AMPHIBIANS. 



559 



always laid hi wafer ; the segmentation is holoblastic, but un- 

 equal. There is usually a metamorphosis in development. 



Huxley was the first to recognise the affinities between Fishes and 

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



Of the characters common to the two classes, the following are 

 important : Gill-slits are functional in respiration, but in Amphibians they 

 may disappear after larval life, the Eustachian tube excepted ; gills are 

 always present, but they may be restricted to the larval stages in 

 Amphibians ; in fishes and larval Amphibians a single ventral aorta 

 leaves the heart ; there is no amnion, and at most a homologue of the 

 allantois (in Amphibians) ; there are only ten pairs of cranial nerves ; 

 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 ; there is a functional pronephros in early stages. 



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 functional allantois. For though the bladder of 

 Amphibians 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 ob- 

 vious differences are the absence of fin -rays and the development of fingers 

 and toes. In the following table the two classes are contrasted :- 



FISHES. 



(lills persist throughout life. 



The swim-bladder functions as a lung 

 in Dipnoi and less markedly in 

 some " Ganoids," but in most cases 

 its respiratory significance is slight. 



The heart is two-chambered (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 (dermotrichia). 



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. 



AMPHIBIANS. 



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, and paired 

 posterior cardinals are seen only in 

 the larva. 



The limbs have digits. 

 There are no fin-rays. 



There are two occipital condyles. A 

 columella runs from the tympanum 

 to a fenestra ovalis in the ear 

 capsule. 



There is no exoskeleton, except in a 

 few cases, and in extinct forms. 



There are posterior nares opening into 

 the cavity of the mouth. 



The cloacal bladder seems to be the 

 homologue of the allantois. 



