Cyclostomata. We owe to Barrois by far the fullest account of the development of the Cyclostomata, but how far his interpretations are to be trusted is very doubtful. The larvæ differ very considerably from the normal larvæ of the Chilostomata and Ctenostomata; the difference being mainly due to the enormous development of the ciliated disc. Barrois has investigated the larvæ of three genera, Phalangella, Crisia, and Diastopora, and states that they very closely resemble each other. The ovum is extremely minute. The segmentation, so far as it has been made out, is regular. During the segmentation growth is very rapid, and eventually there is formed a blastosphere many times larger than the original ovum. The blastosphere becomes flattened, and is converted into a gastrula by bending up into a cup-like form. The gastrula opening is stated to remain as the permanent mouth, which has a terminal and central position. A transverse ring-like thickening is formed round the larva, which probably corresponds with the ciliated ring of previous larvæ; and the body of the larva in front of this ring becomes ciliated. The aboral end of the larva becomes thickened, and grows out into an elongated prominence, which probably corresponds to the ciliated disc. The ring before mentioned becomes at the same time more prominent, and forms a cylindrical sheath for the ciliated disc. At the time when the larva becomes liberated from the maternal cell it has the form of a barrel with a slight constriction in the middle separating the oral from the aboral end. At the centre of the oral face is situated the mouth, leading into a wide stomach, while the aboral end is formed of the ciliated disc enclosed in its sheath. The whole surface is now ciliated. No structure equivalent to the dorsal organ or bud is described by Barrois, but in other respects, if the ciliated disc is really equivalent in the two forms, a general comparison on the line indicated above between this larva and the normal larvæ of the Ctenostomata and Chilostomata seems quite possible. The fixation and subsequent development of the larva take place in the normal manner. Phylactolæmata. The development of the phylactolæmatous Polyzoa has been studied by Metschnikoff (No. 315), who describes the eggs as undergoing a complete segmentation within a peculiar brood-pouch developed from the walls of the body of the parent. After segmentation the cells of the embryo arrange themselves in two layers round a central cavity. The embryo then forms the well-known cyst, from which a colony is formed by a process of budding. _General considerations on the Larvæ of the Polyzoa._