Experimentally-Induced Metamorphosis in Echinus

Abstract

(1) Treatment of advanced larvae of Echinus miliaris with very dilute solutions of HgCl2 (about m/2 X 106 rapidly brings about precocious metamorphosis, a young echinus in one case surviving eight days in spite of the absence of food. (2) This appears to be caused through the differential susceptibility of larval tissues and echinus rudiment. The former are more affected by the poison, begin to dedifferentiate, and can then be readily resorbed by the echinus rudiment. (3) When the echinus rudiment is small, metamorphosis is less rapid and may be incomplete, both larval and echinus tissues being dedifferentiated. Even when metamorphosis in such cases is complete, it is often abnormal, the young echinus possessing small or no tube-feet or spines. Parallel abnormalities are to be found in artificial (thyroid-induced) metamorphosis in anuran tadpoles. (4) It is suggested that a similar mechanism is operative in the normal metamorphosis of echinids, the larval tissues dedifferentiating when the weight of the echinus rudiment causes the organism to sink away from the favorable food and oxygen conditions of the surface. (5) Some dedifferentiation phases are figured.