POLYZOA. 253 



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 indi- 

 cated above between this larva and the normal larvae of the Ctenostoniata 

 and Chilostomata seems quite possible. The fixation and subsequent de- 

 velopment of the larva take place in the normal manner. 



Phylactolaemata. The development of the phylactolsematous 

 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 Larvae of the Polyzoa. 



The different forms of embryo amongst the Polyzoa are repre- 

 sented 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 larva? 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 oral face is 

 situated in all cases the mouth, and in the entoproctous larvae and 

 Cyphonautes also the anus. It thus appears that Cyphonautes, 

 though the larva of an ectoproctous form, is itself entoproctous 

 a fact which tends to shew that the Entoprocta are the more 

 primitive forms. In all the larvae, except possibly those of the 

 Cyclostomata, there is present on the anterior side of the mouth, 

 in the Ectoprocta on the oral, and in the Entoprocta on the 

 aboral side of the ciliated ring, an organ, to which is attached 

 externally a plume of long cilia. This organ has been identified 

 throughout the series in accordance with 

 Hatschek's view as the dorsal organ or 

 rudimentary bud ; but it is well to bear 

 in mind that this identification is of a 

 purely hypothetical character. 



On the aboral side of the ciliated ring 

 there is present in all the larva? an 

 organ, which has been called the ciliated 

 disc, which is probably homologous FlG- 133 A- DlAGRAM OF AN 

 throughout the series. It perhaps re- IDEAL LARVA OF A POLYZOON. 

 mains in the adult of Loxosoma as the m. mouth; an anus; st. sto- 

 cement gland, but not in other forms. mach ; cUiated disc - 



The Polyzoa present a simple and almost certainly degraded 

 organisation in the adult state ; it is therefore more than usually 

 necessary to turn to their larva? for the elucidation of their affinities 



