Cyclic Conditions and Rejuvenation in Hydroids

Abstract

SEVERAL colonies of Tubularia indivisa which have lived in the aquaria for three years are noticed as exhibiting alternating periods of activity and rest. Broadly speaking, the hydranths die off about midsummer and reappear about midwinter, the times in one particular case for the growth of new lengths of hydrocaulus and for the formation of new hydranths being December, 1919, January, 1921, and January, 1922, and in another January, 1920, January, 1921, and December, 1921, the hydranths in each case finally dying off in the intervening periods between the end of May and July. Colonies obtained from moderate depths in January generally show clearly marked new ends to their hydrocauli, such new growth often being an inch or more in length.