236 TOWARDS THE HUMAN FORM 



Sirenidae, which also preserve their three pair of branchiae, 

 and have only two short front legs with three or four toes. 

 These are unquestionably former terrestrial Salamanders, 

 in other words quadrupeds that have again become aquatic. 

 In fact they indicate the normal metamorphosis of the forms 

 destined to become terrestrial by losing the branchiae which 

 they possessed at birth. These gills are subsequently 

 regenerated. The atrophy of the hind-legs can be attri- 

 buted to the lengthening of the tail, which in an animal 

 of seventy centimetres has a length of about twenty-five 

 centimetres. The Amphiumae, whose body is elongated, but 

 whose tail, on the contrary, is short, preserve their hind- 

 legs as well as the others. 



An analogous phenomenon, even more striking, is produced 

 in other Batrachians which have no legs and live underground 

 like worms. They constitute the group of Caeciliidae. About 

 forty species are known, distributed over India, Malaysia, 

 tropical Africa, the Seychelles, South America, and Panama, 

 that is to say, in regions which were all part of the continent 

 of Gondwana in Carboniferous times. These animals were 

 originally aquatic, because their embryos, while still in the egg, 

 acquire magnificent branchiae. Their general characters 

 resemble those of the Stegocephala of that epoch. Certain 

 species have even preserved the scales concealed in the seg- 

 mented folds of their skin. 1 We may therefore ask whether 

 these vermiform Batrachians are not genealogically related 

 with the Dolichosoma. 



The other stegocephalous Batrachians belong to higher 

 types. Their vertebral centra are at first formed of four pairs 

 of elements, the upper ones bearing the arches which surround 

 the spinal cord. They are therefore called temnospondyious. 

 The four pairs are already reduced to three in the vertebrae 

 of the trunk in Archegosaurus, Actinodon, and Euchirosaurus, 

 where only the caudal vertebrae preserve the primitive com- 

 position. The Batrachians become stereospondylous when all 

 the parts are united in one single bone in the form of an hour- 

 glass, with concave bases. They generally have scales only on 

 the ventral surface of the body, thus betraying the influence of 

 friction on the development of the solid parts of the integument. 



1 Ickthyophis, Hypogeophis, Dermophis, C&cilia, Rhinairema, Geotrypetes, 

 Crytopsophis, Gymnophis, Herpele. 



