6 AMPHIBIA 



CHAP. 



assuming evolution to be true, there must have lived countless 

 creatures which were a " rudis indigestaque moles," neither 

 Amphibia nor Eeptilia, in the present intensified sense of the 

 systematist. The same consideration applies equally to the line 

 which leads downwards to the Fishes. But the great gulf 

 within the Yertebrata lies between Fishes and Amphibia, between 

 absolutely aquatic creatures with internal gills and " fins," and 

 terrestrial, tetrapodous creatures, with lungs and fingers and 

 toes. On the side of the fishes only the Dipnoi and the Crosso- 

 pterygii como into consideration. 



The piscine descent of the Amphibia is still proclaimed by 

 the following features. (1) The possession by the heart of a 

 long conus arteriosus, provided with, in many cases, numerous 

 valves, or at least (Anura) one series at the base, another at the 

 beginning of the truncus where the arterial arches branch off; 

 (2) the strictly symmetrical arrangement of these arches; (3) 

 the trilocular heart is still like that of the Lung-fishes or Dipnoi ; 

 (4) the occurrence of as many as four or even five branchial 

 skeletal arches in the larval stage ; (5) the glottis is supported 

 by cartilages which themselves are derivatives of posterior visceral 

 arches; (6) the development of the vertebrae (Stegocephali and 

 Urodela) from four pairs of arcualia, and the formation of the 

 intervertebral joints by a split across the intervertebral ring of 

 cartilage : this feature is unknown in Eeptilia, but it occurs also 

 in Lepidosteus, most probably also in Polypterus ; (7) the hypo- 

 glossal still retains the character of a post-cranial or cervical 

 spinal nerve; (8) the presence of lateral sense-organs; (9) the 

 possession of external gills is of somewhat doubtful phylogenetic 

 value, although such gills occur amongst fishes only in Dipnoi 

 and Crossopterygii. It is not unlikely that in the Amphibia 

 these organs owe their origin to entirely larval requirements, 

 while the suctorial mouth of the larvae of the Anura and many 

 fishes has certainly no ancestral meaning, but is a case of con- 

 vergent development. 



The usual diagnoses of the Amphibia contain the statement 

 that they, or most of them, undergo a metamorphosis, or pass 

 through a larval stage. The same applies to various fishes ; 

 while, on the other hand, the larval (not ancestral) stage has 

 become permanent in the Proteidae and Sirenidae ; and lastly, we 

 cannot well speak of larvae in the viviparous Salamandra atra. 



