DEGRADATION OF ORGANISATION 95 



Disappearance of the Organ of Sight. 



Here again the organ of sight, so useful to the most perfect animals, 

 is entirely extinguished. This organ began to be deficient in some 

 of the molluscs and cirrhipedes and in most of the annelids, and is 

 only found afterwards in the crustaceans, arachnids and insects in a 

 very imperfect state and of little or no use ; after the insects it does 

 not re-appear in any animal. 



Here again, finally, the head altogether ceases to exist, — an essential 

 part of the body of the most perfect animals and the seat of the brain 

 and nearly all the senses ; for the swelling at the anterior extremity 

 of the body of some worms like Taenia is caused by the arrangement 

 of their suckers and is not the seat of a brain nor of any organ of 

 hearing, sight, etc., since there are no such organs in the animals of 

 the neighbouring classes. Hence this swelling cannot be considered 

 as a true head. 



We see that at this part of the animal scale the degradation of 

 organisation becomes extremely rapid, and strongly foreshadows the 

 greatest simplification of animal organisation. 



WORMS. 



Animals with soft elongated bodies, without head, eyes or jointed legs, 

 and no longitudinal cord or circulatory system. 



We now come to worms, which have no vessels for circulation ; in- 

 cluding those known under the name of intestinal worms, and some 

 others not intestinal whose organisation is quite as imperfect. They 

 are animals with soft more or less elongated bodies, which undergo no 

 metamorphosis and are all destitute of a head, eyes and jointed legs. 



The worms should be placed immediately after the insects and before 

 the radiarians, and occupy the eleventh rank in the animal kingdom. 

 It is among them that we note the origin of the tendency of nature 

 to establish the system of articulations, a system that she subsequently 

 carried to completion in the insects, arachnids and crustaceans. But 

 the organisation of the worms is less perfect than that of the insects, 

 since they have no longitudinal cord, head, eyes or true legs, so that we 

 are forced to place them after the insects ; lastly, the new kind of shape, 

 which nature initiates in them on passing from a radiating arrange- 

 ment of the parts to the system of articulations, shows that the worms 

 should be placed even before the radiarians. After the insects, more- 

 over, the plan followed by nature in the animals of preceding classes 

 is lost sight of, viz. that general shape of the animal which consists 



