Before leaving the description of the shell, it is worth while to draw attention to the enormous correlative changes in other organs produced by this case. Nearly the whole organism has been altered. The hard, firm carapace has partly rendered the supporting functions of the vertebral column unnecessary or impossible. In many tortoises, especially in the large land-tortoises, the vertebrae and the capitular portions of the ribs are reduced to mere bony outlines; the reduction to thin paper-like bony lamellae proceeds with age. The iliac bones find a better support in the costal plates; the contact with the sacral ribs is given up, and these ribs fuse partly with the costal plates, or they are absorbed. The whole mass of muscles of the trunk is completely lost in the region of the shell, but traces of them exist in young specimens. Neck, limbs, and tail can in most cases be withdrawn and hidden in the shell. When this is not possible it is due to secondary changes. The neck is withdrawn either by being tucked away {329}sideways (Pleurodira[129]), or by being bent in an S-shaped curve in a vertical plane. In a left-sided profile-view of the animal, the head represents the tail of the S. The neck is withdrawn by long muscles, which are inserted into the ventral side of the middle of the neck, and extend in the shape of vertical ribbons far back into the shell, arising from the centra of some of the middle or even more posterior thoracic vertebrae. Lastly, a few remarks on the PARTIAL REGENERATION, or the mending of injuries to the shell. If part of the horny covering is badly bruised, torn off, or rubbed through, or if part of the shell is crushed, the underlying portion of the bony plate becomes necrotic, and the horny covering also dies so far as its Malpighian layer is destroyed. Soon, however, the uninjured Malpighian cells, around the margin of the wound, multiply, grow into and beneath the injured portion of the bone, and form a new horny layer, casting off the necrotic portion. After several months the deficiency is patched up; new bone has grown in the deeper remaining strata of the cutis, and the outside is covered by a continuous horny layer, without, however, reproducing the original concentric moulding of the shields. In badly crushed shells sometimes almost one-third of the whole shell is thus cast off and mended within one or two years. The regeneration of the forcibly stripped-off shields of _Chelone imbricata_ is described on p. 386. Bitten-off tails and limbs, rather frequent occurrences in water-tortoises, are of course not reproduced, but the wounds are healed and covered again with scaly skin.