BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 103 
been abundantly demonstrated by previous writers. But 
whatever the adult condition of any of the dorsal roots of 
the cranial nerves, whatever the actual condition of olfactory 
nerve, nerve of the ciliary ganglion, fifth, seventh, eighth, 
ninth, and vagus complex, all can, by the consideration of their 
actual development, and of the condition of the various organs 
which are, or would be if present, related to them, be reduced 
to the general schema. 
The divergences between the various nerves are, as might be 
suspected, naturally dependent on the presence or absence of 
gill-clefts in connection with the segment to which the nerve 
belongs. 
For this reason I shall consider the nerves out of their 
natural order, taking those of the true gill-clefts first. Their 
order of treatment will thus be as follows: 
Nerve. Cleft. Segment. 
Seventh. Spiracle, and one absent. Fourth and fifth. 
Ninth. First branchial. Seventh. 
Vagus. Second, third, fourth, and LHighth, ninth, tenth, and 
fifth branchial. eleventh. 
Fifth. Mouth. Third. 
Ciliary. Hypophysis (?). Second. 
Olfactory. Absent. First. 
Auditory. Absent. Sixth. 
In the above list it will be noticed that the cleft of the fifth 
nerve is described as the mouth. This view, which we owe to 
Prof. Dohrn, seems to me to receive very considerable support 
from my researches. I shall refer to the matter subsequently. 
For the ciliary, olfactory, and auditory nerves I have hesi- 
tated to assign clefts, because the evidence for their existence 
is uncertain, and the nature of the three nerves is more easily 
explicable if we regard the clefts as absent or metamorphosed. 
Here suffice it to say, that clefts have been assigned to these 
1 The numbering of the segments is in accordance with those conclusions 
from my researches which appear to me to be fairly certain. Probably the 
facial nerve is a complex of two segmental nerves, and this apart from the 
auditory segmental nerve. If this be the case, then there are eleven segments 
at least from the olfactory nerve to the fourth root of the vagus inclusive. 
