vii.] THE EXTERNAL SKELETON. 247 



Curiously modified thickenings of the epidermis, which 

 take the form of rings, may surround the ends of the tail, as 

 in the Rattlesnake, These being loosely attached, produce a 

 singular sound, a sort of "hissing" rattle, when the tail is 

 vibrated by the excited animal 



Nail-like structures may also be developed from the side of 

 the hand, as in certain Birds (e.g. Palainedea], and in one of 

 the lowest members of man's own class (the Duck-billed 

 Platypus) a hollow horny spur grows 

 upon each ankle. 



The epidermis of the outside of 

 the muzzle and jaws may also be 

 converted into horn, as in the beak FlG . ^.-ORNITHORHYNCUS, 



of Birds and in that of the Turtle. OK DUCK-BIU.ED PLATYPUS. 



As a remarkable exception, the same 



thing may take place in man's own class : this we see by the 

 Duck-billed Platypus, which really has a horny beak very like 

 that of a duck. 



13. These being the principal modifications which investiga- 

 tion shows us to be possible with regard to the epidermis and 

 its appendages, yet other peculiarities may be present in parts 

 belonging to the other ecteronic division, the EPITHELIUM. 



To begin with the mouth, its epithelial lining investing the 

 gums becomes greatly thickened and hardened in persons 

 who have lost their teeth, and who are so unfortunate, or so 

 unwise, as to have failed to replace them by artificial ones. 



In certain beasts, as the Cow and the Sheep, the front part 

 of the upper jaw is always devoid of teeth, and a horny epi- 

 thelial pad is formed there against which the teeth of the 

 front of the lower jaw bite. 



A much more developed structure, though essentially simi- 

 lar, is met with in the Dugong a creature superficially like a 

 Porpoise, but really very different. ' In this creature the front 

 of each jaw is furnished with a dense horny plate, formed 

 (like the horn of the Rhinoceros) of a deposit of horny 

 matter around long processes of the deeper skin layer the 

 deimis. Horny substances in the place of teeth are also 

 developed inside the mouth of the Duck-billed Platypus, 

 and there are horny teeth in the mouth of the Lamprey. 



The maximum of development of this kind of structure is, 

 however, found in the Whalebone Whales. 



The upper jaw in these creatures is furnished with very nu- 

 merous horny plates (termed baleeii), which hang down from 

 the palate along each side of the mouth. They thus form 



