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innervated each by a branch of a segmental nerve of the 
ventral chain, that is by a spinal nerve branch. In Verte- 
brates, on the contrary, the sense-organs of the trunk are 
all innervated by the ramus lateralis of the nervus vagus, 
a cranial nerve which, issuing from the brain, runs along 
the whole trunk and tail, providing on its way every sense- 
organ with a little side-branch. There is no question 
about an innervation by spinal nerves in this case. Only 
the partly transient lateral organs of the head have a 
‘segmental innervation, to which we will refer below 
the lateral organs of Vertebrates are homologous to 
those of Annelids, we must assume that in some way the 
innervation by segmental nerves has been replaced by a 
more concentrated innervation having its centre in the head 
We could imagine, for example, that a plexus-formation by the 
original segmental nerves has begun from the head. This 
plexus extending progressively further backwards, has 
resulted in the disappearance of the segmental communií- 
cation of the lateral sense-organs with the corresponding 
dorsal spinal nerves. This is also what EISIG (1887, p. 53) 
Supposes, when he writes; “Die einfachste Voraussetzung 
wäre die, dass in dem Maasse als das Gehirn seine Function 
als Centralorgan im werdenden Wirbelthiere auf Kosten der 
relativen Selbständigkeit der segmentalen Ganglienknoten des 
Anneliden-Bauchstranges auszuüben fortfuhr (EISIG of course 
‘has in mind DOHRN's’ view, at that time prevalent, that the 
brain is to be compared with the cerebral ganglia, the spinal 
Chord with the ventral ganglion chain, D.). sich zwisct en den 
Asten der die Seitenorgané innervirenden Spinalnerven, zum 
Behufe einer directeren Leitung der Erregungen, successive 
Anastomosen ausbildeten, mit anderen Worten, dass der 
‘Seitennerv oder Ramus lateralis vagi nach dem Principe 
‘eines Collectors zu Stande kam”. Such a plexus- and 
collector-formation is also seen to appear in other cases 
where unity of function aes a more direct communication 
with the brain is required, e.g. in the innervation of the 
dorsal fin and of the ser, limbs of fishes 
As stated by many authors, lastly by KLINKHARDT ( 1905, p. 
475), the ramus lateralis grows out in a backward direction 
within the apneu and only afterwards detaches itself 
from the latte 
Beard versus biste. — BEARD (1885), though formerly (1884) 
Adhering equally to the homology of the lateral organs in 
eel 
in 
