THE LIFE-HISTORY OF NUCULA DE LI'H mODONTA. 323 



fig. H). Sometimes part of a foartli band is present. The 

 end rows of test cells have the cilia evenly scattered over 

 their surfaces. The apical cilia are long and bunched into a 

 sort of whip that precedes the embryo when it swims. In 

 fact^ the embryo resembles that of Yoldia so closely that, 

 except for a difference in size and a slight difference in shape, 

 a description of the surface appearance and movements of 

 one will do very well for the other also. 



The cilia on the embryos of Nucula delphinodonta 

 may then bo regarded as arrested in their development. 

 Life in the protecting brood-sac makes active locomotion 

 unnecessary and even dangerous, inasmuch as active embryos 

 would be likely to find their way out of the brood-sac, and 

 so be exposed to outside dangers. 



The embryos continue to elongate and begin to flatten 

 slightly laterally (fig. 23). In the living embryo, viewed by 

 transmitted light, this stage is marked by the appearance of 

 a light spot near the dorsal margin. A smaller, much less 

 distinct light spot has been present near the ventral margin 

 for some time, and corresponds in position to the cavity that 

 was mentioned as appearing in the mesoderm, ventral to the 

 gut. This space has enlarged considerably (fig. 24), but is 

 covered laterally by rather thick walls of ectoderm and by 

 some mesoderm, so it is not very distinct. The dorsal space 

 is formed by the arching up and flattening out of the cells of 

 the shell-gland, which are now beginning to form the mantle 

 lobes (fig. 20). It is bounded dorsally, laterally, and poste- 

 riorly by the mantle, anteriorly by the mantle and the apical 

 plate, and ventrally by the gut and by the body-wall. A 

 few cells, apparently mesodermal, lie in this space, generally 

 attached to the mantle or to the gut. 



At a little later stage (fig. 25) two fibre-like cells stretch 

 from the anterior end of the gut posteriorly and dorsally to 

 the mantle. They are quite conspicuous in living embryos, 

 and they retain their position until after the test is thrown 

 away. 



About this stage the gut, which has grown posteriorly, 



