Feeding and breathing in lampreys.

A basic problem faced by the agnathans in evolution was how to feed and breathe without jaws. Three solutions are represented by lampreys and their ammocoete larvae, reviewed here, and hagfishes. Lampreys feed upon fish with their suckers and breathe in and out of their branchial gill sacs. Parasitic species of lampreys can be flesh-feeders or blood-feeders, depending primarily on the structure of their teeth. Feeding behavior is characterized by rhythmic rasping, negative pressure pulses in the sucker, and swallowing of fluid into the gut. Ammocoete larvae use a velar pump for unidirectional ventilation and suspension feeding. In both lampreys and ammocoetes the branchial basket is actively compressed for exhalation; branchial expansion and inhalation is by passive elastic recoil, but in ammocoetes water is drawn from the mouth. Central pattern generators for respiration are distributed in the medulla, particularly lateral to Vm, and drive branchial motoneurons in VIIm-IXm-Xm. Trigeminal pattern generators in lampreys may be a holdover from the ammocoete stage, in which they drive nearby velar motoneurons as the primary pump for ventilation. Respiration in lampreys and ammocoetes is stimulated by hypoxia and modulated by reflexes. Metamorphosis from ammocoete to adult lamprey involves extensive remodeling of the head with regression and replacement of most muscles. Trigeminal motoneurons are probably preserved during metamorphosis, as inferred from constant maps of motoneurons in Vm. This hypothesis is supported by analogy with anuran metamorphosis in which V motoneurons are retained and remodeled. In Mallatt's current models, the earliest vertebrates breathed by branchial contractions and valves; jaws initially evolved for better ventilation and later were used for feeding.