50 evolution: the ages and tomorrow 



very peculiar radial forms as adults. Their locomotor sys- 

 tem, which is unique in the animal kingdom, is a closed 

 hydraulic device of great ingeniousness, especially adapted 

 to outlast the shell-closing muscles of an oyster; and they 

 also possess a control over external parasites that other ani- 

 mals might well envy. This control is an arrangement of 

 double-pronged pincers all over the skin, so set up as to 

 pinch off any organisms which try to establish themselves. 

 These two inventions have never been used by any other 

 organisms. 



The Mollusca, too, are slightly peculiar. In derivation 

 they are close to the worms, but they have been able to take 

 a modified body plan and develop it into very successful 

 organisms— clam, snail, octopus, and squid. 



The primitive representatives of all these forms had ap- 

 peared at, or soon after, the opening of the Paleozoic era, 

 some 800,000,000 years ago. During the Paleozoic era many 

 of them followed the plants out onto the land and some 

 very nearly completed their evolution. 



In the meantime the vertebrates appeared. The first ver- 

 tebrate fossil in the record is a crude, armored, fish-like 

 form, called ostracoderm, hardly prophetic of the great 

 destiny of the backboned animals. This fossil probably gives 

 us the type that was quite close to the ancestral origin of the 

 rest of the vertebrates. It was one of the cyclostomes (round 

 mouth), some primitive unarmored representatives of which 

 are still with us, namely, the lamprey and the hagfish. In a 

 study of the origins of the vertebrates we find another living 

 relic, a worm-like animal called Balaiioglossus. Authorities 

 agree that the vertebrates were most probably evolved 

 through forms similar to Balanoglossus and the ostraco- 

 derms, but there is disagreement as to earher original types. 

 There is some evidence for the theory of annelid sources, 

 but there is the stronger possibility that the group to which 

 we belong branched off the ancestral line that also led to 

 the echinoderms (or starfish). 



