444 



AMPHIBIA. 



grows out an allantoic bladder, homologous with the allan- 

 tois in the embryos of higher Vertebrates. 



The ova are small, numerous, usually pigmented, and 

 with yolk towards one pole. They are almost always laid in 

 the water; the segmentation is holoblastic, but unequal. 

 There is usually a metamorphosis in development. It is very 

 difficult to distinguish Amphibians from Fishes, 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. 



From the higher Vertebrates or Amniota, the Amphi- 

 bians are clearly distinguished by the presence of gills in 

 youth at least, and by the absence of an amnion and allan- 

 tois. For though the bladder of Amphibians be homologous 

 with an allantoic outgrowth, it does not function as such, in 

 other words, it does not aid in the respiration or nutrition 

 of the embryo. 



ICHTHYOPSIDA. 



Some of the common characters' — 



Gills are always present, during youth at least. 



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 they may be diminished in the adults. 

 Unpaired fins are almost always represented, during 



youth at least. 



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 respira- 

 tory significance is slight. 



Except in Dipnoi, the heart has two 

 chambers. There is no inferior vena 

 cava. 



The limbs are fins. 



The unpaired fins are supported by fin- 

 rays. 



The skull has 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 and correspond to 



those of Amniota. 

 There are no fin-rays. 



There are two occipital condyles. 



There is no hint of exoskeleton, except in 



a few exceptional cases. 

 There are posterior nares opening into 



the cavity of the mouth. 

 The bladder is the homologue of theallan- 



tois. 



