i82 A HISTORY OF FISHES 



a whole group of muscles is required, some idea will be gained 

 of the vast number of nervous impulses continually going 

 backwards and forwards from sense organs to brain and spinal 

 cord, and from the brain and spinal cord to the muscles and 

 glands. 



In their general plan the sense organs of a fish are not unlike 

 those of higher vertebrates, nostrils, tongue, eyes, and ears all 

 being developed; but whereas certain senses of special import- 

 ance to an animal living in a liquid medium are greatly 

 accentuated, others are very feebly organised. Further, it is 

 possible that fishes possess at least one extra sense unknown to 

 land vertebrates. 



The sense of smell resides in the nasal or olfactory organs, but, 

 unlike the higher vertebrates, the nostrils or nasal openings are 

 never (or scarcely ever) used for breathing purposes. Typically, 

 each nasal organ consists of a somewhat deep pit lined with 

 special sensitive tissue, and in order to provide the maximum 

 of sensitive surface, the lining is generally puckered up into a 

 series of ridges which may be parallel to each other or arranged 

 in radiating fashion like a rosette (Fig. 72B'). The Cyclostomes 

 are unique in possessing a single nostril on the upper surface 

 of the head (Fig. 72A, a'), which in the Lampreys {Petromy- 

 zonidae) leads into a blind nasal sac, but in the Hag-fishes 

 {Myxinidae) actually communicates with the roof of the mouth. 

 In the Sharks and Rays the olfactory organs are invariably 

 large, and, like the mouth, are placed on the lower surface 

 of the head (Figs. 14B; 45B). The single opening of each organ 

 is guarded by valvular flaps, provided with their own cartilages 

 and moved by special muscles. In certain Sharks and Dog- 

 fishes deep oro-nasal grooves connect each organ with the 

 angle of the mouth on the same side. In the majority of Bony 

 Fishes these grooves are wanting, but in the Lung-fishes 

 (Dipneusti) they have been converted into short canals, and the 

 nasal pits communicate with the mouth by true internal 

 nostrils, thus foreshadowing the condition found in higher 

 vertebrates, where they are used for inhaling air for breathing 

 purposes. In Bony Fishes both nasal pits are divided into two 

 separate portions, each with its own opening to the exterior 

 (Fig. 72B). The position of the nostrils varies considerably in 

 difierent fishes; in some the anterior nostril is widely separated 

 from the posterior, in others the two are almost in contact. 

 Occasionally, as in the Cichlids [Cichlidae) and in certain 

 Wrasses {Labridae), the nasal organs each have only a single 



