Evolution of Animals 471 



tion in the line of direct evolution. But we may compare 

 them with living Apoda as follows: 



In Apoda the gills are caducibranchiate; fore and hind limbs 

 are absent; the eyes are rudimentary and without eyelids; 

 the jaws bear teeth; the skeleton is largely cartilaginous and 

 the vertebrae are pseudocentrous (amphicoelous) ; "the hyoid 

 and branchial apparatus is more primitive than in any other 

 recent Amphibia"; there are no supra-occipital, basi-occipital, 

 supra-temporal, or intercalary bones, but the others are strongly 

 and well developed; the alimentary canal, except for the liver, 

 is simple. The outstanding feature here is the well formed 

 skull, evolved doubtless in relation to the burrowing land 

 habit. Tliis also in all probability had to do mth the rather 

 precocious formation of lungs, and so with absorption of the gills. 



The Proteidse seem to be somewhat higher than the Sireni- 

 dae, but present the following characters. The gills are perenni- 

 branchiate; the fore and hind limbs are both developed but 

 often feeble; the digits are 3-2, or 4-4, or 4-5; eyelids are ab- 

 sent; upper and lower jaws bear teeth; the skeleton is largely 

 cartilaginous, and the vertebrae pseudocentrous; there are no 

 supra- or basi-occipitals, supra-temporal, maxillary, nasal, or 

 prefrontal bones; there are three epibranchial cartilages but 

 of the four gill clefts of the larva two remain, the first and the 

 fourth closing; the carpal and tarsal cartilages are few; the 

 alimentary canal is simple. 



The Amphiumidae are caducibranchiate; the fore and hind 

 limbs are developed though feeble, owing probably to degen- 

 eracy, the digits are 3-3; the eyelids are absent; upper and 

 lower jaws bear teeth; the skeleton is in part cartilaginous 

 but to a less degree than in the above, while the vertebrae are 

 still pseudocentrous; there is no median sternal cartilage; the 

 skull is well formed and osseous but there are no supra- or 

 basi-occipitals or intercalary bones; there are three epibranchial 

 cartilages but of the four larval clefts only one remains open; 

 the carpal and tarsal cartilages are few; the alimentary canal 

 is clearly divided into areas and the intestine is convolute. 

 This group is an advance on the two former, but the numerous 



