Pee, eS 
TRANSACTIONS OF SECTION I, 955 
by the VIIth nerve, then, passing into the oral chamber, we find a series of non- 
branchial appendages, viz. the velar and tentacular appendages, supplied by branches 
of the Vth nerve. In fact, by simply considering the tissue between the so-called 
gill-pouches as the segmental unit, we no longer get lost in a maze of hypothetical 
gill-pouches in front of the branchial region, but find that the resemblances between 
the oral and branchial regions, which have led to the endless search for gill-slits 
and gill-pouches, really mean that the oral chamber contains appendages just as 
the branchial chamber, but that the former were not gill-bearing. 
The study of Ammocetes, then, leads directly to the conclusion that the ancestor 
of the vertebrate possessed an oral or prosomatic chamber, which contained a series 
of non-branchial, tactile and masticatory appendages, which were innervated from 
the fused prosomatic ganglia or hind brain, and a branchial or mesosomatic 
chamber, which contained a series of branchial appendages which were innervated 
from the fused mesosomatic ganglia or medulla oblongata. These two chambers 
did not originally communicate with each other, for the embryological evidence 
shows that they are separated at first by the septum of the stomatodeum, and 
also that the oral chamber is formed by the forward growth of the lower lip. 
The phylogenetic test on the side of Limulus and its congeners agrees in a 
remarkable manner with the conclusions derived from the study of Ammoceetes, 
for we see that the variation which has occurred in the formation of Eurypterus 
from Limulus is exactly of the kind necessary to form the oral and branchial 
chambers of the Ammoccetes. Thus, we find with respect to the mesosomatic 
appendages that the free, many-jointed appendages of the crustacean become con- 
verted into the plate-like appendages of Limulus, in which the separate joints are 
still visible, but insignificant in comparison with the large branchis-bearing lamella ; 
then comes the in-sinking of these appendages, as described by Macleod,! to form the 
branchial lamellz, or so-called lung-books of Thelyphonus, and the branchise of 
Eurypterus, in which all semblance of jointed and free appendages disappears and 
the branchiz project into a series of chambers or gill-pouches, each pair of which 
in Thelyphonus open freely into communication. In this way we see already 
the commencement of the formation of a branchial chamber similar to that of 
Ammoceetes. 
So also with the innervation of these mesosomatic appendages, originally a series 
of separate mesosomatic ganglia, each of which innervates a separate appendage ; 
then a process of cephalisation takes place, in consequence of which, in the first 
place, a single ganglion, the opercular ganglion, fuses with the already fused proso- 
matic ganglia, as is seen in the stage of Limulus; then, as pointed out by Lankester, 
in the different groups of scorpions more and more of the mesosomatic ganglia fuse 
together, and so we find the upward variation in this group is distinctly in the 
direction of the formation of the medulla oblongata coincidently with the formation 
of a branchial chamber. 
In a precisely similar way, we find the variation which has occurred in the 
prosomatic appendages leads directly to the formation of the oral chamber and oral 
appendages of Ammoccetes ; for the original chelate and locomotor appendages of 
Limulus become converted into the tactile non-chelate appendages of Eurypterus 
(cf. figs. 4 and 5), and the small chilaria (M) of Limulus, according to Lankester, 
fuse in the middle line and grow forward to form the metastoma of Eurypterus, 
thus forming an oral chamber, into which the short tactile appendages could be 
withdrawn, closely similar in its formation to the oral chamber of Ammoccetes. 
The prosomatic ganglia supplying these oral appendages have already, in Limulus 
(see fig. 4), been fused together to form the infra-cesophageal ganglia or hind brain. 
The phylogenetic test, then, both on the side of the vertebrate and of the inver- 
tebrate, points direct to the conclusion that the peculiarities of the trigeminal and 
vagus groups of nerves are due to their origin from nerves supplying prosomatic 
and mesosomatic appendages respectively. 
2. The anatomical test confirms aud emphasises this conclusion in a most 
striking manner, for we find not only coincidence of topographical arrangement, as 
1 Macleod, Archiv. de Biologie, vol. v. 1884. 
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