FOOD AND GROWTH 59 



That tortoises and turtles can regenerate their horny plates 

 to some extent, provided the deep-seated, or malpighian, layer 

 of the underlying skin be not destroyed, is, however, amply 

 demonstrated. On this subject Dr. H. Gadow writes as fol- 

 lows in the volume on reptiles in the Cambridge Natural His- 

 tory : — 



" If part of the horny covering is badly bruised, torn off, or 

 rubbed through, or if part of the shell is crushed, the underly- 

 ing portion of the horny plates 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 in- 

 jured 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 con- 

 centric moulding of the shields. In badly crushed shells some- 

 times almost one-third of the whole shell is thus cast off and 

 mended within one or two years." 



Should terrapins, as not infrequently happens, lose their 

 tails or limbs by a bite, the missing part is never reproduced ; 

 the stump being merely sealed over. 



One species of terrapin, namely Clemmys leprosa of Europe, 

 is subject to a peculiar disease when living in foul waters, 

 although perfectly healthy when frequenting clear streams ; it 

 is from this diseased condition that it derives its name of 

 leprosa, or "leprous". When living in slimy pools an alga 

 makes its way into the cracks and crevices in the horny plates 

 of the shell, and thus penetrates into the underlying malpighian 

 layer of the skin, or even into the bone itself, which becomes 

 gangrenous in large patches, while the whole shell has a dis- 

 tinctly leprous appearance. The only other known instance of 

 an alga flourishing in the superficial tissues of a vertebrate 

 animal occurs in the sloths (BradypodidcB) among mammals, 

 where it grows in the grooves traversing the coarse hairs, to 

 which it communicates a distinct greenish tinge, believed to aid 

 in rendering the animal inconspicuous in its leafy haunts. 



All vertebrates apparently change from time to time their 

 epidermis and its appendages. In some cases, as in the human 



