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As most of the arachnids live by suction, the jaws are seldom used for mastication. In the scorpion, the apparent representatives of the mandibles of an insect are transformed into a pair of small forceps, and the palpi, so small in insects, are developed into formidable claws: both of these or

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FIG. 222.- Mouth of the Horsefly (Tabanus lineola), magnified: a, antennæ; m, mandibles; mx, maxillæ; mp, maxillary palpi; 76, labrum; 7, labium, or " tongue."

a fang; and the clublike palpi, often resembling legs, have nothing to do with ingestion or locomotion. Both scorpions and spiders have a soft upper lip, and a groove within the mouth, which serves as a canal while sucking

FIG. 223.1
Under Surface of Male Spider,
enlarged: a, c, poison fang; b, teeth on in-
terior margin of mandible, e; f, labium; g,
thorax; h, limbs; i, abdomen; 7, spinnerets;
m, maxillary palpus; d, dilated terminal
joint.

their prey. The tongue is external, and situated between a pair of diminutive maxillæ.

In the ascidians the first part of the alimentary canal is enormously enlarged and modified to serve as a gill At the bottom of this sac, and far removed from

sac.

its external opening, lies the entrance to the digestive Into it the particles of food entering with

tract proper.

the water are conveyed (Fig. 115).

The mouth of vertebrates is a cavity with a fixed roof (the hard palate) and a movable floor (the tongue and lower jaw), having a transverse opening in front,76 and a narrow outlet behind, leading to the gullet. Save in birds and some others, the cavity is closed in front with lips, and the margins of the jaws are set with teeth.

In fishes the mouth is the common entry to both the digestive and respiratory organs; it is, therefore, large, and complicated by a mechanism for regulating the transit of the food to the stomach and the aërated water to the gills. The slits leading to the gills are provided with rows of processes which, like a sieve, prevent the entrance of food, and with valves to keep the water, after it has entered the gills, from returning to the mouth. So that the mouths of fishes may be said to be armed at both ends with teeth-bearing jaws. A few fishes, as the sturgeon, are toothless; but, as a class, they have an extraordinary dental apparatus — not only the upper and lower jaws, but even the palate, tongue, and throat being sometimes studded with teeth. Every part of the mouth is evidently designed for prehension and mastication. Lips are usually present; but the tongue is often absent, or very small, and as often aids respiration as ingestion.

Amphibians and reptiles have a wide mouth; even the insect-feeding toads and the serpents can stretch theirs enormously. True fleshy lips are wanting; hence the savage aspect of the grinning crocodile. With some exceptions, as toads and turtles, the jaws are armed with teeth. Turtles are provided with horny beaks. The tongue is rarely absent, but is generally too thick and short to be of much use. In the toad and frog it

is singularly extensile; rooted in front and free behind, it is shot from the mouth with such rapidity that the insect is seized and swallowed more quickly than the eye can follow. The chameleon's tongue is also extensile. Snakes have a slender forked tongue, consisting of a pair of muscular cylinders, which is solely an instrument of touch.

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FIG. 224.

Mouth of the Crocodile: d, tongue; e, glands; f, inferior, and g, superior, valve, separating the cavity of the mouth from the throat, h.

Birds are without lips or teeth, the jaws being covered with horn forming a beak. This varies greatly in shape, being extremely wide in the whip-poor-will, remarkably long in the pelican, stout in the eagle, and slender in the hummer. It is hardest in those that tear or bruise their food, and softest in water birds. The tongue is also covered with a horny sheath, and is generally spinous, its chief function being to secure the food when in the mouth. It is proportionally largest and most fleshy in the parrots.

The main characteristics of the mammalian mouth are flesh lips and mobile cheeks.77 In the duck-billed

monotremes lips are wanting, and in the porpoises they are barely represented. But in the herbivorous quadrupeds they, with the tongue, are the chief organs of prehension; in the carnivorous tribes they are thin and retractile; while in the whale the upper lip falls down like a curtain, overlapping the lower jaw several feet. As a rule, the mouth is terminal; but in the elephant, tapir, hog, and shrew, the upper lip blends with the nose to form a proboscis, or snout. The mouth is comparatively small in the elephant and in gnawing animals like the squirrel, wide in the carnivores, short in the sloth, and long in the ant-eater. Teeth are usually present, but vary in form and number with the habits of the animal. The ant-eater is toothless, and the Greenland whale has a sieve

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FIG. 225.

Human Tongue and ad- made of horny plates.

jacent parts: a, lingual papillæ;

The

b, papillæ forming V-shaped tongue conforms in size and lines; d, fungiform papillæ; e,

filiform papillæ; g, epiglottis; shape with the lower jaw, and m, uvula, or conical process, is a muscular, sensitive organ, hanging from the soft palate,

n; o, hard palate; r, palatine which serves many purposes, glands, the mucous membrane

lower jaw.

being removed; v, section of the assisting in the prehension, mastication, and swallowing of food, besides being an organ of taste, touch, and speech. Its surface is covered with minute prominences, called papilla, which are arranged in lines with mathematical precision. In the cats, these are developed into recurved spines, which the animal uses in cleaning bones and combing its fur. Similar papillæ occur on the roof and sides of the mouth of the ox and other ruminants. In some animals, as the hamster and gopher, the cheeks

are developed into pouches in which the food may be carried. These may be lined with hair. The tongue is remarkably long in the ant-eater and giraffe, and almost immovable in the gnawers, elephants, and whales.

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3. The Teeth of Animals. Nearly all animals have certain hard parts within the mouth for the prehension or trituration of

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-Sea Urchin bisected, showing masticating

apparatus.

"gastric teeth," as in some marine snails; or the deficiency is supplied by a muscular gizzard, as in birds,

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FIG. 227. - Teeth and Masticatory Apparatus of Gastro- organs, have the pods, enlarged: A, portion of odontophore, or 66 tongue," of Velutina, enlarged; B, portion of odontophore of Whelk (Buccinum undatum), magnified - the entire tongue has 100 rows of teeth; C, head and odontophore

stomach furnished with a powerful set of

of Limpet (Patella vulgata); D, portion of same, greatly
magnified, to show the transverse rows of siliceous teeth. teeth.

The sea urchin is one of the lowest animals which exibits anything like a dental apparatus. Five calcareous teeth, having a wedge-shaped apex, each set in a triangular pyramid, or "jaw," are moved upon each

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