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The shape and arrangement of the teeth and multicuspid laminae of the oral disc and tongue-like piston are described for parasitic lampreys representing the six holarctic genera (Petromyzonli-dae) and each of the monogeneric Southern Hemisphere families (Mordaciidae and Geotriidae). Particular attention is paid to describing the divergent dentitional characters, the location of attack and the size of the oral disc and buccal glands of the blood-feeding Petromyzon marinus and the flesh-feeding Lampetra (Lampetra) fluviatilis and Lampetra (Lampelra) ayresii. The conclusions drawn from these comparisons are used to make suggestions regarding the feeding biology of other parasitic species of lamprey for which less comprehensive data are available.

Compared with P. marinus, the flesh-feeding L.fiuviatilis and L. fluviatils have fewer and smaller oral disc teeth between the circumoral and marginal teeth, a much wider and deeper supraoral. a far larger central cusp on the transverse lingual lamina (which in turn is convex rather than V-shaped) and a smaller oral disc and buccal glands. It is proposed that in the two Lampetra species, the central cusp on the transverse lingual lamina and the interaction of this lamina with the supraoral are adaptations for gouging and cutting out pieces of host tissue. By contrast, the serrated edges of the lingual laminae in P. marinus are used to create a small but deep wound through which a stream of blood is then drawn. This mode of feeding is facilitated by the ability of P. marinus to remain attached for long periods at a single location on the host and to secrete a flow of anticoagulant ‘saliva’ from its relatively large buccal glands.

Since the characteristics of the feeding structures in the parasitic members of the genera lchthyomyzon and Mordacia resemble more closely those of P. marinus than L. fluviutilis and L. ayresii, they would appear to be adapted primarily for the extraction of blood. On the other hand, the reverse is true of Lampetra (Lethenleron) japoniea and Geotria auslralis, indicating that these species ingest predominantly muscle tissue. Species such as Lampetra (Entosphenus) tridentata have an intermediate type of dentition and are apparently more versatile in their feeding habits.

It is concluded that: (i) blood-feeding preceded flesh-feeding in ‘modern’ lampreys; (ii) endemic freshwater parasitic species typically ingest blood; (iii) the ability to feed on flesh developed in populations which had access to estuarine and marine hosts; and (iv) pre-Tertiary forms resembling contemporary lehthyomyzon unicuspsis could have given rise independently to both of the divergent and specialized genera of Southern Hemisphere lampreys (Mordacia and Geolria).