ix REGENERATION SENSE-ORGANS 329 



sideways (Pleurodira 1 ), 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 head represents the tail of the S. The neck is withdrawn by 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 re- 

 generation 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 repro- 

 duced, but the wounds are healed and covered again with scaly 

 skin. 



Sense-organs. The EYE is by far the best developed sense- 

 organ. It is comparatively small. The pupil is round. The 

 iris is mostly dark in terrestrial forms, while in water-tortoises it 

 is often brightly coloured, for instance pale yellow in Chelodina, 

 greenish and mottled with black, pale grey, brown, etc., in 

 various species of Chrysemys. Cistudo presents a curious 

 sexual dimorphism ; the males have red, the females brown, 

 eyes. The sclerotic wall contains a ring of numerous small 

 ossified plates. There is no trace of a pecten. The eye is pro- 

 tected externally by the two lids and the nictitating membrane. 

 In some water-tortoises, notably in Chelodina, the lower lid is 

 transparent. Lacrymal and Harderian glands are present. 



1 7r\evp6i>, side ; deipri, neck. 



