Animals With Complete Digestive Tracts
In zoology, animals above the level of coelenterates and flatworms have a com¬plete digestive tract - one with two openings, a mouth and an anus, as seen using children’s microscopes. In these organisms incoming food material and outgoing wastes do not pass through the same opening. Instead, food can be passed in one direction through a tubular system, which can be divided into a series of distinct sections or chambers; each specialized for a different function. As the food passes along this assembly line, it is acted upon in a different way in each section. The sections, when viewed under children’s microscopes, may be variously specialized for mechanical breakup of bulk food, temporary storage, enzymatic digestion, absorption of the products of digestion, reabsorption of water, storage of wastes, and so on. The overall result is a much more efficient digestive system, as well as a potential for special evolutionary modifications fitting different animals for different modes of existence.
The digestive system of an earthworm, when viewed under children’s microscopes, is a good example of subdivision into specialized compartments. Food, in the form of decaying organic matter mixed with soil, is drawn into the mouth by the sucking actions of a muscular chamber called the pharynx. It passes from the mouth through a short passageway into the pharynx and then through a connecting passage called the esophagus, after which it enters a relatively thin-walled crop that function as storage chamber. Next, it enters a compartment with thick muscular walls, the gizzard, where it is ground up by a churning action; small stones in the gizzard often facilitate the grinding. The pulverized food, suspended in water, now passes into the long intestine, where enzymatic digestion and absorption take place. Finally when viewing the rear of the intestine under a microscope, some of the water involved in the digestive process is reabsorbed, and the indigestible residue is eliminated from the body through the anus.
When you study an earthworm under a microscope, notice that earthworms use extra cellular digestion. Glandular cells in the epithelial lining of the intestine, when studied under a microscope, secrete hydrolytic enzymes into the intestinal cavity, and the end products of digestion-the simple building block compounds-are absorbed.
Extra cellular digestion in animal biology is an adaptation for eating sizable pieces of food; the gizzard in earthworms is obviously another such adaptation. Mechanical breakup of bulk food is common among animals, and a variety of structures that serve this function have evolved. In our own case, food is torn and ground by the teeth. Note when viewing them under a microscope that the grinding or chewing device need not be in the first section of the digestive tract, as in our own case; in earthworms the grinding chamber comes after the crop, which like our stomach functions as a storage organ, and mechanical breakup thus follows temporary storage instead of preceding it. The same arrangement exists even in some vertebrates in animal biology; birds have a muscular gizzard, posterior to the less specialized stomach, in which hard food is ground with rocks and pebbles, which is often called grit.
Some animals, such as bloodsucking and sap sucking insects, have liquid diets. Other animals are filter feeders, straining small particles of organic matter from water. Clams and many other mollusks filter water through tiny pores in their gills; microscopic food particles, trapped in streams of mucus that flow along the gills, are moved along by beating cilia until they enter the mouth.
For some heterotrophs, the answer is yes. Many bacteria and fungi, when studied using a microscope, can flourish on a diet consisting solely of carbohydrate and minerals. They need no protein in their diets, because they, like green plants, can combine inorganic nitrogen with carbon skeletons from carbohy¬drates to make amino acids. In similar fashion, they can synthesize for themselves, all the other classes of compounds necessary for life.
The functional significance of a storage chamber should be clear after a moment’s thought. It enables the animal to take in large amounts of food in a short time, when it is available, and then to utilize this food over a considerable period of time. Such discontinuous feeding makes it possible for the animal to devote much of its time to activities other than feeding, such as searching for a mate, mating, egg-laying, and in some animals, caring for young. Our own stomachs function as storage organs analogous to the earthworm’s crop; they enable us to live well on only three or four meals a day and to devote the rest of our time to other pursuits. A man can survive if his stomach is removed surgically, but he is unable to eat more than a few bites at a time and must therefore eat very frequently. It is not surprising that the vast majority of higher animals have evolved adaptations for discontinuous feeding, thereby gaining time for a behaviorally more varied existence.
