THE PROSOMATIC SEGMENTS OF AMMOCCETES 317 



With the formation of the vertebrate heart from the two longi- 

 tudinal venous sinuses and the abolition of the dorsal invertebrate 

 heart, the function of these tubular muscles as branchial hearts was 

 no longer needed, and their respiratory function alone remained. The 

 last remnant of this is seen in Ammoccetes, for the ordinary striated 

 muscles were always more efficient for the respiratory act, and so at 

 transformation the inferior tubular musculature was got rid of, there 

 being no longer any need for its continued existence. 



The Pal^ostoma, or Old Mouth. 



The arrangement of the oral chamber in Ammoccetes is peculiar 

 among vertebrates, and, upon my theory, is explicable by its 

 comparison with the accessory oral chamber which apparently 

 existed in Eurypterus. According to this explanation, the lower lip 

 of the original vertebrate mouth was formed by the coalescence of 

 the most posterior pair of the prosomatic appendages — the chilaria ; 

 from which it follows that the vertebrate mouth was not the original 

 mouth, but a new structure due to such a formation of the lower lip. 



It is very suggestive that the direct following out of the original 

 working hypothesis should lead to this conclusion, for it is universally 

 agreed by all morphologists that the present mouth is a new forma- 

 tion, and Dohrn has argued strongly in favour of the mouth being 

 formed by the coalescence of a pair of gill-slits. Interpret this in 

 the language of my theory, and immediately we see, as already 

 explained, gill-slits must mean in this region the spaces between 

 appendages which did not carry gills ; the mouth, therefore, was 

 formed by the coalescence of a pair of appendages to form a lower 

 lip just as I have pointed out. 



Where, then, must we look for the pakeostoma, or original mouth \ 

 Clearly, as already suggested, it was situated at the base of the olfac- 

 tory passage, and the olfactory passage or nasal tube of Ammoccetes 

 was originally the tube of the hypophysis, so that the following out 

 of the theory points directly to the tube of the hypophysis as the 

 place where the palseostoma must be looked for. 



This conclusion is not only not at variance with the opinions of 

 morphologists, but gives a straightforward, simple explanation why 

 the palaeostoma was situated in the very place where they are most 

 inclined to locate it. Thus, if we trace the history of the question, 



