THE SENSE-OEGANS AND PEKCEPTION OF FISHES. 23l 



siderably. The posterior nostril of the right side is valvular, and 

 opens outwards. On inspiration water passes into the olfactory 

 chamber through the anterior nostril, and on expiration the posterior 

 nostril opens suddenly as the water passes out through it. The pos- 

 terior nostril of the left side is not valvular, but simple. These re- 

 marks apply also to the dab and the flounder. In all these fishes the 

 nostrils are so arranged that none of them are on the lower surface 

 of the head, but rather on its dorsal side. 



The sole has a different mechanism. It lies on its left side, and 

 both the nostrils of the left side are on the under surface of the head 

 — touching the ground, in fact, when the animal is at rest. On the 

 left side the anterior nostril is large and open, and has a ciliated 

 fold of skin which passes spirally down it ; but the posterior one is 

 very inconspicuous and valvular, being at some distance from the 

 anterior opening. As the sole inspires, water passes in at the an- 

 terior opening, and on expiration it leaves by the posterior nostril, 

 which opens and shuts with a jerk. The anterior nostril of the right 

 side is tubular, and the posterior one is valved, acting like that of the 

 left side. 



In all these fishes, also, the flow of water through the olfactory 

 oi'gans may be suspended though the respiratory movements continue. 



In the tui'bot, which lies on its right side, the left anterior nostril 

 is guarded by a simple triangular flap of skin which projects forward 

 from the posterior edge of the opening and covers it. This possibly 

 forms a valve during life, though I have not had an opportunity of 

 closely examining the currents through the olfactory chamber in a living 

 specimen. The posterior edge of the anterior nostril on the left side 

 is produced into a very large, leaf-like flap of skin, which in dead 

 specimens usually covers the nostril. In one specimen (from Grimsby), 

 however, this fold was reflexed, and lay against the side of the head 

 in such a rigid manner that it could not have been used during life 

 to cover the nostril. 



The right posterior nostril of the turbot is widely open, and the 

 left posterior nostril is also without any special valve, but the skin 

 at its edges nearly meets across the opening. These structures do 

 not materially differ in the brill {R. levis). In one brill (from 

 Grimsby) the two nostrils of the right side were united, forming a 

 common fossa in which the olfactory organ lay. In this specimen 

 the leaf -like fold of skin, usually covering the anterior nostril, was 

 divided into two parts, which were disposed on either side of this fossa. 

 There was no indication that this variation was not congenital. 



The left anterior nostril of Arnoglossus laterna is somewhat tubular, 

 but the left nostrils of Arnoglossus megastoma, which also lies on its 

 right side, were, in a preserved specimen, without valves or flaps of 



