507 



Some of the surface cells seem to crowd into the interior, and 

 others remain at the surface, become vacuolated, and arrange them- 

 selves into rows that surround the embryos. Each embryo seems to 

 be surrounded by five of these rows of surface cells. It will be con- 

 venient to refer to these cells hereafter as test-cells. The test-cells 

 of both species of Nucula are not easily distinguished from each 

 other, and it is possible that those most anterior do not form com- 

 plete rows. 



The test-cells of Yoldia are distinctly rounded, and the cilia on 

 each of the three intermediate rows are collected into a band. The 

 embryos of N. proxima also have three, and frequently part of a 

 fourth band of cilia. Three of the bands correspond, in position, to 

 the bands on the embryos of Yoldia. It has not been determined 

 whether the partial fourth band, when present, is borne on the anterior 

 row of test-cells or on interpolated 

 cells. The embryos of N. delphin- 

 odonta remain evenly ciliated, 

 Fig. 10. 



Certain groups of cells, beside 

 the test-cells, lie at the surface. 

 One of these groups lies at the 

 anterior end of each embryo and 

 carries the apical cilia. The apical 

 cilia of Y. limatula, Fig. 9 ac, and 

 of N. proxima are very long. Those 

 of _N. delphinodonta, Fig. 10, are 



Fig. 10. Surface view of a young 

 embryo of Nucula delphinodonta. 



short and resemble the cilia borne by the test-cells. Beside the cells 

 of the apical plate Y. limatula has a group from which the cerebral 

 ganglia are formed. These cells are connected with the cells of the 

 apical plate, but communicate with the surface, on what may be 

 distinguished as the ventral side, between the first and second rows 

 of test-cells, Fig. 11 eg. 



In both species of Nucula, the cells from which the cerebral 

 ganglia are found, are not distinguishable from the cells that form 

 the apical plate, Figs. 12 and 18. 



At this stage the embryos of Y. limatula and N. proxima swim 

 in more or less definite lines, rotating the while upon their longitu- 

 dinal axes. Rotation is quite rapid but is not always in the same 

 direction, there being frequent changes. During activity, the apical 



