THE AMPHIBIA. 63 



developed villi, or lamellae, which subserve the respiratory 

 function. 



Median fins, formed by prolongations of the integument, 

 supported by one or other kind of skeleton, are very character- 

 istic of fishes, and it is questionable if any fish exists altogether 

 devoid of the system of median fin-rays and their supports, 

 which have been termed inter-spinous bones and cartilages. 

 On the other hand, no vertebrate animal, other than a fish, is 

 known to possess them. 



When the limbs, or pectoral and ventral fins, of fishes are de- 

 veloped, they always exhibit a more or less complete fringe of 

 fin-rays. ISo amphibian is known to possess such rays in its 

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

 extinct Ichtliyosauria 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 Lepiclosiren 

 are exceptions to this rule. 



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

 commonly red, but by a singular exception those of Aiwphioxus 

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

 organization) are colourless. 



Almost all fishes have the heart divided into two auricles 

 and one ventricle ; but Amphioxus, as I have previously stated, 

 is devoid of any special heart, being provided instead with a 

 number of contractile, vascular dilatations ; while Lejndosiren 

 possesses two auricles, and, at the same time, is provided with 

 true lung's. 



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. 



The Amphibia (or Batracliians, Salamandroids, Coecilise, and 

 Labyrinthodonts) resemble fishes, and differ from all other verte- 

 brates 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 exer- 

 cising a respiratory function, or branchiae, are developed from 

 their visceral arches during a longer or shorter period. 



