A large number of larval, metamorphosing, and young feeding adults of the anadromous sea lamprey, Petromyzon marinus L., have been used to provide a detailed description of the external alterations that take place in this species as it changes from a larva to an adult. A total of seven metamorphosing stages (1–7) are described, using as a basis a variety of different morphological characters, such as the appearance of the eye (1–3), the structure of the oral aperture (2–4), the degree of development of the tongue-like piston (4–7), the shape and degree of cornification of the teeth (4–7), and the coloration of the body (5–7). Attention is drawn to the fact that, although the changes in individual characters may vary in their time of initiation and the duration of their development, the overall metamorphic process is relatively synchronous. In contrast to this synchrony, however, the time at which the downstream migration occurs is very variable, a feature which is probably of selective advantage to this species
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