ORDERS OF FISHES. 237- 



CHAPTER XXIV. 



ORDERS OF FISHES. 



The number of diiferent kinds of fishes is so enormous that nothing 

 further will be attempted than merely to give an outline of the 

 leading peculiarities which distinguish the different orders. The 

 classification here adopted is the one now most usually followed, by 

 which the class Pisces is divided into the following six orders : — • 



1. Pharyngohranchii. 



2. Marsipohranchii. 



3. Teleostei. 



4. Ganoidei. 



5. Elasmobranchii. 



6. Dipnoi. 



Order I. Pharyngobranchii. 



This order of fishes includes only a single animal, the anomalous 

 Amphioxus, or Lancelet, the organisation of which differs in almost 

 all its important points from that of all the other members of the 

 class. In fact, the Lancelet presents us with the lowest type of 

 organisation as yet known in the Vertebrata. The Lancelet (fig. 170) 



Pig. 170.— Pharyngobranchii. The Lancelet (Amphioxus lancmlatus), enlarged. 



is an extraordinary little fish, from one and a half to two inches 

 long, which burrows in sand-banks in various seas, but is especially 

 abundant in the Mediterranean. The body is lanceolate in shape, 

 and is provided with a, narrow membranous border, of the nature 

 of a median fin, which runs along the whole of the dorsal and a 

 portion of the ventral surface, and expands at the tail to fortn a 

 lancet-shaped caudal fin. There are no true " paired " fins, repre- 



