ORIGIN OF CHORDATES 429 



hand, the Palaeoniscoids also gave rise to the sturgeons. 

 Chondrosteus (Jurassic) is already like the sturgeon Acipenser. 

 These animals preserve the open spiracle, but the palaeoniscoid 

 structure of the scales is lost. Sturgeons are both fluviatile 

 and marine. 



Another line of evolution from the Palaeoniscoids leads to 

 the higher bony fish or Holostei. These fish lose the open 

 spiracle and their tails assume the homocercal pattern. At 

 the same time the radials of the paired fins become reduced, 

 and the web of the fin is supported mostly by the dermal 

 fin-rays or lepidotrichia. In the median dorsal and ventral 

 fins the lepidotrichia correspond in pairs to the radials, so that 

 the fins can be lowered and raised. Of the Holostei, two 

 groups are primitive. One of these contains Amia, an in- 

 habitant of the rivers of North America. Its lung is still 

 highly vascular and supplied by pulmonary arteries, and in 

 the region of the tail its vertebral column consists of separate 

 hypo- and pleurocentra. The other group contains Lepi- 

 dosteus, likewise an inhabitant of North American rivers. Its 

 scales are of the peculiar pattern known as lepidosteoid, with 

 a covering of ganoin. The lung is vascular, but supplied by 

 arteries from the dorsal aorta. It is worth noticing that the 

 primitive Osteichthyes are almost exclusively inhabitants of 

 fresh water. 



In the remaining highest bony fish or Teleostei (a term 

 not to be confused with Teleostomi), the scales lose the layer 

 of ganoin and become thin and transparent. The lung 

 becomes modified into a swim-bladder, and loses the vascular 

 spongy walls characteristic of a lung. It functions as a hydro- 

 static organ of adaptation to different depths, and this illustrates 

 the fact that the Teleostei are a group which has reinvaded the 

 sea from fresh water. Rivers do not possess sufficient depth 

 to necessitate a swim-bladder. A number of Teleosts, how- 

 ever, are inhabitants of fresh water, to which they have pre- 

 sumably returned from the sea. The Teleosts have radiated 

 into a great many different lines, and are the most successful 

 of the fish. They have become specialised to various modes 

 of life, but they must be regarded as a sterile side branch on 



