THE EMBRYOLOGY OF THE PORIFERA. 217 



The larva is, therefore, to be considered as a two-layered 

 organism from an embryological point of view. 



The layer of flagellated cells consists, according to both 

 authors, of excessively attenuated, columnar cells, each 

 bearing a flagellum. The body of the cell is much thinner 

 than the nucleus, so that for the sake of close packing the 

 nuclei of this layer are obliged to lie at different levels, 

 giving at first sight the appearance of several strata of 

 cells, where in reality there is only a single layer. The 

 nuclei are placed rather far from the outer surface, so that 

 the entire layer of flagellated cells presents the appearance 

 of an inner zone made up of closely -packed nuclei, four 

 or more layers deep, and an outer clear, finely striated 

 zone, the striations corresponding to the outlines of the 

 attenuated cells. Only in Spongilla is the appearance 

 somewhat different, since here the flagellated cells are not 

 so attenuated, being of about the same thickness as their 

 nuclei, so that the latter form a single stratum. 



The accounts of Maas and Yves Delage differ also some- 

 what as regards the relation of the flagellated layer to the 

 inner cell mass. According to the former author the fla- 

 gellated cells form a more or less uniform layer covering 

 the inner mass either completely or partially. The larva 

 of Axinella is remarkable for the occurrence of peculiar 

 gland-like cells placed at intervals between the flagellated 

 cells, which strongly resemble in their characters the cells 

 of the inner mass ; Esperia also has peculiar cells, perhaps 

 glandular, at the anterior end, external to the flagellated 

 cells. According to Delage, on the other hand, the 

 " cellules epidermiques " are entirely internal to the flagel- 

 lated cells in Spongilla, and nowhere exposed on the 

 surface ; their situation is similar in Aplysilla, except that 

 they are exposed at the anterior end, where the inner mass 

 protrudes ; but in Esperia and Re?iiera, on the contrary, 

 they are mixed with the flagellated cells, "and might just 

 as well be said to be external, were it not that they separate 

 a little from one another to permit the necks, with the 

 flagella, of the ciliated cells to pass out between them," 

 while at the hinder end they are exposed and form part 



