336 OILMAN A. DREW. 



small at this stage, consist of a very few cells, and can hardly 

 be distinguished from the surrounding cells. The apical 

 cells acquire cilia about the time that the test cells do (figs. 9 

 and 11), and for some time they cannot be distinguished from 

 them. Later the apical plate may be told by its shape and 

 position (figs. 15 and 24, ap.). 



As development proceeds the test begins to close in over 

 the shell-gland from the sides and anterior end (figs. 10 — 13). 

 Five rows of test cells can now be seen under favourable 

 conditions, but their outlines are very hard to determine. 

 Until the shell-gland is covered, two or three of the posterior 

 rows are incomplete dorsally. A small pore is left near the 

 posterior end, separated from the blastopore by the width of 

 one test cell (fig. 15). The anus comes to lie near this open- 

 ing (tig. 24). 



The five rows of cells are now arranged much as shown in 

 Text-fig. E. From the formation of the test until its ultimate 

 disappearance its cells are evenly ciliated with short cilia. 

 In this respect the embryos differ from those of Yoldia 

 limatula (Text-fig. F) and Nucula proxima (Text-fig. H). 

 Both of these forms have the cilia on each of the three inter- 

 mediate rows of test cells collected into a band. Sometimes 

 a fourth more or less complete band is present. The cilia on 

 the end rows of the test cells of all of the forms are short 

 and evenly scattered over the surfaces of the cells. 



In this connection it is of interest to observe that the cilia 

 on the apical plate of Nucula delphinodonta are short 

 and independent, while those on the apical plates of Yoldia 

 limatula and Nucula proxima are long and buached 

 together. They all seem to have a rather scattering origin, 

 and when animals are killed the cilia become separated from 

 one another. 



In both species of Nucula the embryo differs from that of 

 Yoldia limatula in having a posterior opening in the test, 

 dorsal to the blastopore (fig. 15, and Text-figs. G and H). 

 This difference might easily be accounted for by a slight dif- 

 ference in the closing in of the test over the shell-gland. 



