44 THE ANCESTRY OF VERTEBRATES 



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 homojogization 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 ElSlG, 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 



