A HISTORY Of VERTEBRATES: FISHES 4$\ 



pharynx is a blind sac posteriorly and is separated anteriorly from the 

 mouth cavity by a small flap of tissue. Because of its isolation, the 

 pharynx is often called a respiratory tube. 



The kidneys, as in the frog, are drained by Wolffian ducts. These 

 ducts carry only urine, for sperm or eggs pass from the large median 

 (embryonically paired) testis or ovary into the coelom. A pair of genital 

 pores leads from the coelom into a urogenital sinus, formed by the 

 fused posterior ends of the Wolffian ducts, and thence to the cloaca and 

 outside. The absence of genital ducts may be a very primitive feature. 

 The sexes are separate in the adult lamprey, though sexual differentia- 

 tion occurs rather late in development, and the gonads of young indi- 

 viduals may contain both developing sperm and eggs. 



The eggs are laid on the bottom of streams in a shallow nest, 

 which the lampreys make by removing the larger stones with their 

 buccal funnels (Fig. 22.5). During mating the female attaches to a 

 stone on the upstream side of the nest, and the male to the female, each 

 by its buccal funnel. As the eggs are laid, the sperm is discharged over 

 them. The adults die after spawning. 



Developing sea lampreys pass through a larval stage that lasts five to 

 six years. The larva is so different in appearance from adult lampreys 

 that it was originally believed to be a different kind of animal, and 

 was named Ammocoetes. The ammocoetes larva is eel-shaped, but lacks 

 the specialized feeding mechanism of the adult. It lies within burrows 

 in the mud at the bottom of streams, and sifts minute food particles 

 from water passing through the pharynx. Like the lower chordates, it 

 lias a mucus-producing endostyle to aid in trapping the food. 



Adult lamj^reys injure and kill many other fishes. In recent years 

 the sea lamprey has passed the Niagara barrier, presumably through 

 the Welland Canal, and extended its range from Lake Ontario into 

 the other Great Lakes. The lake fishing industry has been harmed 

 greatly. For example, the lake trout catch in Lake Michigan was 

 6,860,000 pounds in 1943. It began to decrease markedly in 1945 and 

 was a mere 3,000 pounds in 1952. In terms of 1950 prices, the 1943 

 catch was worth $3,430,000; the 1952 catch, $1,500! 



The hagfishes resemble the lampreys in major respects, though 

 differing, of course, in certain details. Hags are believed to be primarily 

 scavengers feeding upon dead fish along the ocean bottom, but they also 

 attack disabled fish of any sort, including those hooked or netted. They 

 burrow into the fish and eat out the inside, leaving little but a bag of 

 skin and bone. They are a commercial nuisance, but their over-all 

 damage is not great, since they are abundant in only a few localities. 



197. Jaws and Paired Appendages 



During the Silurian and Devonian periods, certain descendants of 

 the sluggish ostracoderms acquired paired appendages and jaws, and 

 became more active and predaceous. The earliest fishes of this type are 

 placed in the class Placodermi, and the earliest of these, like their 

 ostracoderm ancestors, were fresh-water forms. 



