466 E. M. PACE. 



between the cells of the two glandular systems^ the other 

 two passin^g to the cells of the ciliated crown. 



The ciliated crown (the " couronne '' of French 

 authors), first visible at this stage, does not consist of a 

 single ring of cells as it is has been described in other 

 Bryozoan larvae, and as figured by Barrois (2) for Flustrella 

 hispida, but of two or three rows of cells, as is shown in 

 PI. 25, figs. 65 a-c {€.). All trace of definite cell walls in the 

 ectoderm has vanished by this time, but certaiuly the ciliated 

 crown contains at least three series of nuclei, corresponding 

 presumably to originally three rows of cells. 



The cells from which the cilia originate lie rather below 

 the aboral groove, and can at will be retracted within the 

 valves of the shell (PI. 25, fig. 6Q). Later on the ciliated 

 crown certainly does have the appearance described by 

 Prouho (20), of a single series of flat discoidal cells with long 

 vibratile cilia, each imbedded in a cuticle, and prolonged below 

 this into a triangular mass of protoplasm (PL 25, fig. 66) ; 

 each of these cells contains a single large nucleus : this 

 appearance does not, however, arise uutil quite late in larval 

 life. No traces of the ciliated crown, or of any specially 

 enlarged cells, are visible before the larva escapes from the 

 vitelline membrane, despite Barrois' assertion to the con- 

 trary. It is evident, therefore, that in Flustrella the 

 ciliated crown is formed by a series of cells, and that it 

 is only late in larval life that these unite to form a single 

 row of large cells. 



The chitinous bivalve shell is developed as a secretion of 

 the aboral ectoderm, and is closely adherent to the latter. 



The aboral groove, which is now strongly developed, 

 occupies its original position above the ciliated crown (PI. 25, 

 figs. 65 a-c, M.G.). It is best seen in transverse section. 



The stomach, as a result of the growth of the larva, has 

 become much more elongated, and, owing partly to this, 

 partly to the absorption of food material, the yolk spherules 

 surrounding the stomach have become much reduced in 

 number. 



