Beard, who, after Froriep, drew attention to this relation of the cranial ganglia to special skin-patches, has compared them with the parapodial ganglia of annelids, _i.e_. ganglia in connection with annelidan appendages; whether we are here obtaining a glimpse of the far-off annelidan ancestry of both arthropods and vertebrates it would be premature at present to say. It is natural enough to expect, on my view, to find evidence of annelidan ancestry in {284}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 Chætopoda. 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 cheliceræ. 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.