The Development of Nereis pelagica Linnæus

Abstract

Larvæ of Nereis pelagica Linnæus were reared from the egg, and the young worms, which developed from them, to the age of one year.The larvæ from an early stage possessed three chætigerous segments. At first after hatching they swam strongly by means of a prototroch, an akrotroch and three paratrochs, but as the cilia gradually disappeared they crawled more and more, finally abandoning swimming altogether.The head developed a pair of tentacles anteriorly and a pair of palps ventrally. A pair of tentacular cirri arose posterior to the prototroch and a little later a second pair ventral to them.The first pair of parapodia lost their bristles, and a lobe which had grown out between the noto- and neuropodial chæta-sacs became a third and posterior pair of tentacular cirri. Much later a fourth pair developed ventral to them.About the time when the larvæ ceased to swim (18.days old) a fourth pair of larval parapodia (third adult pair) appeared, to be followed by a fifth, sixth, etc. At sixty-five days old worms had about twentyone pairs of parapodia, at one year they had sixty and were then about 1-5 cm. long.During larval development a constant succession of bristles was seen. In the earliest stages the bristles were mainly homogomph spinigers with long appendices; later these fell out and were replaced by homogomph and heterogomph falcigers with shorter appendices. Later still these were replaced by bristles of the adult type.It is pointed out that these larvæ differed from those which Herpin reared in that they hatched earlier and instead of crawling as soon as liberated they swam for several days by means of their ciliated girdles.