508 



cilia are closely bunched, Fig. 9 ac, lashed vigorously from side to 

 side, and always precede the embryos in their movements. 



Fig. 11. Median sagittal section of a 36 hour embryo of Toldia limatula. ap apical 

 plate, bl blastopore, eg cells from which the cerebral ganglia are formed, mg mid-gut. 

 ag shell-gland, std stomodseum. t test. 



The embryos of N. delphinodonta remain in the egg cases and 

 are comparatively inactive. 



At a little later stage, the embryos are found to be provided 

 with new ectoderms, which lie just inside the tests. The ectoderms 

 are probably formed from cells that have wandered in from the sur- 



Fig. 12. Median sagittal section of a 25 hour embryo of Nucula proxima. ap apical 

 plate, mg mid-gut. sg shell-gland, std stomodseum. t test. 



