What parts of the tortoise extend at times beyond the shell? Are any of these parts visible when the _shell is closed_? What movements of the shell take place as it is closed? Is the carapace rigid throughout? Is the plastron? =The Skeleton= (Fig. 267).--The _carapace_ is covered with thin _epidermal plates_ which belong to the skin. The bony nature of the carapace is seen when the plates are removed, or if its inner surface is viewed (Fig. 267). It is seen to consist largely of wide _ribs_ (how many?) much flattened and grown together at their edges. The ribs are seen to be rigidly attached to the vertebræ. The rear projections of the vertebræ are flattened into a series of bony plates which take the place of the sharp ridge found along the backs of most vertebrates. Show that the shell of a turtle is not homologous with the shells of mollusks. Does the turtle have shoulder blades and collar bones? Hip bones? Thigh bones? Shin bone (fibia) and splint bone (fibula)? (Fig. 267.) [Illustration: FIG. 267.--SKELETON OF EUROPEAN TORTOISE. _C_, rib plates; _M_, marginal plates; _B_, plastron; _H_, humerus bone; _R_, radius; _U_, ulna; _Fe_, femur.] [Illustration: FIG. 268.--THREE-CHAMBERED HEART OF A REPTILE (tortoise). _a_, veins; _b_, _f_, right and left auricles; _cg_, ventricle; _d_, arteries to lungs; e, veins from lungs; _i_, _n_, two branches of aorta. Compare with Fig. 269 and colored Fig. 2.] Do the plates formed by the ribs extend to the edge of the carapace? See Fig. 267. About how many bony plates form the carapace? The plastron? Do the horny plates outside correspond to the bony plates of the shell? How many axial plates? How many costal (rib) plates? How many border plates? Which plates are largest? Smallest? Do the horny plates overlap like shingles, or meet edge to edge? Is there any mark where they meet on the bony shell? Basing it upon foregoing facts, give a connected and complete description of the structure of the carapace. Compare the skeleton of the turtle with that of the snake, and correlate the differences in structure with differences in habits. [Illustration: FIG. 269.--PLAN OF REPTILIAN CIRCULATION. See arrows.] =Draw= the tortoise seen from the side or above, with its shell closed, showing the arrangement of the plates. Place soft or tender vegetable =food=, lettuce, mushroom, roots, berries, and water, also meat, in reach of the turtle. What does it prefer? How does it eat? It has no lips; how does it drink? Study the =movements= of its eyeballs and eyelids, and the respiratory and other movements already mentioned. State a reason for thinking that no species of land animals exists that lacks the simple power of righting itself when turned on its back. [Illustration: FIG. 270.--REPTILIAN VISCERA (lizard). _lr_, windpipe; _h_, heart; _lu_, lungs; _le_, liver; _ma_, stomach; _dd_, _md_, intestines; _hb_, bladder.]