284 THE ORIGIN OF VERTEBRATES 
vertebrate embryology (as has been so often asserted to be the case), 
seeing that undoubtedly the Arthropoda are an advanced stage of 
Annelida; and, indeed, the way is not a long one when we consider 
Beecher’s evidence that the Trilobita belong to the Phyllopoda, 
certainly a primitive crustacean group, which Bernard derives directly 
from the annelid group Chatopoda. If, then, these plakodal ganglia 
indicate the former presence of appendages, we obtain this result :— 
The foremost ganglion on each side possesses one plakodal ganglion, 
and therefore indicates an anterior pair of appendages, possibly the’ 
chelicere. Then comes the peculiar nerve with four plakodal 
ganglia indicating on each side four appendages close together, 
possibly the endognaths. Then, finally, on each side, the second 
large ganglion with two plakodal ganglia, indicating two pairs of 
appendages, possibly the ectognaths and the metastoma. 
SUMMARY. 
The consideration of the history of the cranial segmentation shows that 
whereas, from the commencement of that history, the evidence for two ventral 
segments supplied by the trigeminal nerve is clear and unmistakable, later 
observers have tended more and more to increase the number of these segments, 
until at the present time the evidence is in favour of at least six, probably seven, 
as the number of segments supplied by the motor part of the trigeminal. 
So, also, the original evidence for the number of dorsal or somatic segments 
limits the number to three, innervated respectively by the oculomotor (III.), 
trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve 
belongs to the facial segment. The muscles which these three nerves supply 
are derived respectively from the walls of the premandibular, mandibular, and 
hyoid coelomic cavities. 
Later evidence points strongly to the conclusion that the oculomotor nerve 
and the premandibular cavity represent not one segment but the fusion of 
four, while the mandibular cavity represents two segments. In addition to 
these, Miss Platt has discovered a still more anterior head-cavity, which she has 
named the anterior cavity, so that the pro-otic segments on this reckoning are 
seven in number, viz. : (1) the anterior cavity, (2,3, 4, 5) the premandibular cavity, 
(6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal 
segments are the eye-muscles, which are all dorso-ventral in position, and are 
not the same as the longitudinal somatic muscles, but belong to a distinct dorso- 
ventral segmental group, the only representative of which at present known in 
the mesosomatic region is the external rectus innervated by the VIth nerve. 
These head-cavities, and these muscles of the vertebrate, resemble the 
corresponding cavities and muscles of the invertebrate to an extraordinary 
