154 ( io ) 



of organs enclosed in a visceral cavity, including a ru- 

 dimental nervous system in the form of a ring. In the 

 Molluscs to this condition is added additional complica- 

 tion, including extensions of the nervous system from 

 the ring as a starting-point, and a special organ for a 

 heart. In the Articulates (crabs, insects,) we have like 

 complications, and a long distinct nervous axis on the 

 lower surface of the body. The last branch or division 

 of animals is considered to be higher, because all the 

 systems of life organs are most complex or specialized. 

 The nervous ring is almost obliterated by a great en- 

 largement of its usual ganglia, thus become a brain, 

 which is succeeded by a long axis on the upper side 

 of the body. This and other points define the Ver- 

 tebrata. 



Plans of structure, independent of the simplicity or 

 perfection of the special arrangement or structure of 

 organs, also define these great groups. Thus the Pro- 

 tozoa present a spiral, the Ccelenterata a radiate, the 

 Echinodermata a bilateral radiate plan. The Articu- 

 lates are a series of external rings, each in one or more 

 respects repeating the others. The Molluscs are a sac, 

 while a ring above a ring, joined together by a solid 

 center-piece, represents the plan of each of the many 

 segments of the Vertebrates which give the members of 

 that branch their form. 



These bulwarks of distinction of animal types are 

 entered into here simply because they are the most in- 

 violable and radical of those with which we have to 

 deal, and to give the anti-developmentalist the best foot- 

 hold for his position. I will only allude to the relations 

 of their points of approach, as these are affected by 

 considerations afterward introduced. 



