THE VERTEBRATE EYE 235 



Cyclostomes, the lamprey and the hag. From these there evolved somewhere 

 in the Upper Silurian period, 350 million years ago,^ the true (gnathous) fishes, 

 possessed of jaws and paired fins. From these primitive fishes three classes 

 radiated : (1) the Placoderms, a motley class mostly with bony armour, which 

 flourished in Devonian times but none of which survived the Palaeozoic era ; 

 (2) the Chondrichthyes, a class of cartilaginous fishes of great age which are now 

 represented only by the Selachians (sharks and rays) and Holocephalians (deep- 

 sea chimtpras) ; and (3) the Osteichthyes, the much larger class of bony fishes. 

 While the Placoderms have disappeared, and the cartilaginous fishes, prolific 

 in the older geological periods, have steadily decreased in importance in more 

 recent times, the bony fishes have shown themselves remarkably adaptive. 

 By the end of the Palceozoic era they had attained almost sole possession of 

 fresh-water streams and lakes ; at that time they had invaded the sc^^s also 

 and rapidly constituted the vast majority of marine forms. 



These bony fishes may be divided into two main sub-groups, each of which 

 has numerous survivors : the Actinopterygii and the Crossopterygii. From the 

 former a series of forms arose in linear progression — the Chondrostei, still with 

 a largely cartilaginous internal skeleton, degenerative representatives of which 

 still survive as the Polypterini (two species of which are extant) and the sturgeons ; 

 the Holostei, provided with bony skeletons, represented today only by two 

 American fresh-water fishes, the bow-fin and the gar-pike ; and eventually the 

 Teleostei, the most specialized of all fishes which include practically all modern 

 species. 



From the early Crossopterygii the Dipnoi (lung-fishes) appeared as an 

 aberrant off-shoot in the lower Devonian period ; of these, three species survive 

 today, swamp -dwelling, mud-loving and eventually air-breathing fishes in which 

 the swim-bladder has been retained as a functioning lung. From the main 

 group, however, a direct line of vertebrate descent continued through the 

 Rhipidistia (a derivative of which exists today as the Coelacanth, Latimeria) ; 

 these fish could already breathe air so that they only had to turn their fins 

 into legs and modify the ear to become Amphibia and survive on land. Develop- 

 ing as tadpole-like aquatic creatures, they underwent this remarkable meta- 

 morphosis as they matured into their adult forms. Initially they lived side-by- 

 side with their cousins, the lung-fishes, in the swamps ; but when the great 

 droughts appeared and the fresh-water pools dried up towards the end of the 

 Devonian period some 300 million years ago, the lung-fishes largely perished, 

 but the Amphibians, capable of creeping and feeding on land, survived. Their 

 first representatives have long become extinct and the class survives today 

 only in three relatively unimportant and highly specialized groups — the frogs 

 and toads (Anura), the salamanders and newts (Urodela) and the worm-like 

 Cfecilians (Apoda). From the highly adaptable primitive types, however, 

 there evolved in the Upper Carboniferous period the first fully terrestrial verte- 

 brates, the most primitive Reptiles, born on land and capable of existing away 

 from water altogether. This spectacular step in evolution was made possible 

 by the development of a large and highly nutrient egg protected by a porous 

 shell so that the young reptile could emerge fully equipped for terrestiial life. 



For many millions of years these primitive reptiles fiourished exceedingly ; 

 emerging on to the hitherto unexploited land, rich in vegetation and food, they 

 spread and gave rise to a multitude of new types, some of them of incredible 

 form and giant size. They still retained, however, the cold-blooded characteristic 

 of their fish and amphibian ancestors, and thus, presumably owing to the climatic 

 changes at the end of the IMesozoic era, this group which had dominated the 



1 See p. 754. 



