc., have their special duties similar to the specialized organs of the many-celled animals to be studied later.
Fig. 19.—Shell of a Radiolarian.
If time and circumstances allow a prolonged study, several additional facts may be observed by the pupil, e.g. Does the paramecium swim with the same end always foremost, and same side uppermost? Can it move backwards? Avoid obstacles? Change shape in a narrow passage? Does refuse matter leave the body at any particular place? Trace movement of the food particles.
Draw the paramecium.
Which has more permanent parts, the amœba or paramecium? Name two anatomical similarities and three differences; four functional similarities and three differences.
The amœba belongs in the class of protozoans called Rhizopoda “root footed.”
Other classes of Protozoans are the Infusorians (in the broad sense of the term), which have many waving cilia (Fig. [17]) or one whiplike flagellum (Fig. [18]), and the Foraminifers, which possess a calcareous shell pierced with holes (Fig. [19]). Much chalky limestone has been formed of their shells. To which class does the paramecium belong?
Protozoans furnish a large amount of food to the higher animals.
CHAPTER III
SPONGES
Suggestions.—In many parts of North America, fresh-water sponges may, by careful searching, be found growing on rocks and logs in clear water. They are brown, creamy, or greenish in colour, and resemble more a cushion-like plant than an animal. They have a characteristic gritty feel. They soon die after removal to an aquarium.
Fig. 21.—Fresh-water Sponge.
A number of common small bath sponges may be bought and kept for use in studying the skeleton of an ocean sponge. These sponges should not have large holes in the bottom; if so, too much of the sponge has been cut away. A piece of marine sponge preserved in alcohol or formalin may be used for showing the sponge with its flesh in place. Microscopic slides may be used for showing the spicules.
Fig. 22.—Section of fresh-water sponge (enlarged).
The small fresh-water sponge (Fig. [21]) lacks the more or less vaselike form typical of sponges. It is a rounded mass growing upon a rock or a log. As indicated by the Arrows, where does water enter the sponge? This may be tested by putting colouring matter in the water near the living sponge. Where does the water come out? (Fig. [22].) Does it pass through ciliated chambers in its course? Is the surface of the sponge rough or smooth? Do any of the skeletal spicules show on the surface? (Fig. [21].) Does the sponge thin out near its edge?
Fig. 23.—Eggs and SPICULES of fresh-water sponge (enlarged).
The egg of this sponge is shown in Fig. [23]. It escapes from the parent sponge through the osculum, or large outlet. As in most sponges, the first stage after the egg is ciliated and free-swimming.
Marine Sponges.—The grantia (Fig. [24]) is one of the simplest of marine sponges. What is the shape of grantia? What is its length and diameter? How does the free end differ from the fixed end? Are the spicules projecting from its body few or many?
Fig. 24.—Grantia.
Where is the osculum, or large outlet? With what is this surrounded? The osculum opens from a central cavity called the cloaca. The canals from the pores lead to the cloaca.
Buds are sometimes seen growing out from the sponge near its base. These are young sponges formed asexually. Later they become detached from the parent sponge.
Fig. 25.—Plan of a sponge.
Commercial “Sponge.”—What part of the complete animal remains in the bath sponge? Slow growing sponges grow more at the top and form tall, simple, tubular or vaselike animals. Fast growing sponges grow on all sides at once and form a complicated system of canals, pores, and oscula. Which of these habits of growth do you think belonged to the bath sponge? Is there a large hole in the base of your specimen? If so, this is because the cloaca was reached in trimming the lower part where it was attached to a rock. Test the elasticity of the sponge when dry and when wet by squeezing it. Is it softer when wet or dry? Is it more elastic when wet or dry? How many oscula does your specimen have? How many inhalent pores to a square inch? Using a probe (a wire with knob at end, or small hat pin), try to trace the canals from the pores to the cavities inside.
Fig. 26.—Bath Sponge.
Do the fibres of the sponge appear to interlace, or join, according to any system? Do you see any fringe-like growths on the surface which show that new tubes are beginning to form? Was the sponge growing faster at the top, on the sides, or near the bottom?
