CHAPTER VI

WORMS

SUGGESTIONS:--Earthworms may be found in the daytime after a heavy rain, or by digging or turning over planks, logs, etc., in damp places. They may be found on the surface at night by searching with a lantern. Live specimens may be kept in the laboratory in a box packed with damp (not wet) loam and dead leaves. They may be fed on bits of fat meat, cabbage, onion, etc., dropped on the surface. When studying live worms, they should be allowed to crawl on damp paper or wood. An earthworm placed in a glass tube with rich, damp soil, may be watched from day to day.

=External Features.=--Is the body _bilateral_? Is there a _dorsal_ and _ventral_ surface? Can you show this by a test with a live worm? Do you know of an animal with dorsal and ventral surface, but not bilateral?

[Illustration: FIG. 69.--An Earthworm.]

Can you make out a head? A head end? A neck? Touch the head and test whether it can be made to crawl backwards. Which end is more tapering? Is the mouth at the tip of the head end or on the upper or lower surface? How is the _vent_ situated? Its shape? As the worm lies on a horizontal surface, is the body anywhere flattened? Are there any _very_ distinct divisions in the body? Do you see any _eyes_?

=Experiment= to find whether the worm is sensitive (1) to _touch_, (2) to _light_, (3) to strong _odors_, (4) to irritating liquids. Does it show a sense of _taste_? The experiments should show whether it avoids or seeks a bright light, as a window; also whether any parts of the body are especially sensitive to touch, or all equally sensitive. What effect when a bright light is brought suddenly near it at night?

Is _red blood_ visible through the skin? Can you notice any _pulsations_ in a vessel along the back? Do all earthworms have the same number of _divisions_ or rings? Compare the size of the rings or segments. Can it crawl faster on glass or on paper?

[Illustration: FIG. 70.--MOUTH AND SETÆ.]

[Illustration: FIG. 71.--EARTHWORM, mouth end above.]

A magnifying glass will show on most species tiny bristle-like projections called _setæ_. How are the setæ arranged? (_d_, Fig. 70.) How many on one ring of the worm? How do they point? Does the worm feel smoother when it is pulled forward or backward between the fingers? Why? Are setæ on the lower surface? Upper surface? The sides? What is the use of the setæ? Are they useful below ground? Does the worm move at a uniform rate? What change in form occurs as the front part of the body is pushed forward? As the hinder part is pulled onward? How far does it go at each movement? At certain seasons a broad band, or ring, appears, covering several segments and making them seem enlarged (Fig. 71). This is the _clitellum_, or _reproductive girdle_. Is this girdle nearer the mouth or the tail?

=Draw= the exterior of an earthworm.

=Dorsal and Ventral Surfaces.=--The earthworm always crawls with the same surface to the ground; this is called the _ventral_ surface, the opposite surface is the _dorsal_ surface. This is the first animal studied to which these terms are applicable. What are the ventral and dorsal surfaces of a fish, a frog, a bird, a horse, a man?

=The name “worm”= is often carelessly applied to various crawling things in general. It is properly applied, however, only to _segmented animals without jointed appendages_. Although a caterpillar crawls, it is not a worm for several reasons. It has six jointed legs, and it is not a developed animal, but only an early stage in the life of a moth or butterfly. A “grubworm” also has jointed legs (Fig. 167). It does not remain a grub, but in the adult stage is a beetle. A worm never develops into another animal in the latter part of its life; its setæ are not jointed.

[Illustration: FIG. 72.--FOOD TUBE of earthworm. (Top view.)]

[Illustration: FIG. 73.--FOOD TUBE AND BLOOD VESSELS of earthworm showing the ring-like hearts. (Side view.)]

=The Food Tube.=--The earthworm has no teeth, and the food tube, as might be inferred from the form of the body, is simple and straight. On account of slight variation in size and structure, its parts are named the pharynx (muscular), gullet, crop, gizzard (muscular), and the long intestine extending through the last three fourths of its body (Fig. 72). The functions of the parts of the food tube are indicated by their names.

[Illustration: FIG. 74.]

=Circulation.=--There is a _large dorsal_ blood vessel above the food tube (Fig. 73). From the front portion of this tube arise several large tubular rings or “hearts” which are contractile and serve to keep the blood circulating. They lead to a _ventral vessel_ below the food tube (Fig. 74). The blood is red, but the _coloring matter_ is in the liquid, not in the blood cells.

