64 INTRODUCTION TO CLASSIFICATION. 



developed, they always exhibit a more or less complete fringe 

 of fin-rays. No amphibian is known to possess such rays in its 

 lateral appendages, but there is some reason to believe that the 

 extinct Ichthyosauria may have been provided with them. 



In most fishes, the nasal sacs do not communicate directly 

 with the cavity of the mouth, but the Myxinoids and Lepidosiren 

 are exceptions to this rule. 



The blood-corpuscles of fishes are always nucleated, and are 

 commonly red, but by a singular exception those of Amphioxus 

 (the Lancelet, which is an exception to most rules of piscine 

 organization) are colourless. 



Almost all fishes have the heart divided into two chambers, 

 one auricle and one ventricle ; but Amphioxus, as I have pre- 

 viously stated, is devoid of any special heart, being provided 

 instead with a number of contractile, vascular dilatations ; while 

 Lepidosiren possesses two auricles, and, at the same time, is 

 provided with true lungs. 



It is useless, therefore, to appeal to the olfactory organ, the 

 blood, the heart, or the respiratory organs, for characters at once 

 universally applicable to, and diagnostic of, fishes, 



XXV. THE AMPHIBIA. 



The Batrachians, Salamandroids, Ciecilise, and Labyrintho- 

 donts resemble fishes, and differ from all other vertebrates in 

 the entire absence of an amnion, and in having only the urinary 

 bladder to represent the allantois. They have red nucleated 

 blood- corpuscles. Yet again, they resemble fishes and differ 

 from all other vertebrates in the fact that filaments exercising a 

 respiratory function, or branchiae, are developed from their 

 visceral arches during a longer or shorter period. 



Some possess median fins, but these are not supported by 

 fin-rays, and their limbs are never fringed with fin-rays. 



Furthermore, in all Amphibia which possess limbs, the skele- 

 ton of these limbs is divisible into parts which obviously corre- 

 spond with those found in the higher vertebrates. That is to 

 say, in the fore limbs there are cartilages, or bones, answering in 

 their essential characters and arrangement to the humerus, radius 



