MOUTH ( AS. a? (loth. in u»Ps, ()I l(. Oen, nml, mouth: connected with bat. menlum, chin, and ultimately with Skt. mukha, face). In an animal. the opening through which food en ters the hotly. Nitt all animals have months, for certain parasitic forms, notably the tapeworm, lack a system for digesting food, and conse quently 1111 opening for its entrance is necessary: in such cases the food is absorbed through the surface of the animal. The simplest form of mouth is that which occurs in the Protozoa, where the foist is taken into the body through a special opening, whieli is fairly constant in position and may be surrounded with cilia, some times long. Many Protozoa, as well as sponges, have no month in any true sense, as the food may be taken in at any part of the body-surface, and therebire flue opening has no fixed position and no regular size or form. The mouth of corals. jelly-fish. sea-anemones. and other ewlenterates is simply an opening, almost al ways central in position. 011 t he lower surface in free-swimming forms, on the upper surface in fixed forms. It is usually circular, lint may be flattcneds and in one group of jelly-fish it is ,lividod up into four or more, sometimes in nunwrable, small openings, by the growing to gether of the lobes on its margin. In the flat worms the mouth is usually circular and is often in the centre of a sucker. hut its position is variable, though it is always 011 the ventral side. It may be near the anterior end of the animal, but it is more often at the centre and is sometimes posterior. Among the vii Hong (dosses of worms the mouth is always anterior, and often terminal, though more often on the lower side of one of the first segments, or, in misegmented forms, in a similar position. lit blood stickers it lies in the centre of a powerful sucker. and is provided with thnw chitinous in vegetable-eating and some carnivorous forms (nemertines) it is a simple opening with out jaws. While ill such active carnivorous worms, as Niue's. it is provided with powerful jaws. jaws, however. are (dearly modified seg ment:1 1 appendages. and are practically feet which have become modified to assist in seizing food or forcing it into the month. In the crus taceans. insect s, spiders, and t he like Iadhro pod31 the mouth is more or less terminal. and is provided with lateral jaws: sometimes as many as eight pairs of appendages are modified to serve for this purpose. The structure and arrangement of the month parts in insects are very complicated. and are of great importance in classification. Ponighly they may be grouped as
hiting. when there are freely movable lateral jaws seize and rut or tear the food; piercing, when the various parts 11 re more or less milted to form a sh arp. piereing instrument, which penetrates animal or plant membranes in search of the Ilnid food. which is then slicked; suelthug, when the parts are united to form a suctorial 111w, with no adaptation for piereing, Among eehinoderms the month shows consider able variety of form. In mollusks the mouth is generally anterior and often ventral. but it is frequently terminal. In Mc devil- fish, and the like, it is in the centre of the foot. In the clams and other lamellibranehs there are special organs of sense, known as labial paips, on each side of the mouth, hut there is no tongue, while in all other mollusks a tongue covered with teeth and known as the radula is present. The cephalopods (squids and the like) have powerful jaws, arranged like the beak of a par rot, but in all other mollusks the jaws are small and rathey weak, sometimes three in num ber, or they may be wholly wanting, as in the lamellibranchs.
In the vertebrates we find the formation of the mouth is used by some writers to divide them into two contrasted groups. the eye/msqumes or round-mouths, and the gnalhostomcs or jaw months. The round-mouths include only three or four genera, and are characterized by the absence of jaws, the mouth serving as a sucking organ, The surface of this mouth-sucker bears characteristic horny teeth. All the other ver tebrates are gnathostomes, provided with verti cally tmwing upper and lower jaws. one or both of which. except in birds. turtles, and some whales, bear teeth. There are also a tongue and various glands, notably the salivary glands. True lips, provided with muscles. are characteristic of mammals, hut are also found in dipnoid fishes. In many mammals the sides of the buccal cavity —that is, the space outside the jaws—are en larged to form cheek-pouches, of use as food reservoirs. The origin of the vertebrate month has been a matter of much discussion, and is closely associated with the still more funda mental question of the origin of the skull. Ap parently, however, the jaws arise as modifica tions, of the first branchial arch. which heeomes divided into two parts, the proximal giving rise to the quadrate bone, which gives rise to an anterior process, forming a sort of primary upper jaw•; the distal part is the cartilage of Meekel, the basis of the lower jaw.
See TEETH; TONGUE; GLAND; SKULL.