324 THE ORIGIN OF VERTEBRATES 



supplied seven appendages, which appendages did not parry branchiae, 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 Ammocoetes. 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 

 Ammocoetes ; here there is no sign of any suctorial appai'atus. 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 

 palfeostracan 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 modification of appendages, as so frequently occurs 

 in the arthropods. 



The study of Ammocoetes indicates that the velum and lower lip correspond 

 to the metastoma of the Eurypterid. i.e. the chilaria of Lunulas, 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 contimiation of 

 the dwindling process, already initiated in the Eurypterid. would easily result in 

 the tentacles of Ammocoetes. 



The nasal tube of Ammocoetes. 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 palaeostoma, 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 appendag-es of the pala?ostracan 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. 



