ORGANS OF SMELL. 229 



mucous membrane ; in other Fishes we even meet with tufted rami- 

 fications of the folds. The olfactory nerve usually dilates close 

 against the nasal cavity of its side into a considerable bulb, or it 

 forms, as in the Plagiostomi, an elongated knot corresponding to the 

 chief mucous fold, and from this its branches proceed along the 

 plaits of the mucous membrane and their subdivisions ; frequently 

 the olfactory nerve divides previously to this into several branches ; a 

 branch of the fifth pair passes also as an accessory nerve to the or- 

 gans of smell. 



Peculiar arrangements of the organs of smell are exhibited by the 

 Cyclostomi, for in them the nasal cavity is single, but the two fami- 

 lies composing this order differ in some respects from each other. 

 In that family to which Petromyzon and Ammoccetes belong, we 

 find a single nasal aperture or spiracle upon the head, which leads 

 into a rather narrower nasal canal, that finally expands into the 

 single nasal cavity lined by plicated mucous membrane j upon the 

 bottom of this cavity is found a more membranous, contractile 

 flask-shaped coecal pouch, an appendage that does not communicate 

 with the pharynx, but with the nasal cavity by a small opening. 



In the second family, the Myxinoidae, the palate is perforated in 

 a remarkable manner, and in Myxine there is a broad naso-palatal 

 opening. In Bdellostoma the lesser naso-palatal opening lies above 

 a fold of the mucous membrane at the extremity of the palate, and 

 leads freely into the naso-palatal duct. The external nasal aperture 

 communicates by means of the nasal tube formerly described as com- 

 posed of cartilaginous rings, with the naso-palatal opening. 



As regards the Amphibious Fishes, typified by the Lepidosiren, 

 the two species of this genus at present known appear to comport 

 themselves differently as regards the structure of the olfactory 

 organs. In L. annectens the nasal depressions are imperforate ; the 

 olfactory nerve passes through a cartilaginous ethmoidal plate to 

 the organ of smell, which always consists of a sac lined by trans- 

 versely-plicated mucous membrane ; an external opening is merely 

 found beneath the upper lip. L. paradoxa seems to agree complete- 

 ly with Siren and Proteus among the Amphibia, for in it we find a 

 posterior opening upon the internal side of the upper lip, and the mu- 

 cous membrane disposed in folds. 



The organ of smell, recently discovered in Amphioxus, is of a still 

 more simple kind. It consists of a single depression, situated to the 

 left side, which terminates freely in a small cup, and rests directly 

 upon the central axis of the nervous system. It presents upon its 



