1. Larvae of Branchiomma vesiculosum Montagu were obtained from artificial fertilizations, and reared through metamorphosis and for some weeks afterwards.
2. The larvae are extremely yolky and do not feed until after metamorphosis. They swim by means of a broad prototroch, and are provided with a pair of cup-shaped eye-spots, a large head vesicle, and two anal vesicles. There is a broad neurotroch but no telotroch. The mouth is open but the anus is closed. In the last swimming stages rudiments of the adult branchial apparatus appear as a pair of lobed swellings, behind the eyes but in front of the prototroch. Behind the latter the collar rudiments appear. There are usually three or four chaetigerous segments marked out when metamorphosis sets in.
3. The metamorphosing larva settles on the bottom, and secretes for itself a tube of mucus. The prototroch and larval tissues (episphere) of the head clump together to form a large snout-like structure. This gradually breaks up into small pieces which are thrown off one by one as they are formed. With the loss of the prototroch and other larval head tissues the adult part of the head becomes joined on to the trunk. At the same time the branchial rudiments branch to form pinnules, which are directed forwards so that their bases overlap the eyes. The anus opens, and the young worm begins to feed. Metamorphosis occupies about four days.
4. The larval stages and the metamorphosis have been studied in histological, and to a limited extent in cytological, detail. Apart from the curious metamorphosis, which to some extent recalls that of Owenia and Polygordius, the development shows no striking features.
5. After metamorphosis the worm elongates by the addition of setigers in front of the pygidium. These are at first of the thoracic type (dorsal bristles, ventral uncini), but when the ninth and succeeding setigers appear they show abdominal constitution (dorsal uncini, ventral bristles) from the first. Thus no setigers change over from abdominal to thoracic constitution as they do in the development of young Serpulid worms.
6. Coincident with the loss of the neurotroch at metamorphosis the mid-dorsal line of the head and trunk of the young worm becomes ciliated to form the faecal groove of the adult. This strip of cilia continues on the ventral surfaces of the ninth and succeeding setigers, after passing round the right side between the eighth and ninth bristle segments. For a time the intersegmental groove between these segments is ciliated on the left side as well.
7. The branchial rudiments, which began to branch before metamorphosis, continue steadily to branch and grow afterwards. The pinnules are supported by an internal skeleton of thick-walled cells to the base of which the main dorsal and ventral longitudinal muscles of the body become attached. The most dorsal branch of each branchial rudiment, lying close to the mid-dorsal line, forms one of the so-called palps of the adult and is not supported by an internal skeleton.
8. The manner in which the young worms build their first sandy tube is described.
9. Early larvae of Sabella pavonina (Savigny) are described. They closely resemble those of Branchiomma.
10. The present position of embryological knowledge concerning the Polychaetes is very briefly summarized. It is shown that Sabellid larvae are more closely related to Serpulids than to those of any other family.