THE PROSOMATIC SEGMENTS OF AMMOCGTES 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 PALZOSTOMA, OR OLD Mourn. 
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 J have pointed out. 
Where, then, must we look for the paleeostoma, 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 paleostoma 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 palzostoma was situated in the very place where they are most 
inclined to locate it. Thus, if we trace the history of the question, 
