1. Sabellaria alveolata Linnæus has been reared for the first time from the egg through the pelagic stages and metamorphosis to early bottom stages. The chief features in the development of the external characters are described and special attention is drawn to the existence of peculiar grasping-cilia on the telotroch. The manner in which the young worm builds its first tube of sand grains is also described.

2. Sabellaria spinulosa Leuckart has similarly been reared for the first time from the egg to early bottom stages. Its development is very similar to that of the preceding species, but during the later pelagic life the two can easily be separated, as the primary palese of the crown, which appear among the long provisional bristles of the first pair of parapodia, show a marked specific difference. The early stages cannot be distinguished.

3. A larva which is probably that of the rare Pallasia murata Allen is described.

4. A tentative key to the British Sabellarian Larvaæ is drawn up.

5. The observations of previous workers are discussed. It is shown that Sabellarian larvaæ have frequently been described as the larvaæ of Spio or of Polydora, in spite of the fact that the early stages of Sabellaria had been obtained more than once from artificial fertilisations. Caullery was the first to point out the error, and he is fully confirmed in this paper.

6. It is shown that the caudal appendage of a full-grown Sabellaria alveolata possesses internally a series of fifty or sixty transverse septa.