254 The Study of Animal Life part in 



mydoselachus) is said to be very closely allied to types which occur 

 in the Old Red Sandstone. Allied to the Elasmobranchs, but 

 sometimes kept in a separate division, are two genera, the Chimcera 

 or King-of-the-Herrings, and Callorhynchus, its relative in Southern 

 Seas. 



(2) The Ganoid fishes are almost, if not quite, as ancient as the 

 Elasmobranchs, but their golden age, long since past, was in Devonian 

 and Carboniferous ages. There are only some seven different kinds 

 now alive. Two of these are the sturgeons (Acipenser) and the 

 bony pike (Lepidosteus). The latter has a bony skeleton ; the 

 sturgeon is in part gristly. An armature of hard scales is very 

 characteristic of this decadent order. 



(3) In Permian times, when Reptiles were beginning, a third 

 type of fish appeared, of which the Queensland mud-fish (Ceratodus) 

 seems to be a direct descendant. In this type the air-bladder is 

 used as a lung, thus suggesting the transition from Fishes to Am- 

 phibians. Perhaps this order was always small in numbers ; now- 

 adays at least there are only two genera — Ceratodus, from the 

 fresh water of Queensland, and Protopterus, from west and tropical 

 Africa ; while another form, sometimes called a different genus 

 (Lepidosiren), is recorded from the Amazons. Double-breathers or 

 Dipnoi we call them, for they do not depend wholly upon gills, but 

 come to the surface and gulp air into their air-bladder. Mud- 

 fishes they are well named, for as the waters dry up they retire into 

 the mud, forming for themselves a sort of nest, within which they 

 lie dormant. 



(4) In the Chalk period the characteristically modern fishes 

 (Teleosteans), with completely bony skeletons, began. Herring 

 and salmon, cod and pike, eel and minnow, and most of the com- 

 monest fishes belong to this order. Heavy ironclads yield to swift 

 gunboats, and the lithe Teleosteans have succeeded better than the 

 armoured Ganoids. 



The wedge-like form of most fishes is well adapted for rapid 

 swimming. Most flat fish, whether flattened from above down- 

 wards like the gristly skate, or from side to side like the flounders 

 and plaice, live at the bottom ; those of eel - like shape usually 

 wallow in the sand or mud ; the quaint globe-fish float passively. 

 The chief organ of locomotion is the tail ; the paired fins help to 

 raise or depress the fish, and serve as guiding oars. In the climb- 

 ing perch they are used in scrambling ; in the flying fish they are 

 sometimes moved during the long swooping leaps. In eels and 

 pipe-fish they are absent ; in the Dipnoi they have a remarkable 

 median axis. The unpaired fins on the back and tail and under 

 surface are fringes of skin supported by rays. 



Fishes are often resplendent in colours, which are partly due to 



