100 THE EVOLUTION OF MAN. 



latter the distinguishing characteristic features already 

 mentioned. We cannot therefore say that the Amphioxus is 

 the progenitor of the Vertebrates ; but we may certainly say 

 that the Amphioxus of all known animals is nearest allied 

 to this progenitor ; both belong to the same limited family 

 group, to the lowest Vertebrate class, that of the Skull-less 

 Animals (Acrania). In the human pedigree, this group 

 forms the ninth stage of the ancestral chain, the first among 

 Vertebrate ancestors. From this Skull-less group was 

 developed the Amphioxus on the one side, and on the other 

 the parent-form of the Skulled Animals (Craniota). 



The comprehensive group of the Skulled Animals 

 includes all known Vertebrates, with the single excep- 

 tion of the Amphioxus. All these Skulled Animals 

 possess a distinct head, inwardly specialized from the 

 trunk, and this contains a skull, enclosing a brain. This 

 head also carries three of the higher sense-organs, which are 

 partially wanting in the Skull-less Animals (nose, ears, and 

 eyes). At first, the brain appears in a very simple form, as 

 an anterior bladder-like extension of the medullary tube 

 (Plate XI. Fig. 16, m x ). This, however, is soon distributed by 

 several tranverse grooves — first into three, and afterwards 

 into a series of five consecutive brain-bladders. In the 

 formation of the head, skull, and brain, together with the 

 higher sense-organs, lies the most essential advance made 

 by the skulled parent-form beyond its skull-less ancestors. 

 Other organs, however, also soon rose to a higher grade 

 of development; a compact centralized heart appeared, a 

 more perfect liver and kidneys; and in other directions 

 also important advance was made. 



The Skull-less Animals may be primarily subdivided 



