232 THE SENSE-ORGANS AND PERCEPTION OE FISHES. 



skin of any kind. The riglit anterior nostril, however, has a very 

 large loose flap of skin. The olfactory folds are but slightly deve- 

 loped, and are mere ridges on the floor of the olfactory chamber. 

 Their number in the left organ of a large specimen was thirty {v. 

 fig. 3). 



The olfactory structures of Miiller's top-knot {Zeugopterus punctatus) 

 are so abnoi^mal and reduced that it will be best to reserve any state- 

 ment about them until the homologies of the parts are more clear. 



The olfactory organs themselves in fishes are composed of the well- 

 known folds bearing the sensory and supporting cells of the epithe- 

 lium. On this occasion I propose to give an account of the general 

 structure of these organs, deferring the description of the histology 

 until a full comparison can be made between the olfactory elements 

 of the fishes which hunt by scent with the same parts in those which 

 seem not to use their olfactory organs for this purpose. 



The arrangement of the olfactory folds differs in the various fishes. 

 Roughly speaking, they are built up on one of four types, or on some 

 plan intermediate between them. 



(1) In the skate and dog-fish the plates are arranged in a radiating 

 manner on the inside of a hollow capsule, like the septa of an orange. 

 In this case the free internal edges of the plates do not bear sensory 

 cells, but are fibrous supporting tissues. 



(2) The conger and eel have the plates of the organ arranged in 

 two rows on each side of a central raphe, upon which the two rows 

 are folded longitudinally so as to form the lining of the olfactory tube. 

 The olfactory organ of the sole, though a much less considerable 

 structure, is arranged on a similar plan ; for on it the longitudinal 

 raphe is depressed so as to form a groove from which the plates rise 

 up on each side. The number of plates in an eel one and a half feet 

 long was about thirty-eight pairs in each organ. As already men- 

 tioned, the number and size of these plates increase with the growth 

 of the animal. 



(3) The third type of olfactory organ, of which the second is a 

 modification, is that most commonly found among fishes. In it the 

 plates are fitted together in a radiating manner, forming a convex 

 eminence in the olfactory chamber. The whole organ is either 

 circular (as in Cottus and Motella mustela) or elliptical (as in the 

 mackerel), according to the number and shape of the plates of which 

 it is composed. 



In all the Teleosteans hitherto mentioned most of the plates are 

 placed at right angles to the long axis of the body, and each organ 

 essentially consists of two rows of such plates united in the middle ; 

 for the circular collection of radiating plates of Cottus, &c., only 

 differs in degree from the more common elliptical one. 



