THE LIFE-HISTOKY OF NUCULA DELFfllNODONTA. 341 



animal matter that they retain their forms after the lime salts 

 have been dissolved away. The material is quite tough^ and 

 frequently causes much trouble in cutting series of sections. 



Each valve of the shell of fully adult animals has from ten 

 to twelve teeth in the series anterior to the cartilage pit and 

 five or six posterior to it (figs. 50 and 51). All of the teeth 

 are more or less conical, pointed, curved toward the dorsal 

 margin of the shell, and distinctly grooved on the side away 

 from the cartilage pit. Each series of teeth forms a ridge 

 some distance from the dorsal margin of the shell, which 

 disappears dorsal to the adductor muscle-scar. The teeth of 

 the two valves interlock so completely that it is frequently 

 quite impossible to separate the valves without breaking 

 some of them. The cartilage pit is large and deep. The 

 adductor muscle-scars and pallial lines are faintly marked. 



Mantle. 



The shell-gland is formed early. About the time that the 

 gut is formed it consists of a number of large cells that lie 

 near the blastopore, on what may be distinguished as the 

 dorsal side of the embryo. Its cells do not seem to bear 

 cilia, but only preserved material was at hand for the deter- 

 mination of this point. The surrounding ciliated cells, those 

 that form the test, begin to grow over the shell-gland from 

 the sides and anterior end (figs. 10, 11, 12, and 13, sg.). At 

 the same time the shell-gland flattens slightly, and the cells 

 along its margins push up and form a slight ridge, that keeps 

 the surface of the shell-gland separated from the overgrowing 

 test. Soon after the shell-gland is covered by the test, it 

 arches dorsally, and the two come to lie close together (figs. 

 17 and 18, sg.). As the embryo flattens laterally the shell- 

 gland arches dorsally still more (fig. 20), and a space appears 

 between it and the intestine. This space seems to be formed 

 by the multiplication and flattening of the cells of the shell- 

 gland, which arches dorsally and becomes separated from the 

 intestine. Lateral folds (fig. 20, m.), the beginnings of the 



