676 EVOLUTION OF ANIMALS Part V 



have a conspicuous air or gas bladder that lies in the dorsal part of the body 

 cavity parallel to the backbone. In some species, it is connected with the 

 pharynx by an open duct; in others, as in the perch, by a solid strand of tissue 

 (Fig. 33.11). Its transparent walls are plentifully supplied with blood vessels 

 and it is filled with oxygen, nitrogen, and carbon dioxide in varying propor- 

 tions, evidently originating from the blood. African lung fishes use the air 

 bladder as a lung. In certain deep-sea fishes, the unusually large proportion of 

 oxygen that has been identified in the bladder has been taken to be an extra 

 insurance against its sparsity in deep water. The bladder also aids the fish 

 in lifting and holding its body at one or another level of water as a pickerel 

 hangs in ambush, motionless among the pond weeds. The sharks lack air blad- 

 ders, but they have the lifting capacity of relatively enormous livers stocked 

 with oil (Fig. 33.7). 



Fishes make sounds with their air bladders. Undersea noises were heard in 

 full strength by means of radar during World War II. Certain regions, one of 

 them the Chesapeake Bay, were at that time a bedlam of racket obstructing 

 any other sounds. The croaker ( Micropogon ) listened to by means of a hydro- 

 phone proved to be guilty of the noise, made by the contraction of a muscle on 

 the capacious bladder that acted as a resonator. Croakers are edible fishes com- 

 mon along our southern Atlantic coast. It has been estimated that their 

 population in Chesapeake Bay was at one time about 300 million. The male 

 weakfish (squeteague) can set its bladder in vibration and produce sounds. 

 The fresh-water drum or sheepshead is able to grunt by muscles working on 

 the gas bladder. Although fishes lack true vocal organs, they have joined the 

 world's chorus with what means they have. 



Circulation. The heart with its auricle and one ventricle lies in the peri- 

 cardial sac ventral to the pharynx. It is located far forward in the body, con- 

 tains venous blood only, and pumps it to the gills, from whence it goes directly 

 to other parts of the body (Fig. 33.12). In all other vertebrates, the blood is 

 returned from the respiratory organs to the heart before it is distributed to 

 the body. Fish blood is under low pressure, is relatively thick, and does not 

 flow easily. 



Reproduction. Almost all fishes multiply abundantly, many of them 

 enormously. The sexes are separate and in the majority fertilization occurs in 

 the open water. Mackerel gather in great assemblies of males and females and 

 the water swarms with sex cells. Herring do likewise and a single female 

 produces 30,000 to 2,500,000 eggs per year. Counts and calculations by the 

 U. S. Bureau of Fisheries credit the halibut with 2,000,000 eggs per year and a 

 codfish with 9,000,000. 



In central New York state, brook trout (Salvelinus fontinalis) begin to 

 ascend to the spring-fed headwaters of streams in September. There the males 

 and females first congregate in the deeper pools below the spawning grounds. 



