146 Darwin, and after Darwin. 



described. I allude, of course, to Amphioxus, which 

 is by far the most primitive or generalized type of 

 vertebrated animal hitherto discovered. Indeed, we 

 may say that this remarkable creature is almost as 

 nearly allied to a worm as it is to a fish. For it has 

 no specialized head, and therefore no skull, brain, 

 or jaws : it is destitute alike of limbs, of a centralized 

 heart, of developed liver, kidneys, and. in short, of 

 most of the organs which belong . to the other 

 Vertebrata. It presents, however, a rudimentary back- 

 bone, in the form of what is called a notochord. Now 

 a primitive dorsal axis of this kind occurs at a very 

 early period of embryonic life in all vertebrated 

 animals ; but, with the exception of Amphioxus, in 

 all other existing Vertebrata this structure is not 

 itself destined to become the permanent or bony 

 vertebral column. On the contrary, it gives way to, 

 or is replaced by, this permanent bony structure at 

 a later stage of development. Consequently, it is very 

 suggestive that so distinctively embryonic a structure 

 as this temporary cartilaginous axis of all the other 

 known Vertebrata should be found actually persisting 

 to the present day as the permanent axis of Amphioxus. 

 In many other respects, likewise, the early embryonic 

 history of other Vertebrata refers us to the permanent 

 condition of Amphioxus. In particular, we must 

 notice that the wall of the neck is always perforated 

 by what in Amphioxus are the gill-openings, and that 

 the blood-vessels as they proceed from the heart are 

 always distributed in the form of what are called 

 gill-arches, adapted to convey the blood round or 

 through the gills for the purpose of aeration. In all 

 existing fish and other gill-breathing Vertebrata, this 



