Such plakodal ganglia, then, may give valuable indication of non-branchial segments as well as of branchial segments. The researches of Kupffer on the formation of the trigeminal ganglia in Ammocoetes are the chief attempt to find out from the side of the sensory ganglia the number of segments originally belonging to the trigeminal. The nature and result of these researches is described in my previous paper (_Journal of Anatomy and Physiology_, vol. xxxiv.), and it will suffice here to state that he himself concludes that the trigeminal originally supplied five at least, probably six, segments. As I have stated there, the evidence as given by him seems to me to indicate even as many as seven segments. In the full-grown Ammocoetes, as is well known, there are two distinct ganglia belonging to the trigeminal, the one the ganglion of the _ramus ophthalmicus_, the other the main ganglion. According to Kupffer the larval Ammocoetes possesses three sets of ganglia, not two, for between the foremost and hindmost ganglion {283}he describes a nerve (_x._, Fig. 113), with four epibranchial ganglia, which do not persist as separate ganglia, but either disappear or are absorbed into the two main ganglia (Fig. 113). This discovery of Kupffer's is very suggestive, for, as already stated, a transformation takes place when the Ammocoetes is 5 mm. long, so that the arrangement of the parts before that period is distinctly more indicative of the ancestral arrangement than any later one. If we use the name plakodal ganglia to represent that part of these ganglia which was originally connected with the skin, then Kupffer's researches assert that in the larval Ammocoetes there were seven such plakodal ganglia, one in front belonging to the foremost trigeminal ganglion, two behind, parts of the hindmost ganglion, and four in between, which do not exist later as separate ganglia. [Illustration: FIG. 113.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4 MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.) _A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._, notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper lip; _l.l._ lower lip; _vel._, septum between oral and respiratory cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four epibranchial ganglia.] In accordance with the views put forward in this book, a possible interpretation of these plakodal ganglia would be given as follows:--