ORIGIN OF CHORDATES 427 



large estuaries, and that the evolution of the early true chordates 

 took place in such surroundings. 



The ciliary method of feeding which these animals possessed 

 limited the size of the particles of food which they could ingest, 

 and the size to which they could grow. 



The earliest chordates of which fossil remains are known 

 are the Ostracoderms (from the upper Silurian and Devonian), 

 which recent work has shown to be related to the Cyclostomes, 

 especially as regards the brain, auditory organs, and blood- 

 vessels. This raises some interesting problems, because the 

 Ostracoderms possessed denticles, bone, and paired fins, all 

 of which structures are lacking in Cyclostomes. The mode of 

 life of Petromyzon, Bdellostoma, and Myxine is undoubtedly 

 degenerate with their sucking mouth, but they would be more 

 degenerate than otherwise expected if they had lost the struc- 

 tures possessed by the Ostracoderms. The curious fossil 

 Palasospondylus (from the Devonian) may be related to these 

 forms, in all of which evolution had proceeded far enough for 

 the formation of a definite head. 



The first true fish appear to have been cartilaginous 

 (together, the cartilaginous fish are called Chondrichthyes) and 

 related to the Selachii. Among them may be mentioned 

 Acanthodes (upper Silurian), Cladoselache (Devonian) in- 

 teresting for the structure of its paired fins, and Pleuracanthus 

 (Permian). All these forms had true biting jaws, two pairs of 

 paired fins, and heterocercal tails. True Selachii related to 

 Heterodontus (the Port Jackson Shark) appeared in the 

 Carboniferous. At the present day, the Selachii are repre- 

 sented by the true sharks (and dogfish), and by the rays (Raia, 

 Torpedo), which have become adapted to living on the sea 

 bottom and have become flattened in consequence. Their 

 pectoral fins have expanded and fused with the sides of the 

 body. The gill-slits are on the under surface, the spiracle 

 is above. One member of the rays, Pristis the saw-fish, 

 has returned to an active mode of life. The angel-fish 

 Rhina is intermediate in form between the sharks and the 

 rays. 



Another group of cartilaginous fishes diverged in the 



