ORGAN OF SMELL IN FISHES. 329 



Osseous Fishes they are situated on the sides of the snout, iu a 

 cavity formed by the nasal, fig. 75, 15, the prefrontal, 14, the 

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

 are covered externally by the skin, which is usually pierced by 

 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 nostrils the posterior is usually open, the 

 anterior closed, as by a sphincter or a valve : the anterior aperture 

 is often produced into a tubular process, as in the Loach, which 

 acts, either by muscular power or by some modification of form, 

 as a valve. Both apertures in some Lophioid Fishes are bell- 

 shaped and pedunculate. In some Siluri a tentacle is continued 

 from the external nasal tube. When the nasal sac is round, the 

 pituitary plica? radiate from its centre : when the sac is elongated, 

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

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

 Perch, figs. 131, o /, & 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, 19, but, like the Polypterus and Lepidosteus, each nasal 

 sac has a double aperture : the Lepidosiren has an elongated nasal 

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

 apertures, fig. 186, ol, upon the under part of the thick upper lip, 

 but neither of these communicate witli the mouth. In some 

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

 smooth part : 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 backward, expanding, 

 as far as the back part of the palate, w^here it ends blindly. The 

 prolongation of the single nasal cavity in the Lamprey is analogous 

 to this. 



In the Plagiostomes the nasal cavities are situated beneath the 

 snout, in the Sharks, figs. 30 & 63, b : beneath the fore part of 

 the head, behind the base of the rostrum, in the Saw-fish (Pristis), 

 fig. 65 : or near the angles of the mouth, as in the Chimera and 

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

 cavity has a single and commonly wide opening, defended by 

 valvular processes, supported by peculiar cartilages more or less 

 intimately connected with the proper olfactory cartilaginous sacs, 

 and representing the superadded cartilages of the ( alse nasi ' in 

 higher Yertebrata. 1 They have their proper muscles : whence we 

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



1 See the description of these 'nasenfliigelknorpel' in xxi. p. 171. 



