1909.] OLFACTORY ORGAN OF TELEOSTEAN FISHES. 661 



Furthei" specialization can be seen in the Salmons and 

 Herrings, where the sac, though still a general extension of a 

 great part of the olfactory chamber, is definite in shape, and enters 

 into functional relations with the skeleton of the jaws. The 

 next stage is shown by Orestias, Ojyhiocephalus, Anableps, 

 Bovichthys, the opening of the sac becoming restricted to a 

 definite circumscribed hole in a fairly definite and constant 

 position in the wall of the olfactory chamber. Finally, tlio 

 position and number of the sacs and their mode of connection 

 with the olfactory chamber became crystallized in the Acantho- 

 pterygii, resulting in the definite ethmoidal and lachrymal sacs 

 more or less characteristic of this group. 



In certain fishes the accessory sacs have apparently been further 

 specialized for the production of mucus. This point has been 

 specially dealt with by Kyle in the case of Pleuronectids. To the 

 fishes mentioned by him should be added Merlucciits and Zeus, in 

 both of which the accessory sac and olfactory chamber were filled 

 with an abundance of coagulated mucus. 



Physiological Summary, 



From oui" knowledge of the structure of the olfactory organ, it 

 may be concluded that there are at least three means by which a 

 curi-ent of water may be brought to play upon the laminae of the 

 rosette. The first is by the action of cilia within the anterior 

 nosti'il and upon the lining membrane of the olfactory chamber 

 (Bateson, p. 230). This seems to be the only way in the Eels, 

 which are among the few fishes shown by Bateson to hunt their 

 food by smell, and is probably also in many other cases one of the 

 agents, though not the most important, in ensuring a constant 

 gentle flow of water over the rosette. 



The second method is by the deflection of water into the nose- 

 cavity daring forward progression. This may be effected by the 

 position and slope of wide-open nostrils to the horizontal, as in 

 Esox, but more frequently it is brought about by a hood or screen 

 upstanding behind the anterior nostril (Gadids, Carps). In this 

 case the force of the current is under the control of the fish, and 

 varies directly with the pace at which it is moving. In noses of 

 this type a further refinement is frequently met with in the form 

 of an internal flap that conducts the water-current right down 

 into the centre of the rosette. 



The third method is by the alternate dilatation and compression 

 of accessory sacs connected usually with the hinder part of the 

 olfactory chamber. 



In the majority of cases these are acted upon by the movements 

 of the premaxilla and maxilla, occasionally {Capros, Clupea) by 

 those of the mandible, or (Siluroids) of the palatine bar, and in 

 several cases (where the lachrymal sac extends upon the buccal 

 membrane) by the general pressure of the water in the mouth 

 during expiration. 



