BRANCHIAL SENSE ORGANS OF ICHTHTOPSIDA. 199 



nerve, ganglion, and thickening of a gill-bearing segment, is the 

 absence in the olfactory segment of any prge- or post-branchial nerves. 



Fig. 2 shows us a ganglion fused with an epiblastic sensory thicken- 

 ing and connected with the brain by a short nerve stalk. In fact, it 

 is the picture of a branchial sense organ and its associated ganglion. 



The facts of development here given, which accord so marvellously 

 with the development of the other cranial segmental nerves, certainly 

 render necessary a modification of Marshall's view as to the nature of 

 the olfactory organ, and in fact a modification in the sense of the 

 above passage, in which the nose is regarded not as a gill-cleft, but as 

 the sense organ of a gill-cleft. 



Marshall based his views firstly on the correspondence in anatomical 

 and histological structure between the nose and other gill-clefts ; 

 secondly, on the frequent occurrence of two branches of the olfactory 

 nerve, one on each side of the supposed cleft ; and he further compared 

 the Schneiderian folds of the nasal mucous membrane, as Stannius 1 

 had previously done, to the folds of a gill. 



The facts of development, as stated by Marshall, have been here 

 admitted, but at the same time slightly extended, and in such a wise 

 that the development of the olfactory nerve and organ is shown to 

 agree very closely with that of the nerve, ganglion, and branchial sense 

 organs of any other cranial segmental nerve. 



But now as to the relationships of the branches of the olfactory 

 nerve to the supposed cleft, and as to the nature of the branches them- 

 selves. 



In its earliest development the olfactory nerve shows nothing that 

 can really be homologised with the post-branchial branch of a cranial 

 nerve. Such a resemblance, when present at all, is only existent in 

 much later stages, 



But the post-branchial branch of a cranial nerve, whenever developed, 

 is, par excellence, concerned with the innervation of the gill muscula- 

 ture, and if it contains sensory fibres its main portion is motor. There 

 is nothing like a gill-musculature, even in early stages, connected with 

 the olfactory organ. 



No one has yet described an arterial arch, gill-cartilage, or muscula- 

 ture in connection with the supposed nasal visceral arch. The 

 Schneiderian folds have indeed, in Elasmobranchii and other forms, a 

 certain resemblance to gill-folds, but this alone would not be sufficient 

 i Stannius, ' Lehrbuch cler Vergleichenden Anatomie,' ii. Theil 



