The morphological development of the branchial crown and uncinigerous tori, growth, and demographic structure were observed in juveniles of Owenia fusiformis Delle Chiaje, 1841 from the Bay of Seine (eastern English Channel). Growth becomes significant only after one and a half months of benthic life; individual growth is highly variable and results in a progressive widening of the size distribution of the population over time. From six initial branches present at the post-larval stage, seven stages of branchial development can be distinguished. The branchial crown develops by successive dichotomous divisions and growth of the new branches. The number of dichotomous divisions (D) increases with total length (LT) and age (A): D = 1.718 ln(LT) − 0.291; D = 0.987 ln(A) − 1.471. The number of uncinigerous tori (T) increases with total length (LT) and age (A): T = 4.272 ln(LT) + 7.272; T = 4.477 ln(A) − 2.479. The acquisition of fully grown characters after metamorphosis was progressive and depended more on total length of the juvenile than on age. At any given age, there was an important variability in investigated characters because of differential growth of juveniles. Ecological consequences resulting from interactions between morphological development and growth with respect to recruitment are discussed.