390 ZOOLOGY. 



ordinary fishes, and is subdivided into six orders by cha- 

 racters in general not very important. These orders are 

 Acanthopterygii, Malacopterygii abdominales, Malacopterygii 

 sub-branchiales, JVJalacopterygii apodes, Lophobranchii, and 

 Plectognathi. 



Fig. 398. Common Cod. 



495. The plectognathi differ from all other fishes in the 

 conformation of their mouth, for in them the upper jaw-bone 

 is united to the cranium. This family comprises the coffres 

 (Fig. 399), which have the body covered with a kind of cuirass 

 with osseous compartments ; the diodon or globe-fish, and 

 the tetrodon, which by swallowing the air become inflated 

 like a ball, are examples of this class. 



496. The lophobranchii are characterized by the struc- 

 ture of the gills. These, instead of resembling the teeth of a 

 comb, divide into little rounded tufts, fixed in pairs along the 

 branchial arches. The syngnathus and the hippocampus 

 belong to this family. 



Fig. 399. Trunk Fish (Ostracion). 



497. The acanthopterygii comprise all osseous fishes 

 with the upper jaw moveable and comb-formed branchiae, and 

 in whom the first fin is supported by osseous and spiniform 

 rings (Fig. 386). The perch, the mackerel, the tunny (Fig. 

 400), the sword-fish, and nearly three-fourths of all known 

 fishes belong to this family. 



