CHAPTER VI 

 MOUTHS AND JAWS 



P'orm of mouth in Cyclostomes, in Selachians. Jaws of Selachians, of 

 Bony Fishes. Form and position of mouth in Bony Fishes. Modifica- 

 tions of lips. Weevers and Star-gazers. Gar Pikes, Gar-fishes, and 

 Half-beaks. "Sword-fishes." Tube-mouths. Fishes with protractile 

 mouths. Flat-fishes. 



The importance of the part played by hunger and the conse- 

 quent need for food in the daily life of a fish is obvious, and is 

 reflected in the form of the mouth, jaws, teeth, and so on, 

 structures which present more diverse modifications than any 

 other organ of the body. As will be explained in the present 

 chapter, such modifications may generally be shown to be 

 more or less intimately associated with the mode or conditions 

 of life, the manner of obtaining food, and the nature of the diet 

 itself. 



The C /clostomes (Lampreys and Hag-fishes) differ from all 

 other fishes in having a rounded, funnel-like mouth placed at 

 the end of the head, which, although supported by special 

 cartilages, is entirely devoid of true biting jaws. The mouth 

 of the Lamprey [Petromyzon) acts as a sucker, by means of 

 which it attaches itself to other fishes, devouring them by 

 sucking their blood and rasping off their flesh with the horny 

 teeth on the muscular piston-like tongue (Fig. 45A). At one 

 time the absence of jaws was regarded as the result of degenera- 

 tion consequent upon the adoption of semi-parasitic habits, 

 but a detailed examination of some very ancient extinct forms 

 now known to be ancestral to the modern Cyclostomes has 

 shown that these also lacked true jaws {cf. p. 344). The Lam- 

 prey is able to strike its suctorial mouth against the skin of its 

 prey, and becomes so firmly attached that it is rare indeed for 

 the victim to shake off its persecutor before succumbing from 

 loss of blood. The amazing strength of the sucker may be tested 

 by allowing a Lamprey in an aquarium to attach itself to the 

 hand or arm, and it will be found almost impossible to detach 

 the fish without lifting it from the water. While engaged in 

 feeding the Lamprey is carried about by its victim, and it is by 



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