ORGAN OF SMELL IN FISHES. 329 



Osseous Fislies tliey are isituated cm the sides of the snout, in a 

 cavity formed l)y the nasal, fig. 75, 15, the prefrontal, u, the 

 lacrymal, 73, the premaxillary, 22, and the vomer. The capsules 

 arc covered externally by the skin, which is usually jjierced hy 

 two openings for each sac : the Chromides, and all the Wrasses 

 with ctenoid scales, have a single opening for each nose-sac ; 

 where there are two ncistrils the posterior is usually o])cn, the 

 anterior ch)sed, as liy a sphincter or a valve : the anterior aperture 

 IS often produced into a tubular jirocess, as in tlie Loach, which 

 acts, either by muscular i)Ower or by some modification of form, 

 as a valve. Both apertures in S(nne Lophioid Fishes are bell- 

 shaped and 2'ediinciilate. In some Siluri a tentacle is continued 

 from the external nasal tulie. Wlien the nasal sac is round, the 

 pituitary plicaj radiate fi-om its centre : when the sac is elongated, 

 it is usually traversed by an axial jiartition with a row of folds on 

 each side ; and tliere are transitional arrangements, as in the 

 Perch, figs. 131, ol, & 134. In a few Fishes these folds are 

 further complicated by secondary processes. The Sturgeon pre- 

 sents the radiated type of the olfactory organ with secondary folds, 

 fig. 125, 10, but, like the Polypterus and Lepidostciis, each nasal 

 sac has a double aperture : the Le]iidosireii has an ehmgated nasal 

 sac, with the biserial arrangement of jiituitary folds, and with two 

 apertures, fig. 186, ol, upon the iiiider jiart of the thick upper lip, 

 but neither of these communicate with the moiitli. In some 

 Osseous Fishes the olfactory sac is divided into a jilicated and a 

 smooth jiart : the former exercising the sense-function, the latter 

 that of a reservoir. In the Mackerel this extends down to the 

 palate : in the Wolf-fish the reservoir passes liackward, expanding, 

 as far as the back part of the palate, where it ends blindly. The 

 prolongation of the single nasal cavity in tlie Lamprey is analogous 

 to this. 



In the Plagiostomes the nasal cavities are situated beneath the 

 snout, in the Sharks, figs. 30 & 63, h : lieneath the fore part of 

 the head, behind the base of the rostrum, in the Saw-fish {Fristis), 

 fig. Gl) : or near the angles of the mouth, as in the Chimcera and 

 the Rays, where a groove extends to the mouth. Each olfactory 

 cavity h^s a single and commonly wide opening, defended by 

 valvular processes, sujjported by peculiar cartilages more or less 

 intimately connected with the proper olfactijry cartilaginous sacs, 

 and representing the superadded cartilages of the ' ala3 nasi ' in 

 higlier Vertebrata.' They have their proper muscles : whence we 

 must conclude that these Fishes scent as well as smell, i. e. actively 

 ' Sec tlie descripliou of these 'naseutlugclknorpel ' in xxi. p. 171. 