=Nervous System.=--Between the ventral blood vessels is a _nerve cord_ composed of two strands (see Fig. 75). There is a slight swelling, or _ganglion_, on each strand, in each segment (Fig. 75). The strands separate near the front end of the worm, and a branch goes up each side of the gullet and enters the two pear-shaped _cerebral ganglia_, or “brain” (Fig. 75).

[Illustration: FIG. 75.--GANGLIA NEAR MOUTH and part of nerve chain of earthworm.]

=Food.=--The earthworm eats earth containing organic matter, the inorganic part passing through the vent in the form of circular casts found in the morning at the top of the earthworm’s hole. What else does it eat?

The earth worm needs no teeth, as it excretes through the mouth an _alkaline fluid_ which softens and partly digests the food before it is eaten. When this fluid is poured out upon a green leaf, the leaf at once turns brown. The starch in the leaf is also acted upon. The snout aids in pushing the food into the mouth.

=Kidneys.=--Since oxidation is occurring in its tissues, and impurities are forming, there must be some way of _removing impurities from the tissues_. The earthworm does not possess one-pair organs like the kidneys of higher animals to serve this purpose, but it has numerous pairs of small tubular organs called _nephridia_ which serve the purpose. Each one is simply a tube with several coils (Fig. 76). There is a pair on the floor of each segment (Fig. 76). Each nephridium has an inner open end within the body cavity, and its outer end opens by a pore on the surface between the setæ (Fig. 78). The nephridia absorb waste water from the liquid in the _celom_, or body cavity surrounding the food tube, and convey it to the outside.

[Illustration: FIG. 76.--TWO PAIRS OF NEPHRIDIA.]

=Respiration.=--The skin of the earthworm is moist, and the blood capillaries approach so near to the surface of the body that the oxygen is constantly passing in from the air, and carbon dioxid passing out; hence it is constantly breathing through all parts of its skin. _It needs no lungs_ nor special respiratory organs of any kind.

[Illustration: FIG. 77.--Sperm (_sp_) and egg glands (_es_) of earthworm.]

=Reproduction.=--When one individual animal produces both sperm cells and egg cells, it is said to be hermaphrodite. This is true of the earthworm. The egg cell is always fertilized, however, not by the sperm cells of the same worm, but by sperm cells formed by another worm. The openings of these ova or _egg glands_ consist of two pairs of small pores found on the ventral surface of the fourteenth and fifteenth segments in most species (see Fig. 77). There are also two pairs of small _receptacles_ for temporarily holding the _foreign sperm cells_. One pair of the openings from these receptacles is found (with difficulty) in the wrinkle behind the ninth segment (Fig. 77), and the other pair behind the tenth segment. The _sperm glands_ are in front of the ovaries (Fig. 77), but the _sperm ducts_ are longer than the _oviducts_, and open behind them (Figs. 77, 78). The worms exchange sperm cells, but not egg cells. The reproductive girdle, or _clitellum_, already spoken of, forms the case which is to hold the eggs (see Fig. 71). When the sperm cells have been exchanged, and the ova are ready for fertilization, the worm draws itself backward from the collarlike case or clitellum so that it slips over the head. As it passes the fifteenth and sixteenth segments, it collects the ova, and as it passes the ninth and tenth segments, it collects the sperm cells previously received by touching another worm. The elastic, collar-like clitellum closes at the ends after it has slipped over the worm’s head, forming a _capsule_. The ova are _fertilized in this capsule_, and some of them hatch into worms in a few days. These devour the eggs which do not hatch. The eggs develop into complete but very small worms before the worms escape from the capsule.

[Illustration: FIG. 78.--Side view showing setæ, nephridia pores, and reproductive openings.]

=Habits.=--The earthworm is omnivorous. It will eat bits of meat as well as leaves and other vegetation. It has also the advantage, when digging its hole, of _eating the earth_ which must be excavated. Every one has noticed the fresh “casts” piled up at the holes in the morning. As the holes are partly filled by rains, the casts are most abundant after rains. The chief _enemies_ of the earthworm are moles and birds. The worms _work at night_ and retire so early in the morning that it takes a very early bird to catch a worm. Perhaps the nearest to an intelligent act the earthworm accomplishes is to _conceal the mouth of its hole_ by plugging it with a pebble or bit of leaf. They _hibernate_, going below danger of frost in winter. In dry weather they burrow several feet deep.

