{317}With the formation of the vertebrate heart from the two longitudinal 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 Ammocoetes, 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 PALÆOSTOMA, OR OLD MOUTH. The arrangement of the oral chamber in Ammocoetes 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 formation, 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 palæostoma, or original mouth? Clearly, as already suggested, it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of Ammocoetes 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 palæostoma must be looked for.