228 PISCES. 



by the microscope ; and this is the more remarkable, as these struc- 

 tures appear to occur generally in the auditory organs of the Inverte- 

 brate animals. 



No vestige of an organ of hearing has hitherto been found in Am- 

 phioxus, so that we have an example among the Vertebrata of a 

 creature completely destitute of the auditory function. 



The Amphibious Fishes have a more complete organ of hearing. 

 At least in Lepidosiren annectens a large labyrinth has been found 

 with three small semicircular canals, and two great sacs containing 

 otoliths, but without a trace of tympanic cavity. 



Organs of Smell. 



IN all Fishes we find distinctly developed organs of smell. They 

 consist of cavities situated at the anterior extremity of the snout, in 

 front of the eyes, and beneath the nasal bones ; they are bounded by 

 the maxillary bones and vomer, present an elongated, oval, or round 

 form, and open by two nasal apertures lying one behind the other ; 

 the anterior opening is occasionally, as in Muraena, lengthened out 

 to form a short contractile tube ; the posterior is in some instances 

 removed to a distance from the first, and patulous. This is the gen- 

 eral arrangement of these parts in the Osseous Fishes ; the nasal 

 passages are scarcely ever perforate throughout, as in the air-breath- 

 ing animals, though in several species of Conger (as in the Am- 

 phibia that breathe by branchiae) they are found to open internally 

 beneath the upper lip. Minor peculiarities frequently occur in the 

 structure of these organs, as in Lophius piscatorius, where the nasal 

 cavities exhibit the form of two small bell-shaped sacs situated be- 

 neath the upper lip, and lined by folded mucous membrane. In the 

 Plagiostomi the nasal depressions are very large, and can be closed 

 by means of a membranous or cartilaginous operculum ; they are 

 placed inferiorly near to the angles of the mouth, and the operculum 

 can be raised by muscles. The mucous membrane lining them is 

 very vascular, provided with crypts that secrete a copious mucus, 

 and disposed in exceedingly delicate and frequently very vascular 

 folds. In some instances these folds radiate from a projecting mid- 

 dle elevation of the mucous membrane ; more frequently, however, 

 they arise like the teeth of a comb, in the form of ridges transversely 

 directed, from the sides of a middle longitudinal fold ; the transverse 

 plications in the Plagiostomi are further provided with stellated folds 

 upon their walls, in order to increase the extent of surface of the 



