Abstract
All previous attempts to estimate the length of larval life of lampreys have involved ammocoetes of brook lampreys only. Knowles (1941) estimated a larval life of 3 years, Schultze (1930) placed the figure at between 3 and 4 years, while IvanovaBerg (1931) came to the conclusion that brook lampreys metamorphose after 5 years. Hardisty (1944) also estimated an ammocoete life span of 5 years in Lampetra planeri (Bloch). Hardisty (1951) mentions the possibility that the larval life of L. planeri might vary slightly according to environmental conditions. For this reason the author has included yet another analysis of L. planeri yearclasses along with the completely new study of L. fluviatilis (L.) and Petromyzon marinus L. ammocoetes included in the present paper. Ammocoetes were collected from various rivers and streams in order to obtain pure samples of each of the three species. Since the methods employed in identifying the ammocoetes (MacDonald, in press) had not been perfected when this experiment was started, all mixed samples were rejected. Large-scale collection of living ammocoetes was made possible by the generous loan of a portable D.C. electric stimulator from the Ministry of Agriculture, Fisheries and Food. Finally, three pure samples of each species were obtained, each sample having been taken at a different time of year. Ammocoetes thus taken were preserved on the spot in
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