DICESIIVE SYSTEMS AND METABOLISM Green plants build their tissues from inorganic ma- terials by the photosynthetic process, using energy from the sun (Chaps. 1, 12). The food of animals is obtained by eating plants or other animals. It serves two purposes, as a fuel to supply bodily energy and as a source of materials for growth and repair. After being obtained (feeding), it is broken down into simpler chemical substances (digestion) and then is taken into the cells and tissues of the body (absorp- tion), where itis utilized (metabolism). 5-1 Feeding Animals differ widely in their food habits. Some insects feed on the tissues or juices of a single species of plant or the blood of one kind of animal, but most animals take several or many kinds of food. Cattle, deer, rodents, and insects that eat plants are herbivorous; cats, sharks, flesh flies, and many marine animals whose food consists entirely or largely of other animals are carnivorous; humans, bears, rats, and others that eat various plant and an- imal materials are general feeders, or omnivorous; vultures and some insects that eat dead animals are scavengers; and certain frogs, lizards, birds, and mammals that feed primarily on insects are insec~ tivorous. Paramecia and certain other protozoans, some sea anemones, certain fishes, and tadpoles, that feed on small particles, living or dead, such as plankton, are termed microphagous feeders. In con- trast, most higher animals, including humans, that use larger materials are macrophagous feeders. A few animals feed on fluids, like the mosquitoes and ticks that suck blood and the aphids that pump in plant juices,The digestive mechanism in various animals (Fig, 5-1) differs in general form, structural details, and physiologic processes according to the nature of the food, manner of life, and other factors, All means for taking and using food are essentially alike in that materials from the external environment are brought into intimate contact with intemal membranous sur- faces where digestion and absorption can take place. 5-2 Invertebrates permanent structures for taking or digesting food An amoeba pushes out lobes (pseudopodia) at any part of its one-celled body to surround an item of food; the latter is taken into a fluid-filled food vacuole in the cytoplasm for digestion (Fig. 15-4), In paramecia and other ciliate protozoans a per- manent external oral groove lined by beating cilia Many protozoans have no ‘Chopler § Digestive Systems and Metabolism 87 carries food particles to a definite “cell mouth,” where they pass into food vacuoles and are digested (Fig. 15-22), Many animals, from protozoans to lower chordates, draw food to the mouth in a current of water by use of cilia ‘The microscopic food of sponges is captured by and digested in flagellated collar cells that line cer- tain interior canals of the animal; digestion is thus ntracellullar, as in protozoans. In intracellular diges- tion, small food particles that are engulfed by phago- cylosis at the cell surface become surrounded by fluid-filled vacuoles into which digestive enzymes, probably carried chiefly by lysosomes, are released. Cnidarians have a definite mouth connected to a saclike digestive (gastrovascular) cavity within the body that is lined by a tissue layer of special diges- tive cells (Fig. 17-2). The flatworms (except tape- worms) have a mouth and a branched digestive tract Figure 54 Types of digestive systems in animals and protistans; diagrammatic. A. Amoeba, food enters at any place on coll sutace 'B. Paramecium, with defrite cell mouth, C. Hydra, mouth and sacike digestive cavity. D. Pranario, mouth and branched ‘gestive act Dut no anus, E, Earthworm, tubular digestive tract having specialized sections, complete wih terminal mouth anc crus. F. \Venebrate, complete andiparty coled tract with specialized parts and digestive glands. vent at base ot fil mouth esophagus branched digestive EARTHWORM, SALAMANDER.88 Patt] General Animal Biology (gastrovascular cavity) extending to all parts of the body (Fig, 18-1). In both the latter groups the tract is coelenteronic (incomplete) in that foods enter and undigested residues pass out the same opening, the mouth. In the coclenterates and flatworms, food that has entered the digestive tract is acted upon by en- zymes secreted from gland cells in the interior linin, This is extracellular digestion, in a digestive cavity, such as occurs in all higher animals; some partly di gested food, however, is taken into cells lining the cavity for intracellular digestion. In most other invertebrates the digestive tract is essentially a tube within the body. It opens to the outside (mouth, anus) and is separated from the in- terior body spaces by selectively permeable mem- branes. It is termed enteronic (complete) because food enters the mouth and passes through various organs for storage, digestion, or absorption and any residues pass out the anus at the opposite end of the system. The parts differ in structure in animals be- longing to various groups (Chaps. 19 to 26), but the names applied to them give some indication of the function of each part. An earthworm, for example, has a mouth with fleshy lips to grasp food, a muscu- lar pharynx that sucks in the food and lubricates it by mucous secretions, a slender esophagus to carry food on to the dilated crop for storage, a muscular- walled gizzard where food is ground against parti- cles of sand, and a long intestine with pouchlike lat- eral extensions providing a large surface for absorp- tion of digested portions (Fig. 22-3). Undigested residues pass out the anus at the posterior end of the body. Jaws with teeth occur in the mouths of some other annelid worms, in squids and octopuses, in sea urchins, and in many arthropods. The mouth in most mollusks hasa radula (Fig. 21-16) bearing many fine horny teeth that serve to rasp off particles of food. The mouth parts of arthropods are modified appendages; those of insects are adapted for either chewing or sucking (Chap. 25; Table 25-2). 