THE INTEGUMENT AND THE EXOSKELETON 105 



:ion must have been the same as that of the scales which pre- 

 ceded them, that this original direction would have been 

 retained after the loss of the scales, and that the hereditary 

 transmission of this may account for at least a general plan 

 underlying the variations occurring in the mammals of the 

 present day. The existence of individual variations, known 

 to be considerable in man and certain domestic animals, points 

 to a diminution. of the original functional importance, which 

 has become no longer sufficient to retain the various features 

 at a definite standard. 



Aside from the formation of horny scales, feathers, and hair, 

 the epidermis produces numerous other organs composed of 

 keratin, and fitted for various uses. The most of these appear 

 as isolated instances to subserve a particular purpose in a 

 small group of animals, but in one case, that of claws or 

 nails, the organs are possessed by both Sauropsida and Mam- 

 malia and form a strictly homologous series throughout, which 

 presents some interesting modifications. 



The first employment of this substance in this locality 

 appears to be 'in the dog-fish, where the fins are lengthened 

 beyond the limits of the fish skeleton by numerous horn 

 threads, set close together and forming two series, overlapping 

 the cartilaginous rays on each side. Otherwise there is little 

 use of keratin among fishes and almost none at all among 

 amphibians, unless there be included a certain form of wart 

 found in toads and due to the local thickening of the stratum 

 corneum. One species of salamander also (Siren) possesses 

 horny plates in the mouth, serving the purpose of teeth. In 

 turtles, a dorsal carapace and a ventral plastron are formed 

 from parts of the endoskeleton, with the addition of dermal 

 elements, and these are covered by large plates of keratin, the 

 so-called " tortoise-shell." The jaws of the same animal are 

 also covered with horny plates equipped with a sharp cutting 

 edge, and a precisely similar formation produces the charac- 

 teristic beak of birds, although it is hardly to be supposed that 

 the two structures are genetically connected. Aside from 

 the coat of imbricated scales, many reptiles possess horns, 



