144, JOHN BEARD. 
Nature oF THE Mout. 
A few words may be here said on the bearing of these 
researches on the nature of the mouth. 
Dohrn! first suggested that the mouth was primitively a 
pair of gill-clefts, which have coalesced and come to open 
medianly. He afterwards showed? that it arises in Teleostei as 
two lateral depressions just like gill-clefts. In the preceding 
pages I showed that in Elasmobranchs there was a primitive 
branchial sense organ over the angle of the mouth, and with 
this sense organ an associated ganglion, the Gasserian, and 
also, that just as in the nerves of other gill-clefts a supra- 
‘branchial nerve was afterwards developed from this ganglion in 
connection with the extension of the branchial sense organs of 
the mouth cleft. I need hardly say that I see in these facts a 
strong additional support for Dohrn’s view. 
SEGMENTATION OF THE HEap. 
Admittedly this is one of the most difficult problems in 
Vertebrate morphology, and I cannot flatter myself that I am 
nearer a solution of it than other zoologists. But it. may be 
remarked that the tendency of recent researches has been to 
increase the number of segments recognisable in the Verte- 
brate head. In ordinary sharks with five true gill-clefts, 
Marshall and Van Wijhe recognised nine segments, but Van 
Wijhe rejected Marshall’s olfactory segment, and Marshall did’ 
not regard the hyoid as composed of two segments. I should 
increase the number to at least eleven in sharks with four roots 
to the vagus, and apparently Dohrn would agree with this 
number, but his segments might not be quite the same. 
Indeed, at present it is impossible to solve the problem with 
any degree of probability, and it is a question whether it ever 
will be solved. Hence the following table is only a tenta- 
1 Dohrn, ‘ Ursprung der Wirbelthiere.’ 
2 Dohrn, “Studien, &c.,” ‘ Mittheil. a. d. Zool. Station zu Neapel,’ Bd. iii 
I. “Der Mund der Knochenfische.” 
