TRANSACTIONS OF SECTION I. 961 
The separate part of the Vth nerve which supplies the velar appendage passes 
within it from the dorsal to the ventral part of the animal, and then, as Miss 
Alcock has shown, turns abruptly forward to supply the large median tentacle. 
This extraordinary course leads directly to the conclusion that this median 
tentacle, which is in reality double, constitutes, with the velum of each side, the 
true velar appendages. 
Again, on each side of the middle line there are in Ammoceetes four large 
tentacles, each of which possesses a system of muscles, muco-cartilage, and blood- 
spaces, precisely similar to the median ventral tentacle already mentioned. Each 
of these is supplied, as Miss Alcock has shown, by a separate branch of the 
motor part of the Vth nerve (see fig. 6), and each branch is comparable with the 
branch supplying the large velar appendage. 
That such tentacles are not mere sensory papillz surrounding the mouth, but 
have a distinct and important morphological meaning, is shown by the fact that 
they are transformed in the adult Petromyzon into the remarkable tongue and 
suctorial apparatus: a modification of oral appendages into a suctorial apparatus 
which is abundantly common among Arthropods. 
Finally, the Vth nerve innervates the visceral muscles of the lower and 
upper lips of Ammoccetes. In order, then, for the story to be complete, the 
homologues of the lower and upper lips must also be found in the system of 
prosomatic appendages of forms like Limulus and Eurypterus. The lower lip, 
like the opercular or thyroid appendage, possesses a plate of muco-cartilage, and, 
as already mentioned, falls into its natural place as the metastoma of the old 
Eurypterus-like form, by the enlargement and forward growth of which the oral 
chamber of Ammoccetes was formed. The meaning of the upper lip will be con- 
sidered with the consideration of the old mouth tube. The comparison of the 
metastoma of Eurypterus with the lower lip of Ammoccetes demonstrates the 
close resemblance between the oral chambers of Eurypterus and Ammoccetes. In 
order to obtain the condition of affairs in Ammoccetes from that in Eurypterus, it 
is only necessary that the metastoma should increase in size, and that the last 
oral appendage, the large oar-appendage, should follow the example of the other 
oral appendages, and be withdrawn into the oral cavity, and so form the velar 
appendage. 
Thus we see that, just as the mesosomatic appendages of Limulus can be traced 
into the branchial and thyroid appendages of Ammoccetes through the inter- 
mediate stage of forms similar to Eurypterus, so also the prosomatic appendages 
and chilaria of Limulus can be traced into the velar and tentacular appendages 
and lower lip of Ammoccetes through the intermediate stage of forms similar to 
Eurypterus. 
3. Lastly comes the ontogenetic test. The concordant interpretation of the 
origin of the motor part of the Vth, of the VIIth, IXth, and Xth nerves giver by 
the anatomical and phylogenetic tests must explain and be illustrated by the facts 
of the development of Ammoccetes. 
We see :— 
1. The oral chamber of Ammoccetes is known in its early stage by the name ot 
the stomatodeum, and we find, as might be anticipated, that it is completely 
separated at first from the branchial chamber by the septum of the stomatodzum. 
2. This septum is the embryological representative of the basal part of the 
operculum, and demonstrates that originally the operculum separated the oral and 
branchial chambers, 
3. Subsequently these two chambers are put into communication by the break- 
ing through of this septum, illustrating the communication between the two 
chambers by the separation of the median basal parts of the operculum. 
4, The velar appendages, the tentacular appendages, the lower lip, all form as 
out-buddings, just as the homologous locomotor appendages are formed in arthropods. 
5. The branchial bars are not formed by a series of inpouchings in a tube of 
uniform thickness, but, as Shipley! has pointed out, by a series of ingrowths at 
» Loe. cit. 
