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. 



In 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 polycbietan 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 pleura' 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 paheostracan 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 Pakeostracan stage (Fig. 167, C). 



