POLYZOA. 305 



and states that they very closely resemble each other. The ovum is ex- 

 tremely 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 larva? ; 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 larvte of the Ctenostomata and Chilostomata seems quite possible. 

 The fixation and subsequent development of the larva take place in the 

 normal manner. 



Phylactolaemata. The development of the phylactolaama- 

 tous Polyzoa has been studied by Metschnikoff (No. 815), 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 Larva of the Polyzoa. 



The different forms of embryo amongst the Polyzoa are 

 represented in figs. 130 B, 131, 132, and 133 in what I regard as 

 identical positions, and fig. 133 A is a figure of what may be 

 regarded as an idealized larval Polyzoon. In all the larvae there 

 is present a ciliated ring, which separates an oral from an aboral 

 face, and is apparently homologous throughout the series. In 

 the adult it is probably represented by the lophophore. On the 



B. II 2O 



