514 



the openings of the stomodseum and the intestine, and to the edges 

 of the apical plate. 



A corresponding stage in development is reached by the em- 

 bryos of N. proxima, in about 60 hours. They closely resemble 

 the embryos of Y. limatula in external appearance, but are much 

 smaller. They still swim rapidly but have periods of rest. Internally 

 they closely resemble the embryos of N. delphinodonta. 



At the stage indicated by these descriptions, a decided change 

 in the appearance and life of the embryos is brought about, by the 

 stripping off, and casting away of the tests. With Y. limatula this 

 takes but a very few minutes. The embryos settle to the bottom, the 

 apical cilia shrivel, and the test-cells break apart and, frequently, go 

 to pieces at once. 



^' 



m 







Fig. 19. 



Fig. 20. 



Fig. 19. Reconstruction of an embryo of Yoldia limatula, at a stage during casting. 

 Represented as seen from the right side, with the right shell-valve and mantle-lobe removed. 

 aa anterior adductor muscle, eg cerebral ganglion. / foot, g rudiment of gill, int in- 

 testine, ot otocyst. pa posterior adductor muscle, pg pedal ganglion, r pouch that leads 

 to the cerebral ganglia, rl right lobe of the digestive gland, std stomodasum. ( adhering 

 test-cells, vg visceral ganglion. 



Fig. 20. Transverse section of an embryo of Nucula delphinodonta, soon after the 

 test has been cast away. Taken through the stomach and the disorganized digestive 

 glands, m mantle, sto stomach, as cells of the disorganized digestive glands. 



Casting includes, beside the test, the stalk that extends from it 

 to the cerebral ganglia, the apical plate and its connection, and the 

 stomodreum, from the blastopore to the position of the definitive mouth. 

 These parts, together with a few adhering test-cells that have rounded 

 up, may not sever their connection with the body for a half hour or 

 more, Fig. 19, but they finally break just below the cerebral ganglia, 

 and casting is complete. 



