60 ZOOLOGY 



that their limbs are in the form of fins supported by spines 

 termed fin-rays, whereas the limbs of all the rest are con- 

 structed on the same principle as the human limb. As the 

 late Sir William Dawson has said, there is more resemblance 

 between the limb of the lowest newt and the limb of man than 

 between the limb of the newt and the fin of a fish. The Keptiles, 

 Birds, and Mammals are bracketed together as Amniota, be- 

 cause in the development of the embryo a portion of the egg 

 forms a hood-like structure ensheathing the body of the embryo. 

 This hood, termed the amnion, or popularly the caul, is cast 

 off at birth, and the young animal enters on its free life in a 

 form closely resembling the adult. Amphibia, a subdivision 

 which includes frogs, toads, and newts, are distinguished from 

 Amniota not only by the total absence of an amnion, but by 

 the fact that they begin their free life as little fish with 

 fins which only gradually change into four-footed creatures. 

 Amphibia are often popularly confused with Keptilia, since a 

 newt is in external form rather like a lizard. It is well to 

 remember, therefore, that the skin of a true Reptile is dry 

 and scaly, and that its toes terminate in nails, whereas an 

 Amphibian is nailless and has a moist slimy skin. Reptiles, 

 Birds, and Mammals are at once distinguished from one another 

 by the character of their skins. In all three groups the skin 

 produces horny structures which serve as a protection for it. 

 The epithelium forming the skin is several layers thick, and 

 the cells forming the outer layers dry up and give rise to the 

 horny structures mentioned. In Reptiles these are scales not 

 to be confounded with the scales of fish, which are not formed 

 in the epithelium but in the connective tissue beneath the skin, 

 and the epithelium is continued over them. In Birds the 

 horny structures are feathers, and in Mammals hairs. The 

 wing of a Bird is of course only the arm and hand of the animal. 

 In the hand only the thumb and two fingers are developed, 

 and to the back of these fingers and to the forearm specially 

 long feathers are attached which form a supporting membrane 

 like the plane of an aeroplane. The Bat has also a wing 

 which is formed from the hand, but in this case the supporting 

 membrane of the wing consists of a web of skin stretched 

 between the elongated fingers. 



Of the enormous economic importance of the Vertebrata 



