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wholly hypothetical. GEGENBAUR (1887) points, in his dicus- 
sion of MARSHALL’s view, to the olfactory grooves of 
Cephalopods which are just as little derived from gill-slits and 
to which those of Vertebrates show a certain resemblance, 
though he does not think of an homologization of the 
two. KUPFFER (1890) attempted to show that the olfactory 
epithelium develops from a placode extending across the 
middle line and hence has an unpaired character. KOLTZOFF 
(1902) demonstrated the inadequacy of this view and the 
paired nature of the olfactory organ is now universally 
admitted. 
The monorhinism of Cyclostomes, though present from 
the beginning of development, is to be considered as a 
secondarily acquired character in view of the fact that the 
olfactory nerve is double from the beginning. In Elasmo- 
branchs we evidently find a more primitive disposition. 
Here, as in higher Vertebrates, the olfactory grooves origin- 
ate in front of the cerebral plate or, if somewhat later, 
on both sides of the neuropore, between the latter and 
the mouth, i.e. on the ventral side of the snout or prosto- 
mium. Now, if we look in Annelids at the same spot, 
that is here at the dorsal half of the prostomium, we find a 
pair of ciliated pits, closely resembling the olfactory grooves 
of Craniates. 
Ciliated pits in Annelids and Molluscs. — These ciliated 
pits, nuchal grooves, olfactory pits etc. as they are design- 
ated by different authors, have a very wide distribution 
among Annelids, much more so indeed than the statocysts. 
In his memoir on Oligognathus bonelliae SPENGEL (1881) 
has for the first time given a survey of all that was known 
about them up till that time. In Capitellids EISIG (1887) 
found them regularly present and carefully investigated their 
structure, They here represent a pair of transverse grooves 
situated dorsally at the base of the prostomium. Out of them 
a club-shaped, vigorously ciliating, organ can be everted 
like the finger of a glove. Though EISIG, with most authors, 
considers it as an olfactory organ, yet, until its function 
has been proved by experiments, he keeps to the neutral 
name ciliated organ. Between the ciliated organs and the 
posterior lobes of the cerebral ganglia very intimate rela- 
tions exist: in Notomastus and Mastobranchus the only 
function of the latter is the innervation of the ciliated organs, 
so that the hinder of the two pairs of ganglia of nearly 
