246 



CLASSIFICATION OR VERTEBRATES. 



bones, called epineurals, epicentrals, or epipleurals, accordingly 

 as they are articulated to neural arch, centrum, or ribs. 



The mouth is usually armed with teeth ; and these may occur 

 not only on the bones which form the edge of the jaws (pre- 

 maxillary, maxillary, dentary), but on those which form the roof 

 of the mouth (palatine, vomer, pterygoid), and also on the pha- 

 ryngeal bones. The alimentary canal usually has the regions 



well differentiated, and in 

 the ganoids a spiral valve 

 occurs in the intestine. 

 Pyloric caeca, from a sin- 

 gle one to two hundred 

 in number, are common. 

 There is no cloaca, as the 

 urogenital ducts always 

 open behind the vent. 



A spiracle occurs only 

 in some ganoids. The 

 gills differ from those of 

 the lower fishes in the 

 reduction of the inter- 

 branchial septum, so that 

 the gills themselves pro- 

 ject beyond the arch into 

 the opercular cavity like 

 the teeth of a comb. In 

 the lophobranchs the gill 

 filaments are replaced by 

 curious tufts. An air- 

 bladder or swim-bladder is usually present. It arises as an 

 outgrowth from the dorsal side of the oesophagus (except in 

 Polyptems), which soon becomes differentiated into bladder 

 and duct. In the lower forms the duct remains open through- 

 out life (ganoids and physostomi), but in the physoclisti it 

 closes later, and the bladder loses all connection with the 

 exterior. In many ganoids and some teleosts the inner sur- 

 face becomes plicated. In most forms it receives its blood- 

 supply from the aorta direct, or by way of the coeliac axis ; but 



FIG. 247. Relations of gill clefts in a 

 teleost. O, operculum, enclosing a branchial 

 chamber. 



