324 THE ORIGIN OF VERTEBRATES 
supplied seven appendages, which appendages did not carry branchie, but were 
originally used for purposes of locomotion as well as of mastication. 
Such appendages clearly no longer exist in the higher vertebrates, the 
muscles of mastication only remaining ; but in the earliest fish-forms they must 
have existed, as, indeed, is seen in Ptericthys and Bothriolepis. Judging from 
all the previous evidence some signs of their existence may reasonably be 
expected still to remain in Ammoccetes. Such is indeed the case. 
In the adult Petromyzon the trigeminal nerve innervates specially a 
massive suctorial apparatus, by means of which it holds on to other fishes, or 
to stones in the bottom of the stream. There is here no apparent sign of 
appendages. Very great, however, is the difference in the oral chamber of 
Ammoceetes; here there is no sign of any suctorial apparatus, but instead, a 
system of tentacles. together with the remains of the septum or velum, which 
originally closed off the oral from the respiratory chamber. These tentacles 
are the last remnants of the original foremost prosomatic appendages of the 
paleostracan ancestor. Like the lateral eyes they do not develop until the 
transformation comes, but during the whole larval condition their musculature 
remains in an embryonic condition, and then from these embryonic muscles 
the whole massive musculature of the suctorial apparatus develops ; a sucking 
apparatus derived from the moditication of appendages, as so frequently occurs 
in the arthropods. 
The study of Ammoccetes indicates that the velum and lower lip correspond 
to the metastoma of the Eurypterid, 7.e, the chilaria of Limulus, while the large 
ventral pair of tentacles, called the tongue, correspond to the ectognaths of the 
Eurypterids, and probably to the oar-like appendages of Ptericthys and 
Bothriolepis. From these two splanchnic segments the suctorial apparatus 
in the main arises; the motor supply of these two segments forms the mass of 
the trigeminal nerve-supply, and the nerves supplying them, the velar nerve and 
the tongue-nerve, are markedly separate from the rest of the trigeminal nerve. 
The rest of the tentacles present much less the sign of independent 
segments. In their nerves, their muco-cartilaginous skeleton, and their 
rudimentary muscles, they indicate a concentration and amalgamation, such 
as might be expected from the concentrated endognaths. The continuation of 
the dwindling process, already initiated in the Eurypterid, would easily result in 
the tentacles of Ammoceetes. 
The nasal tube of Ammoccetes, which originates in the hypophysial tube, 
corresponds absolutely in position and in its original structure, to the olfactory 
tube of a scorpion-like animal. From this homology two conclusions of 
importance follow: (1) the old mouth, or paleostoma, of the vertebrate was 
situated at the end of this tube, therefore, at the termination of the infundi- 
bulum ; (2) the upper lip, which by its growth, brings the olfactory tube from 
a ventral to a dorsal position, was originally formed by the foremost sternites 
or endostoma, or else by the sterno-coxal processes of the second pair of 
prosomatic appendages of the paleeostracan ancestor. 
In strict accordance with the rest of the comparisons made in this region, 
the pituitary body shows by similarity of structure, as well as of position, that 
it arose from the coxal glands, which were situated at the base of the four 
endognaths. 
