530 



AMPHIBIA. 



Huxley was the first to recognise the aFlinities between Fishes and 

 Amphibians, and to unite the two classes under the title Ichthyopsida. 



Of the common characters of the two classes, we may emphasise the 

 following : — gills are always present, Imt may be restricted to the larval 

 stages ; there is no amnion, and at most a homologue of the allantois ; 

 there are lateral sensory structures, such as the " branchial sense- 

 organs" and those of the " lateral line," but these may be diminished 

 in the adults ; unpaired fins are almost always represented, but may 

 not persist in the adult life. 



From the higher Vertebrates or Amniota the Ichthyopsida are clearly 

 distinguished by the presence of gills (in youth at least) and by the 

 absence of amnion and allantois. For though the bladder of Amphi- 

 bians may be homologous with an allantoic outgrowth, it does not function 

 as such, i.e., it does not aid in the respiration or the nutrition of the 

 embryo. 



It is more difficult to distinguish between Fishes and Amphibians, 

 more especially if we include the Dipnoi in the former class. The most 

 obvious differences are the absence of fin-rays and the development of 

 fingers and toes. In the following table the two classes are con- 

 trasted : — 



Ami'hibians. 



Gills persist throughout life. 



The swim bladder functions as a lung in 

 Dipnoi and less markedly in some 

 Ganoids, but in most cases its respira- 

 tory significance is sliuht. 



The heart is two-cbambered (incipiently 

 three-chambered in Dipnoi)- There 

 is no inferior vena cava, except in 

 Dipnoi. 



The limbs are fins. 



The unpaired fins are supported by fin 

 rays. 



The skull has, in most cases, one occipital 

 condyle. 



There is usually an exoskeleton of scales 

 or scutes. 



Except in Dipnoi, the nasal sacs do not 

 open posteriorly into the mouth. 



There is no certain homologue of the 

 allantois. 



Gills may disappear as the adult form is 



attained. 

 Lungs are always developed in the adults. 



It is doubtful whether they are 



directly comparable with the swim 



bladder. 

 The heart has three chambers. There is 



an inferior vena cava. 



The limbs have digits. 

 There are no fin rays. 



There are two occipital condyles. 



There is no exoskeleton, except in a 



few exceptional cases, and in e.xtinct 



forms. 

 There are posterior nares opening into 



the cavity of the mouth. 

 The bladder seems to he the homologue 



of the allantois. 



The Ft'og as a type of Amphibians, 



The common British frog {Rajui te;nporaria) and the 

 frequently imported Continental species [R. esculenta) agree 

 in essential features. A black patch on the side of the 

 head behind the ear distinguishes our British species ; the 

 males of the edible frogs have special resonating sacs, and 

 there are other trivial differences. 



