254 CLASSIFICATION OF VERTEBRATES. 



rise to the blastoderm, which gradually extends over the yolk. 

 A peculiarity in development is the formation of the central 

 nervous system as a solid cord or 'keel,' in which a lumen 

 appears later by splitting. In many teleosts a peculiar saccular 

 structure, known as Kupffer's vesicle, occurs near the hinder 

 end of the alimentary tract. It disappears before hatching, and 

 its significance is not yet understood. 



The teleosts first appear in the triassic, and in the creta- 

 ceous they exceed the ganoids in number, while to-day the 

 group includes the vast majority of the forms some fifteen 

 thousand in all commonly known as fishes. 



The classification of the group is in a very unsatisfactory 

 condition, especially as regards the vast order acanthopteri, 

 where as yet it is almost impossible to frame sub-orders, owing 

 to the extent to which the different families intergrade, and the 

 limited degree to which we can ascertain the lines of descent. 

 The trouble is largely based upon the fact that many lines have 

 persisted, so that our artificial systems cannot easily be applied. 

 The teleosts have apparently had two lines of descent, one 

 leading from sturgeon-like ancestors to the Ostariophysi (for 

 it is difficult to believe that the peculiar Weberian apparatus 

 has been evolved twice in the history of the fishes), and the 

 other from some Amia-like form to the isospondyli, and these 

 to the apodes on the one hand, and to the acanthopteri and other 

 orders. For convenience the division Physostomi is retained 

 for all those fishes, the ostariophysi excepted, in which the duct 

 of the air-bladder remains open permanently ; the term Physo- 

 clisti is frequently employed for the remaining groups in which 

 the duct is closed, as well as for those in which the bladder 

 itself has disappeared. 



ORDER I. OSTARIOPHYSI. 



Teleosts with the anterior vertebrae modified into a ' Web- 

 erian apparatus ' connecting the inner ear with the large swim- 

 bladder. Fins without spiny rays, or at most with a single 

 spine in front of pectorals and dorsals ; ventrals abdominal in 

 position ; duct to air-bladder persisting. 



