OKDEES OF FISHES. 



237 



CHAPTEE XXIV. 



OEDEES OF FISHES. 



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

 further will be attem])ted than merely to give an outline of tlie 

 leading peculiarities which distinguish the different orders. The 

 clas.sification here adopted is the one now mo.st usually followed, by 

 which the class Pisces is divided into the f<illowing si.x orders : — 



1. Phary'iujohranrhii. 



2. Marsipi)hrandiii. 

 .3. Teleostei. 



4. Ganoidei. 



5. Elasmohranc/ni. 

 (i. Dipnoi. 



Order I. Pn.vRyNGOBRANciiii. 



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

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

 all its impoi'tant points from that of all the other members of the 

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

 organisation as yet known in the Vertfbi'aiu. The Lancelet (fig. 170) 



Fi,^. 170.— Pli.iryngohrancliii. Th^'^jar\&<^\&t (Ampliioxvs h'vcmlatu^), enlarged. 



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

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

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

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

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

 portion of the ventral sui-face, and ex)iands at the tail to fomi a 

 lancet-shaped caudal fin. There are no tnie "]iaired " fins, repre- 



