juvenile hormone as the result of which different further phylogenetic changes have arisen secondarily. (1966, p. 135)

Finally, Costlow (1968, p. 37) has suggested that gigantism and neoteny in the larvae of some crustaceans may be elucidated by experiments on the extirpation of eyestalks in other species. The hormonal control of development in crustaceans is poorly known, but molting is initiated by a hormone released from the Y-organ following a decrease in concentration of molt-inhibiting hormone secreted by the X- organ-sinus gland complex of the eyestalks. If both eyestalks are removed prior to the third day in third-stage zoeal larvae of the mud crab Rhithropanopeus harrisii, one or two supernumerary zoeal stages may be interpolated before metamorphosis to the first crab stage (Costlow, 1966).

Amphibian Paedomorphosis and the Thyroid Gland

The endocrine control of paedomorphosis in salamanders has been recognized ever since Gudernatsch demonstrated the thyroid control of amphibian metamorphosis in 1912. In one early experiment, for example, Hoskins and Hoskins (1919) removed the thyroid anlage before its differentiation began and obtained giant larvae (two to three times the size of controls) that never metamorphosed, in both Ambystoma punctatum and Rana sylvatica.⁷

Some early and naive hopes for a simple mechanism of paedomorphosis, and a science of rejuvenation for the aged, succumbed to the complexities of thyroid response by paedomorphic salamanders. These salamanders form a graded series, from species with rare neotenics, through paedomorphic populations transformable only under unusual laboratory conditions, to perennibranchiate salamanders that have never been induced to metamorphose. This structural series is matched by decreasing sensitivity to thyroid hormones. The series is purely structural and not evolutionary (the causes of facultative neoteny need not be viewed as first steps in the evolution of perennibranchiates). But the continuity of causes does argue for an evolutionary initiation of paedomorphosis as a rapid (and probably facultative) response to immediate demands of the environment. The series includes:

1. Facultative paedomorphs. Lynn and Wachowski (1951), Brunst (1955), and Kollross (1961) reviewed earlier work on successful induction of metamorphosis by thyroxin and various organic iodines in facultative paedomorphs of several Ambystoma species.⁸Neotenic A. tigrinum populations from cold Rocky Mountain lakes can be induced