Fig. 27.—Bath Sponge.
Fig. 28.—Bath Sponge.
Burn a bit of the sponge; from the odor, what would you judge of its composition? Is the inner cavity more conspicuous in a simple sponge or in a compound sponge like the bath sponge? Is the bath sponge branched or lobed? Compare a number of specimens (Figs. [26], [27], [28]) and decide whether the common sponge has a typical shape. What features do their forms possess in common?
Fig 29.—Skeleton of a glass sponge.
Sponges are divided into three classes, according as their skeletons are flinty (silicious), limy (calcareous), or horny.
Some of the silicious sponges have skeletons that resemble spun glass in their delicacy. Flint is chemically nearly the same as glass. The skeleton shown in Fig. [29] is that of a glass sponge which lives near the Philippine Islands.
The horny sponges do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibres of spongin, a durable substance of the same chemical nature as silk (Figs. [30] and [31]).
The limy sponges have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form.
Fig. 30.—A horny sponge.
Fig. 31.—Section of horny sponge.
The commercial sponge, seen as it grows in the ocean, appears as a roundish mass with a smooth, dark exterior, and having about the consistency of beef liver. Several large openings (oscula), from which the water flows, are visible on the upper surface. Smaller holes (inhalent pores—many of them so small as to be indistinguishable) are on the sides. If the sponge is disturbed, the smaller holes, and perhaps the larger ones, will close.
The outer layer of cells serves as a sort of skin. Since so much of the sponge is in contact with water, most of the cells do their own breathing, or absorption of oxygen and giving off of carbon dioxide. Nutriment is passed on from the surface cells to nourish the rest of the body.
Reproduction.—Egg cells and sperm-cells are produced by certain cells along the canals. The egg cell, after it is fertilized by the sperm cell, begins to divide and form new cells, some of which possess cilia. The embryo sponge passes out at an osculum. By the vibration of the cilia, it swims about for a while. It afterwards settles down with the one end attached to the ocean floor and remains fixed for the rest of its life. The other end develops oscula. Some of the cilia continue to vibrate and create currents which bring food and oxygen.
The cilia in many species are found only in cavities called ciliated chambers. (Figs. [22], [32].) There are no distinct organs in the sponge and there is very little specialization of cells. The ciliated cells and the reproductive cells are the only specialized cells. The sponges were for a long time considered as colonies of separate one-celled animals classed as protozoans. They are, without doubt, many-celled animals. If a living sponge is cut into pieces, each piece will grow and form a complete sponge.
Fig. 32.—Microscopic plan of ciliated chamber. Each cell lining the chamber has a nucleus, a whip-lash, and a collar around base of whip-lash. Question: State two uses of whip-lash.
That the sponge is not a colony of one-celled animals, each like an amœba, but is a many-celled animal, will be realized by examining Fig. [32], which shows a bit of sponge highly magnified. A sponge may be conceived as having developed from a one-celled animal as follows: Several one-celled animals happened to live side by side; each possessed a threadlike flagellum (E, Fig. [32]) or whip-lash for striking the water. By lashing the water, they caused a stronger current (Fig. [25]) than protozoans living singly could cause. Thus they obtained more food and multiplied more rapidly than those living alone. The habit of working together left its impress on the cells and was transmitted by inheritance.
Cell joined to cell formed a ring; ring joined to ring formed a tube which was still more effective than a ring in lashing the water into a current and bringing fresh food (particles of dead plants and animals) and oxygen.
Few animals eat sponges; possibly because spicules, or fibres, are found throughout the flesh, or because the taste and the odour are unpleasant enough to protect them. Small animals sometimes crawl into sponges to hide. One sponge grows upon shells inhabited by hermit crabs. Moving of the shell from place to place is an advantage to the sponge, while the sponge conceals and thus protects the crab.
Special Report: Sponge “Fisheries.” (Localities; how sponges are taken, cleaned, dried, shipped, and sold.)