The Life-Cycle of the PolychaeteNereis Virens

Abstract

Adult males ofNereis virens(Sars) spawn while swarming in mid-water and show epitokous changes which include enlarged parapodia, natatory chaetae and histolysis of the body wall. Females spawn while in their burrows and do not show such changes. An artificial rise in temperature can induce premature spawning. Seminal fluid is ejected through pygidial pores, but no such pores occur in the female. Both lunar period and temperature rise seem to be involved in initiating spawning, which generally occurs in early May in the Thames populations. Gravid worms show a peculiar method of swimming in which the anterior region is held rigid. A pair of flagellar vanes and other features characterize the fine structure of the spermatozoon. The mature oocytes bear microvilli, and some express a jelly-like layer after fertilization which traps excess spermatozoa. Many unit-membraned vesicles, of uncertain function, occur in the periphery of the nucleus of the mature oocyte. At fertilization the acrosomal contents are discharged on to the surface of the oocyte. The structure and behaviour of the monotrochophore, trochophore and nectochaete larvae, and the details of the nature and rate of development of such structures as the parapodia, cirri, palps and jaws, differ in some ways from the known larval development of certain other nereids. A very brief planktonic phase occurs in larval life, and young worms migrate from the sublittoral to the littoral regions.