BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 143 
ferent success. That Balfour was right in regarding the cranial 
nerves as more primitive than the spinal is probable enough, 
but at the same time it is very questionable whether the spinal 
nerves ever had the same primitive characters as the cranial. 
Dohrn’s idea that the anus arose from a pair of coalesced 
gill-clefts may be rejected without more ado, for there seems to 
be no evidence for it. Not so, however, his mode of regarding 
the mouth as a pair of coalesced gill-clefts, that is probably 
true. In dealing with the relations of head and trunk the 
vexed question of anterior roots of cranial nerves crops up, and 
with it the nature of the head cavities. I have no observations 
to record on the so-called anterior roots of cranial nerves 
except on the hypoglossus, which has certainly nothing to do 
with the cranial nerves, as Dohrn has pointed out. Van Wijhe 
regarded the hypoglossus as made up in Elasmobranchs of three 
anterior roots of the vagus. In this point my researches agree 
with those of Dohrn and Froriep. The hypoglossus has 
nothing to do with the vagus. 
Froriep’s! account of the development of the former in Mam- 
malia seems to hold good also for Elasmobranchs. As in 
Mammalia the hypoglossus of Elasmobranchs is derived from 
the anterior roots of the first three spinal nerves. The posterior 
roots are developed in the embryo, but afterwards abort. I 
have not figured them, because the spinal nerves really lay 
beyond the scope of this work. 
As to the head cavities themselves, their persistence in the 
anterior part of the head may, as other observers have stated, 
be due to their functional connection with the eyes, that they 
once occurred in all the segments of the head is probable 
enough, though with what organs they were originally con- 
nected is not so plain. Possibly from their muscular nature, 
and the apparent absence of sensory elements, even in develop- 
ment, in their nerves, they may have been the muscles of 
neural parapodia. That they had nothing to do with the gill- 
clefts themselves is pretty certain. 
1 Op. cit., pp. 5 and 48. 
