Procceilini/s, _ 17 



The last and highest group of fishes is the Dipnoi, a word 

 meaning "two breathings," in allusion to the fact that they 

 have acquired lungs to breathe with in addition to their gills, 

 their lungs being derived from the air-bladder of ordinary 

 lower fish. The heart, too, is an advance upon that of other 

 fish, for it contains an extra chamber, called the left auricle, 

 into which the blood flows after being purified in the lungs. 

 But though the Dipnoi seem to be in advance with regard to 

 these matters, it is a curious thing that, with regard to a 

 spinal column and limbs, they are decidedly behindhand. 

 For the back-bone is a gristly rod, Hke that of the Lamprey, 

 and the limb is a simple-jointed rod, with a pair of fine fin- 

 rays on each joint. The habits which give to these fishes the 

 name of "Mud-fish" are thus described by Prof. Huxley: — 

 "The Lepidosiren of the East and West Coasts of Africa has 

 a habit in the dry season, when the water vanishes, of bur- 

 rowing into the mud, taking care to leave a hole communi- 

 cating with the atmosphere. Here the fish remains enclosed 

 in mud, which, by drying, becomes of brick-like hardness, 

 and it is said that in this state they have been brought into this 

 country, where warm water is only required to disintegrate 

 the clay, when the fish emerges, active and voracious." 



This completes the fishes. And now let me introduce to 

 you a little fish-like animal. 



This little creature has a rounded fish-like body, through 

 which, instead of a jointed back-bone, a gristly uotochord 

 runs, and, like the lower fish, it has no limbs. To enable it 

 to get about, however, it is furnished with a long tail, which 

 supports a broad fin, and by waving this to and fro it 

 progresses quickly through the water. Like the lower fish, 

 too, it has holes on each side of its neck through which two 

 pairs of breathing-tufts or gills are situated just behind the 

 holes. It has a rounded, sucking mouth, like a Lamprey. It 

 also has the ordinary type of fish-heart— two chambers, 

 auricle and ventricle. In fact it is a fish. But as this fish 

 grows, changes take place. The gristly notochord separates 

 into joints, and becomes bony : it is converted into a true 

 spinal-column. Next two pairs of little limbs make their 



