452 THE ORIGIN OF VERTEBRATES 
commencement of the liver. This simple liver-diverticulum became 
the tubular liver of Ammoccetes, and formed, curiously enough, not 
a glandular organ of the same character as the liver of the higher 
vertebrates, but a hepato-pancreas, like the so-called liver of the 
arthropods, which also is a special diverticulum of the gut, or rather 
the main true gut of the animal. In both cases the liver is the chief 
agent in digestion, for in Ammoccetes the liver-extract is very much 
more powerful in the digestion of proteids than the extract of any 
other organ tried by Miss Alcock. Subsequently in the vertebrate 
the gastric and pancreatic glands arise and relieve the liver of the 
burden of proteid digestion. 
It is, to my mind, somewhat significant that the liver on its first 
. formation in the vertebrate should have arisen as a digestive organ of 
the same character as the so-called liver in the arthropods ; whether 
it originally belonged to any separate segment is in our present state 
of knowledge difficult to say. 
CONCLUSION, 
Tn conclusion, I will endeavour to illustrate crudely the way in 
which, on my theory, the notochord and vertebrate gut may have 
been formed, the agencies at work being in the main two, viz. the 
dwindling of appendages as mere organs of locomotion, and the 
conversion of a ventral groove into a tube. 
I imagine that, among the Protostraca, forms were found some- 
what resembling trilobites with markedly polychetan affinities ; 
which, like Apus, possessed a deep ventral groove from one end of 
the body to the other, and also pleural fringes, as in many trilobites. 
This might be called the Trilobite stage (Fig. 167, A). 
This groove became converted into a tube and so gave rise to the 
notochord, while the appendages were still free and the pleure had 
not met to form a new ventral surface. This might be called the 
Chordate Trilobite stage (Fig. 167, B). 
Then, passing from the protostracan to the paleostracan stage, 
the oral and respiratory chambers were formed, not communicating 
with each other, in the manner described in previous chapters, a 
ventral groove in the metasomatic region being the only connection 
between respiratory chamber and cloaca. This might be called the 
Chordate Paleostracan stage (Fig. 167, C). 
