Description of Young Ammocoetes belonging to Two Species of Lampreys: Petromyzon marinus and Entosphenus lamottenii

The description of young ammocoetes, 19–68 mm in length, belonging to Petromyzon marinus and Entosphenus lamottenii, is given. The principal differences between species consist in the intensity and pattern of the pigmentation. For identification, in decreasing order of importance, the different sections of an ammocoete are as follows: the dorsal surface of the head and trunk, the lateral side of the head, and the tail region. Among the other characters are described the peritoneum, which is pale, unpigmented in E. lamottenii and darkly pigmented in P. marinus; and the precursor of the tongue, the dark pigmentation of which is found in 98% of E. lamottenii and only in 11% of P. marinus.