1 88 THE DARWINIAN THEORY 



This brings us to the consideration of the lower 

 group of Vertebrates — Acrania. 



Amphioxus (see Fig. 20). — This is a small fish-like 

 animal from one and a half to two inches in length, 

 living in the sand, and found at Messina, Naples, 

 on our own shore, in Australia, and elsewhere. It 

 possesses a notochord, a large number of gill-slits, 

 and a central nervous system along its dorsal surface. 

 Amphioxus is an animal of very primitive character, 

 differing from other Vertebrates in the following 

 points : The skeleton is of extreme simplicity, con- 

 sisting of an elastic rod, the notochord. It has no 

 limbs, no skull, no ribs, no brain, and no ears. The 

 eye is a single pigmented spot at the anterior end of 

 the nervous system. The alimentary canal is 

 straight, and there are a hundred or more gill-slits 

 (eight being' the greatest number in fish). The 

 heart is a straight tube, and the liver a simple diver- 

 ticulum from the alimentary canal. 



Yet Amphioxus is obviously a Vertebrate, for 

 although it differs in almost all the above points 

 1 from adult Vertebrates, it resembles the embryos of 

 Vertebrates, and may be said to halt at an early 

 stage in development. All Vertebrates pass through 

 stages in which there are no limbs ; in which the 

 notochord is the only skeletal structure, and the 

 brain merely the anterior end of the neural tube ; 

 in which paired eyes and ears are not yet formed ; 

 in which the heart is a simple tube, the alimentary 

 canal straight, and the liver a simple outgrowth 

 from it. 



The position of Amphioxus with regard to Verie- 



