WORaiS TO VERTEBRATES 57 



them are largely or entirely parasitic, and in connec- 

 tion with this mode of life have undergone modifica- 

 tions and degeneration which make it exceedingly dif- 

 ficult to decipher their descent or relationships. 



Four of these branches have reached great promi- 

 nence in numbers and importance. One or two others 

 were formerly equally numerous and have since become 

 almost extinct ; so the brachiopoda, which have been 

 almost entirely replaced by mollusks. The same may 

 very possibly be true of others. For of the amount of 

 extinction of larger groups we have generally but an 

 exceedingly faint conception. Indeed in this respect 

 the worms have been well compared to the relics which 

 fill the shelves of one of our grandmother's china-closets. 



The four great branches are the echinoderms, mol- 

 lusks, articulates, and vertebrates. The echinoderms, 

 including starfishes, sea-urchins, and others straggled 

 early from the great army. We know as yet almost 

 nothing of their history ; when deciphered it will be as 

 strange as any romance. The vertebrates are of course 

 the most important line, as including the ancestors of 

 man. But we must take a little glance at mollusks, 

 including our clams, snails, and cuttle-fishes ; and at 

 the articulates, including annelids and culminating in 

 insects. The moUuscan and articulate lines, though 

 divergent, are of great importance to us as throwing a 

 certain amount of light on vertebrate development; 

 and stiU more as showing how a certain line of devel- 

 opment may seem, and at first really be, advantageous, 

 and still lead to degeneration, or at best to but pai-tial 

 success. 



When we compare the forms which represent fairly 

 well the direction of development of these three lines, 



