THE LIFE-HISTORY OF NUCULA DELPHTNODONTA. 339 



body in each of these forms is such as might be produced if 

 the body of Nucula were to grow and protrude to a corre- 

 sponding extent. In such a case the test of Nucula would 

 occupy a corresponding position to that occupied by the 

 velum in the other forms. 



Apical Plate. 



At an early period the cells of the apical plate cannot be 

 distinguished from those that form the test, but as develop- 

 ment proceeds they become marked off as a rather definite 

 plate at the anterior end of the embryo (figs. 11, 16, and 24, 

 ajp.). This plate is relatively large and thick, and extends 

 posteriorly as far as the stomach. The cells from which the 

 cerebral ganglia are formed lie ventral to it (figs. 15 and 24, 

 eg.). Beneath the test the cerebral ganglia and the apical 

 plate are in contact. 



The cells of the apical plate are evenly ciliated with short 

 cilia, like those borne by the test cells (fig. 16). In this 

 respect this species differs from both Nucula proxima and 

 Yoldia limatula. Both of these forms have long apical 

 cilia (Text-figs. G and H) that during life are bunched 

 together (Text-fig. F). Nucula pr ox i ma has an apical plate 

 that in extent may be compared to that of Nucula del- 

 phinodonta, but the apical plate of Yoldia is comparatively 

 very small. The short, diffuse cilia on the apical plate of 

 Nucula delphinodonta are probably the result of the 

 conditions that make active locomotion at this stage both 

 unnecessary and dangerous. (See what is said regarding this 

 under the head of Test.) Certainly most of the apical plate, 

 and probably all of it, is cast away when the test is shed (figs. 

 34 and 35). 



Shell. 



Some lime salts are deposited soon after the cuticle of the 

 shell begins to be secreted, which takes place about the time 

 that the lobes of the mantle begin to form (fig. 20). When 

 the test is shed (figs. 34 and 35), the shell- valves are white, 