5-3 Vertebrates' The digestive system of almost every vertebrate has the following essential parts (Figs. 5-2, 5-3): (1) The mouth and mouth cavity com- monly have teeth to grasp, tear, or chew food and a tongue (fishes excepted) that may help in capturing or manipulating it; in most land vertebrates the sali- vary glands secrete saliva to lubricate the food and start digestion. (2) The pharynx contains gill slits in fishes and some aquatic amphibians but has no di- rect digestive function. (3) The esophagus (gullet) is a flexible tube carrying food past the region of the heart and lungs. (4) The stomach is a large pouch where food is stored and some digestion occurs. (5) The small intestine, a long, slender, coiled tube, is the principal region for digestion and absorption. (6) ‘The large intestine (colon) is the portion where water and salt absorption occurs, some cellulose is partly digested by bacteria, and undigested residues are formed into relatively dry masses (feces) for expul- sion through (7) the cloaca, which ends with (8) the anus or vent. In addition, the cloaca is an exit for excretory wastes and sex cells in sharks, amphibians, reptiles, and birds, but these enter it by separate openings; the cloaca is absent in most mammals. 2 Fora comparison of the digestive and other organ systems in the various classes of vertebrates, see the figures on “general structure” in Chaps, 28 to 34. Figure 5-2 Diagram of structure and activities in he digestive tract of vertebrate, Wow lines Incicate glanculor areas ‘mouth > storage centers > digestion “® digestion anc absorption large intestine smal intestine > residues pass thru and out gall bladder (bile storage)imestine (colon) small intestine ureter rectum — badder Figure 53 The human digestive system. All vertebrates have two large digestive glands, the liver and pancreas, connected by ducts to the upper part of the small intestine. ‘Typically all vertebrates have teeth in both upper and lower jaws except the living birds, turtles, and a few specialized forms in other classes. Most fishes, amphibians, and reptiles have simple, slender, co! cal teeth attached to the bone surfaces (Chaps. 29 to 32). In the absence of flexible lips, their teeth serve mainly to grasp the food. Some birds peck or tear food with their beaks, but many swallow it intact. Among mammals, however, the teeth of an individ- ual usually are of several kinds, differentiated for cutting, piercing, shearing, or grinding according to the materials used as food (Secs, 34-8, 34-19). Struc- turally a tooth has a hard outside enamel, a filling of softer dentine, and a central living pulp supplied Chapter § Digestive Systerns and Metabolism 89 with blood vessels and nerves (Fig. 5-4). Teeth of mammals and some reptiles have the root of each set ina socket of the bony jaw. 5-4 Food and digestion The plant and animal foods taken by animals consist of proteins, carbohy- drates, and fats, together with vitamins, minerals, and water, The water and inorganic salts can be ab- sorbed from the digestive tract without change, but the organic materials must be altered before they can bbe utilized, Some foods are subjected only to chemi- cal alteration, as with the microscopic organisms taken as food by protozoans and other small animals, the fluids of plants sucked up by bees and aphids, the blood pumped in by parasitic worms, leeches, or insects, and the larger prey taken by cnidarians and starfishes. Many other animals have the capacity to reduce food physically. This must be done before chemical digestion can proceed effectively. It is ac- complished by teeth in the mouth or elsewhere (pharynx of some fishes, stomach of crayfishes) and by grinding in the gizzard of earthworms and birds. Flesh eaters such as the sharks, large fishes, snakes, hawks, owls, cats, and others bolt down their food intact or in large pieces, and its physical reduction is accomplished by muscular and chemical action in the stomach. Other fishes and the herbivorous mammals that eat plant materials chew their food thoroughly before it can be digested. Insects and Figure 5-4 Enlargedsectionothumantoothinthejaw. Com pore Fig. 34-13, ame dentine ——aum pulp cavity blood vessels ‘and nerves dentine connective tissue AVY ot on yhe—90 Part) General Animal Biology many land vertebrates have salivary glands that pro- vide secretions to moisten the food while it is being chewed and swallowed. 5-5 Digestive enzymes The chemical aspects of digestion involve the reduction of complex organic substances in the food into simpler molecules that can be passed through cells of the digestive epithe- lium to enter the fluids and cells of the body. Proteins are reduced to amino acids, fats to fatty acids and glycerol, and carbohydrates to simple sugars (mono- saccharides) such as glucose. These changes are per- formed by the digestive enzymes (Sec. 2-19). These enzymes are produced by all animals from protozoans to mammals, but not the same number or kinds of enzymes are present in every sort of animal. The food in a vacuole within a protozoan changes gradually in form and size as it is acted upon by enzymes. The reaction of the vacuole changes from acid to alkaline during the process, as can be shown by indicator dyes. The cytoplasm therefore has the ability to secrete enzymes and also substances to change the acidity (pH) of fluid in the vacuole. Among lower invertebrates, enzymes are secreted by cells in some or all parts of the digestive tract, but in higher animals only by glands or cells in certain portions of it. In the vertebrates, some are produced in the salivary glands, others regularly in the stom- ach and small intestine, and most by the pancreas (Fig. 5-2).