CHARACTERISTICS OF MARINE FISHES 165 



odors in the water. However, this sense, as in hearing, may be assisted by the 

 lateral line organs. 



The Lateral Line. Most fishes have this plainly visible line, provided with 

 pores, extending from the head or shoulder to the base of the caudal fin or even 

 to the end of that fin. Tubes are connected with the pores in the lateral line, which 

 contain numerous nerves, indicating that they are sense organs. Their function is 

 not well understood. It is rather generally believed, however, that they are useful 

 in detecting disturbances in the water. In detecting disturbances, soxmd waves, or 

 odors in the water the lateral line organs may aid the fish in finding food and 

 in avoiding enemies. Nevertheless, many fish, including herrings, shad, alewives, 

 and mullets, have no lateral line and surely do not need one. 



Tactile Organs. Special organs of touch, useful in exploring the bottom for 

 food and perhaps also in avoiding enemies, are present in several groups of fishes. 

 Tactile organs consist of barbels and generally of some sort of modified paired 

 fins. Catfish, goatfish, codfish, croakers, etc. have barbels attached somewhere to 

 the snout or lower jaw. Some other groups, such as blennies and scorpion fish, 

 have barbels over the eyes or on the body, principally along the lateral line. 

 Barbels may be long and slender, as in catfish and goatfish, or short, as in croakers 

 and codfish. In some species they are single, as in the codfish (Fig. 10-21), few, as 

 in goatfish and catfish (Fig. 10-22), or numerous, as in the croakers (Fig. 10-23). 



The sea robins (Triglidae) and threadfins (Polynemidae) have feeler-like 

 appendages attached to the pectoral fins, and the cusk eels (Ophidiidae) and 

 some of the blennies (Blenniidae) have modified ventral fins that serve as tactile 

 organs. 



The Gills. Gills are to fish what lungs are to air-breathing vertebrates-organs 

 of respiration. They are set behind the cavity of the mouth and consist of bony 

 arches to the rear of which slender reddish filaments are attached. It is through 

 these slender filaments that free oxygen in the water is absorbed and carbon 

 dioxide is given off. On the anterior edge of the bony arches (generally known 

 as gill arches or as branchial arches) is a series of projections, which may be few, 

 short, and stubby at one extreme, or numerous, long, and slender at the other. In 

 those species in which they are few and short, they serve no useful purpose; but 

 in others that have numerous long close-set gill rakers, as, for example, the 

 menhaden (Brevoortia) they are used in straining from the water small organ- 

 isms upon which the fish feed. 



The number of branchial arches present varies somewhat among the diflFerent 

 groups of fishes. The vast majority of bony fishes, however, have 4 pairs, with a 

 single opening on each side of the head. Sharks and skates and rays, with few 

 exceptions, have a separate opening, known as a "gill-slit," to each gill arch. 



The Air Bladder. The air bladder or swim bladder is an elongated sac, with a 

 thin semitransparent wall, lying close to the backbone in the abdominal cavity. 

 It is in a measure comparable with the lungs of air-breathing vertebrates. Never- 

 theless, a "normal" air bladder seems to have nothing to do with respiration, its 

 chief purpose presumably being that of providing buoyancy and helping the 

 animal to rise and sink in the water. It is to be remembered, however, that many 

 fish, such as the flatfish, sharks, and skates and rays, have no air bladder. Even 

 such an active swimmer as the common mackerel, Scomber scombrus, has no 



