‘AMPHIBIANS. 579 
mentation is holoblastic, but unequal, 
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 dim- 
inished 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, ze. it does not aid in the respiration or the 
nutrition of the embryo. ‘ 
_ Itis 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 contrasted :— 
There is usually a 
FISHES. 
AMPHIBIANS. 
Gills 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. 
There are no true posterior nares. 
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, 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 ~ 
