273 
this view, we still see that the “head-prolongations” 
(Kopffortsatz) of Amphioxus also produce muscles and, as 
we have seen, thee can be no question about considering 
the latter as a distinct pair of somites, which would lie in 
front of the neuropore, ie. the old mouth of the Annelids. 
Nature of the trigeminus. — Opinions differ again as to 
the double nature of the trigeminus which must be assumed 
to furnish a dorsal root to both the premandibular and 
the mandibular segment. According to VAN WYHE (1882) we 
must distinguish the ramus ophthalmicus profundus, together 
with the ganglion ciliare, from the rest of the trigeminus, 
together with the ganglion Gasseri, these being the dorsal 
roots respectively of the praemandibular and ot the mandibu- 
lar segment. This view, suggested already, though with some 
reserve, by GEGENBAUR (1872, p. 290), has found a general 
acceptance, ZIEGLER also, as we have seen, accepts it. Truly 
the ramus ophthalmicus profundus could be considered oniy 
as a very incomplete segmental nerve, which has lost 
its typical branches and which. in contrast to the other 
head nerves, has no relation to any gill-sht (as the trige- 
minus ll has to the mouth). Both ganglia, moreover, arise 
in close connection with each other and at their first 
appearance the double nature is as a rule far from obvious. 
In higher Vertebrates there is little doubt that the insig- 
nificant ciliary ganglion, situated over the eye, is nothing 
but a portion detached from the ganglion Gasseri, but it 
is questionable, if here it may be compared to the ciliary 
ganglion of Anamnia. 
BALFOUR (1878, p. 214), pointing to tbe striking simi- 
larity in the arrangement of the branches of the trigeminus 
to the facialis, writes: “1 was at first inclined to regard 
the anterior branch of the fifth (ophthalmic) as representing 
a separate nerve, and was supported in this view by its 
relation to the most anterior of the head-cavities, but the 
unexpected discovery of an exactly similar branch in the 
seventh nerve has induced me to modify this view, and 
l am now constrained to view the fifth as a single nerve, 
whose branches exactly correspond with those of.the seventh”, 
In the same way GEGENBAUR, who was originally (1872, p. 
290), inclined to consider the trigeminus as a double 
nerve, though he remarks: “doch darf auch die Annahme 
einer mächtigeren Entfaltung eines einzigen Spinalnerven 
nicht ausgeschlossen werden”, afterwards (1887, p‚ 51) 