=The muscular coat= beneath, and much thicker than the skin, consists of two layers: an outer _layer runs around the body_ just beneath the skin, and an inner, thicker _layer of fibers runs lengthwise_. The worm crawls by shortening the longitudinal muscles. As the bristles (_setæ_) point backward, they prevent the front part of the body from slipping back, so the hinder part is drawn forward. Next, the circular muscles contract, and the bristles preventing the hind part from slipping back, the fore portion is pushed forward. Is the worm thicker when the hinder part is being pulled up or when the fore part is being thrust forward? Does the earthworm pull or push itself along, or does it do both? Occasionally it travels backward, _e.g._ it sometimes goes backward into its hole. Then the bristles are directed forward.

The right and left halves of the body are counterparts of each other, hence the earthworm is _bilaterally symmetrical_. The lungs and gills of animals must always be kept moist. The worm _cannot live long in dry air_, for respiration in the skin ceases when it cannot be kept moist, and the worm smothers. Long immersion in water is injurious to them, perhaps because there is far less oxygen in water than in the air.

Darwin wrote a book called “Vegetable Mold and Earthworms.” He estimated that there were fifty thousand earthworms to the acre on farm land in England, and that they bring up eighteen tons of soil in an acre each year. As the acids of the food tube act upon the mineral grains that pass through it, the earthworm renders _great aid in forming soil_. By burrowing it makes the soil more _porous_ and brings up the subsoil.

Although without eyes, the worm is sensitive to light falling upon its anterior segments. When the light of a lantern suddenly strikes it at night, it crawls quickly to its burrow. Its sense of touch is so keen that it can detect a light puff of breath. Which of the foods kept in a box of damp earth disappeared first? What is indicated as to a sense of taste?

Why is the bilateral type of structure better adapted for development and higher organization than the radiate type of the starfish? The earthworm’s body is a double tube; the hydra’s body is a single tube; which plan is more advantageous, and why? Would any other color do just as well for an earthworm? Why, or why not?

The _sandworm_ (Nereis) lives in the sand of the seashore, and swims in the sea at night (Fig. 79). It is more advanced in structure than the earthworm, as it has a distinct head (Fig. 80), eyes, two teeth, two lips, and several pairs of antennæ, and two rows of muscular projections which serve as feet. It is much used by fishermen for bait. If more easily obtained, it may be studied instead of the earthworm.

[Illustration: FIG. 79.--SAND WORM × ²⁄₃ (Nereis).]

=There are four classes in the branch Vermes=: 1) the _earthworms_, including sandworms and leeches; 2) the _roundworms_, including trichina, hairworms, and vinegar eels; 3) _flatworms_, including tapeworm and liver fluke; 4) _rotifers_, which are mere specks in size.

[Illustration: FIG. 80.--HEAD OF SANDWORM (enlarged).]

The =tapeworm= is a flatworm which has lost most of its organs on account of its parasitic life. Its egg is picked up by an herbivorous animal when grazing. The embryo undergoes only partial development in the body of the herbivorous animal, _e.g._ an ox. The next stage will not develop until the beef is eaten by a carnivorous animal, to whose food canal it attaches itself and soon develops a long chain of segments called a “tape.” Each segment absorbs fluid food through its body wall. As the segments at the older end mature, each becomes full of germs, and the segments become detached and pass out of the canal, to be dropped and perhaps picked up by an herbivorous animal and repeat the life cycle.

The =trichina= is more dangerous to human life than the tapeworm. It gets into the food canal in uncooked pork (bologna sausage, for example), multiplies there, migrates into the muscles, causing great pain, and encysts there, remaining until the death of the host. It is believed to get into the bodies of hogs again when they eat rats, which in turn have obtained the cysts from carcasses.

=Summary of the Biological Process.=--An earthworm is _a living machine which does work_ (digging and crawling; seizing, swallowing, and digesting food; pumping blood; growing and reproducing). To do the work it must have a continual _supply of energy_. The energy for its work is set free by the protoplasm (in its microscopic cells) undergoing a destructive chemical change (_oxidation_). The waste products from the breaking down of the protoplasm must be continually removed (_excretion_). The broken-down protoplasm must be continually replaced if life is to continue (the income must exceed the outgo if the animal is still growing). The microscopic cells construct more protoplasm out of food and oxygen (_assimilation_) supplied them by the processes of nutrition (eating, digesting, breathing, circulating). This protoplasm in turn oxidizes and releases more energy to do work, and thus the cycle of life proceeds.