THE GENEALOGY OF ANIMALS 77 



worms are flat and sac-like, with bilateral sym- 

 metry and the power to move about, but not 

 tubular, as are the true worms. They are blood- 

 less, like the celenterates and sponges. 



From the flat worms developed the annelid 

 worms, animals perforated by a food canal and 

 possessing a body cavity filled with blood sur- 

 rounding this canal. The body cavity is the space 

 between the walls of the body and the alimentary 

 canal, the cavity which in the higher animals 

 contains the heart, liver, lungs, kidneys, etc. The 

 worms and all animals above them have this 

 cavity. The worms and all animals above them 

 also have, as an inheritance from the flat worms, 

 bodies with bilateral symmetry that is, bodies 

 with two halves similar. This peculiarity was 

 probably acquired by the flat worms, and so 

 fastened upon all subsequently evolved species, as 

 a result of pure carelessness. It probably arose 

 out of the habit of using continually, or over and 

 over again, the same parts of the body as fore and 

 aft. It has been facetiously said that if it had not 

 been for this habit, so inadvertently acquired by 

 these humble beings so long, long ago, we would 

 not to-day be able to tell our right hand from our 

 left. In the worm is found the beginning of 

 that wonderful organ of co-ordination, the brain. 

 The brain is a modification of the skin. It may 

 weaken our regard for this imperial organ to know 

 that it is, in its morphology, akin to nails and 

 corns. But it will certainly add to our admiration 

 for the infinite labours of evolution to remember 



