146 Darwin, and after Darwin. 
described. I allude, of course, to Amphiorus, 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 Amphioxis, 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 cartilagingus 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 
