NERVOUS SYSTEM OF FISHES. 201 



with a valve. In the Lamprey the flask-shaped nasal sac opens upon 

 the top of the head : a simple membranous tube is coiatinued from 

 the expanded bottom of the sac, which dilates as it descends, but 

 terminates in a blind end at the hypophysial vacuity {fig. 26. hy) of 

 the base of the skull, where the mucous membrane of the palate 

 passes over it entire and imperforate.* 



In all Fishes, save the Dermopteri, the olfactory organs are 

 double : and they have no communication with the mouth. In 

 Osseous Fishes they are situated on the sides of tlie snout, and are 

 covered 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 ; the anterior aperture in 

 the biperforate sacs is often produced into a tubular process, which 

 acts either by muscular power, or some modification of form, as a valve. 

 It is provided with a moveabh; cartilage in the Conger ; and the 

 tubular nostrils of the Cyclopterus are in perpetual motion in the 

 living fish. Both apertures in some Lophioid fishes are bell-shaped 

 and pedunculate. In some Siluri a tentacle is continued from the 

 external nasal tube. AVhen the nasal sac is round, the pituitary 

 plicaj radiate from its centre : when the sac is elongated, it is usually 

 traversed by an axial partition with a row of transverse folds on 

 each side. In a few Fishes these folds are further complicated by 

 secondary pi'ocesses. The Sturgeon presents the radiated type of the 

 olfactory organ with secondary folds {fig. 43. 19) ; but, like the 

 Polypterus and Lepidosteus, each nasal sac has a double aperture. 

 The Lepidosiren has an elongated nasal sac, with the bi-serial 

 arrangement of pituitary folds, and with a double ajDcrture {fig. 54. 

 ol) ; but neither of these communicate with the mouth : the peculiar 

 position of the nasal sacs on the under part of the thick upper lip, 

 may have deceived the German naturalists who have affirmed the 

 reptilian nature of this animal on the erroneous supposition that the 

 posterior aperture of the nasal sac communicated with the mouth : the 

 cartilaginous capsule of the sac is fissured, or barred, reminding one 

 of the more complex nasal cartilage in the Myxine.f In the Pla- 

 giostomes the nasal cavities ai'c situated beneath the snout, near the 

 angles of the mouth, especially in the Rays : each cavity has a 

 single and commonly wide opening, defended by valvular processes, 



* XXI. p. 4:5. ; figs. ii. and iv. 



f In the first specimen of l.cpidnsiroi avnccteiis wliich served for my description 

 of its anatomy in 18:59, partial (leeomi)ositioii of the npjjer Hp had destroyed the 

 soft inemhrane extended over the mouth of the olfactory sac, which led nie to 

 the belief that it had but one opening ; the second, or posterior opening, is outside 

 the_maxillary teeth. 



