230 THE SENSE-ORGANS AND PEEOEPTION OF FISHES. 



would be coBsistently larger than in those which seek food by 

 sight, but this is only partially true. For though the great develop- 

 ment of tbese parts in the eel, conger, and Elasmobranchs is accom- 

 panied by an acute sense of smell, yet in the rocklings, the loach, 

 and the sole, which also seek their food by scent, the olfactory 

 organs cannot be said to be proportionally more developed than 

 they are in forms which feed by sight, such as the plaice and the 

 pollack. 



Nostrils. — In all forms {? Zeugopterus punctattis) examined, two 

 nostrils occur on each side in the manner characteristic of most 

 fislies. Of these two nostrils the anterior is to some extent tubular 

 in all the fishes (except the Elasmobranchs) whicb seek their food 

 by scent {v. p. 235) . This tube is most developed in the conger, in 

 which it is simple and straight, projecting beyond the surface of the 

 nose. In the eel this anterior portion of the nostril is formed of 

 two flaps of skin. In the rocklings it takes the form of a very 

 short tube, the dorsal edge of whicb is produced into a long barbel. 

 The anterior nostril of the loaches resembles that of the rockling, 

 but the tube stands up more vertically from the head, and the pos- 

 terior edge of the aperture is not produced into a barbel, but is bent 

 over it to form a kind of bood. This latter form of nostril is also 

 found in nearly the same shape in Lepadogaster. In all these forms 

 the posterior nostril is a simple opening without a valve. 



These tubular nostrils are ciliated as well as the olfactory epithe- 

 lium itself, and a current is thus caused which enters by the ante- 

 rior and leaves by the posterior opening. In the majority of fishes 

 rhythmical oscillations of the water in the olfactory pits also occur, 

 but these are not present in any of the animals mentioned below as 

 seeking food by scent, except Lepadogaster. 



In the ordinary round fislies {Gadidse, Lahrida^, &c.) the two 

 nostrils are placed close together. Through these openings currents 

 are no doubt caused by the cilia on the olfactory organs themselves, 

 but tbe principal movement of water in the olfactory chambers is 

 an oscillatory movement which occurs rhythmically, keeping time 

 witli and being probably dependent on the respiratoiy movements 

 of the fish. The former movements may nevertheless be suspended 

 while the respiratory movements continue. It did not appear that 

 in these fishes the water entered or left by either nostril in parti- 

 culai", but rather that it oscillated in and out through both of them 

 at once. 



In the flat-fishes the arrangement of this current is more compli- 

 cated. The plaice may be described as having the structure which 

 is found in most of them. This fish lies on its left side. The an- 

 terior nostrils are tubular, that of the right side being produced con- 



