BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 



103 



seas. It could obtain aud secrete in a solid form, the carbonate 

 of lime in solution in the sea-water, and with that secreted solid 

 calcareous matter build a surrounding habitation exactly as 

 does the coral animal of to-day. The Echinodermata has lost 

 two Orders since Palaeozoic times, but each of the existing three 

 Orders, Asteroidea, Echinoidea and Crinoidea, were represented 

 in the seas of that epoch, and the little PaloMster of the 

 Silurian seas was quite like a little star-fish of our southern 

 shores. 



The early fishes had a peculiar structure, but it was not a 

 structure peculiar to Palaeozoic times, for there is the same 

 structure to be seen in many living fishes. This was the 

 extension of the backbone to the end of one of the lobes of the 

 tail, the other lobe being merely a fin lobe. And with this 

 unsymmetrical tail the earliest fishes had an exterior coating of 

 bony plates instead of scales. But in the sturgeon this type of 

 fish still lives, and not in tropical waters or under exceptionally 

 warm conditions, since sturgeons are often cau2;ht off British 

 coasts and, as is well known, abound in the Eussian Caspian and 

 Volga. The sharks are also representative of the heterocercle 

 tailed fishes, as they are called, but the majority of recent fishes 

 have equal lobed or homocercle tails. Even the peculiar 

 Dipterus of Devonian age has its li^^.ng representative in the 

 both lung and gill-possessing Ccratodus of Australia. 



Insects quite like those now living abounded in Pakeozoic 

 times, for cockroaches, crickets, beetles, dragon-flies, etc., were 

 plentiful, and no less than 239 species of Orthoptera have been 

 taken from Carboniferous strata. There are besides, in the 

 Jurassic rocks, remains of earwigs, grasshoppers, white-ants, 

 may-flies, and that genus of Diptera we know so well, the fly. 



But perhaps the most striking example of persistency of 

 form and structure and the continuance of the same physio- 

 logical power implying the same function of the same organs, 

 is afforded by the little Lingula, a genus of the Class. 

 Brachiopoda. The fossil, Linguldla Davisii is in sutficient 

 numbers in one of the divisions of the Cambrian rocks to uive 

 it the name Lingula Flags, and the Lingula is now living in 

 abundance in the China seas. These two species are essentially 

 the same animal. Their general form and size are similar, the 

 character of the horny shell, in composition and structure, of 

 both, was similar, and thus we see that the animal of Cambrian 

 times was morphologically and physiologically allied to the 

 Lingula of our own day. As might be expected, the Lingula is 

 found fossil in many formations between the Lingula Flags 



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