THE LIFE-HISTORY OF NUCTTLA DELPHINODONTA. 337 



The ciliated embryos of Nucula delphinodonta^ unlike 

 those of the other two forms^ are not able to swim freely in 

 the water. At the most they are barely able to move on the 

 surface of a glass dish. This is probably the result of their 

 being carried in a protecting brood-sac. It seems but 

 natural that the bands of strong cilia and the apical tuft of 

 cilia would not be developed by embryos such as these^ 

 were there no need for active locomotion, and where active 

 locomotion would be dangerous. It is for the best interest 

 of embryos that they remain in the brood-sacs, where they 

 are protected from many enemies. Were they capable of 

 active movement, many would probably escape and perish. 

 In the two related forms, Nucula proxima and Yoldia 

 limatula, the embryos have to depend on their own 

 activities for their existence. 



It is highly probable that the embryos of the ancestors of 

 Nucula delphinodonta led an active, free-swimming 

 existence. The rearing of embryos in protecting brood-sacs 

 is very possibly connected with the present life of the animal 

 beneath the surface of the mud, and, in any case, has prob- 

 ably been acquired at a comparatively recent day. Again, 

 the test in its present condition is of no appreciable value to 

 the embryo, and no doubt is to be regarded as a vestige of a 

 once functional organ. 



Young embryos of Nucula delphinodonta when taken 

 from the brood-sacs do not live well, and it is accordingly 

 difl&cult to determine how long the test is retained. As near 

 as could be judged, it seems to be retained about two weeks. 

 Its cells then begin to break apart near the posterior end of 

 the embryo, and many of them move toward the anterior 

 end, where they remain attached to the apical plate and the 

 stomodseum (fig. 34). Sometimes most of the cells of the test 

 seem to thus accumulate at the anterior end, but they fre- 

 quently become detached and go to pieces before reaching 

 this position. In any case they, together with the apical 

 plate, and the stomodaeum, to the position of the future 

 mouth, are finally thrown away (fig. 35). In many cases the 



