262 EMBRYOLOGY OF TEREBRATULINA. 



The expression of mine which occurs above, that the embryology of Lingula when 

 known would agree with that of Discina, was in type long before I became aware of Mr. 

 MCrady's discovery. 



The absence of setas in the embryo, with the opacity of the valves, is an evidence that it 

 was considerably advanced, for Muller described some of the larva observed by him as hav- 

 ing lost the larger bristles. 



In closing this brief memoir I wish to express my thanks to Mr. Richard Rathbun, who 

 accompanied me to Eastport, and who assisted me in securing materials for study. 



To Mr. Edward Burgess I am again under many obligations during the preparation of 

 the paper. 



My thanks are also due to Mr. Chas. A. Walker, who has with great fidelity and patience 

 reproduced accurately my drawings upon steel. 



EXPLANATION OF PLATES. 

 PLATE VIII. 



Fig. 1. A cluster of eggs from the genital band. 



Fig. 2. A single egg encased in its capsule. 



Figs. 3, 4, 5. Eggs from the peri visceral cavity immediately after their escape from the pallij.1 sinuses. 



Fig. 6. Highly magnified portion of the eggshell. 



Figs. 7, 8, 9, 10 11. Eggs in various stages of segmentation. 



Fig. 12. First ciliated stage. 



Figs. 13, 14, 15, 16, 17. Successive stages of transverse division of embryo, showing long tuft of cilia at 

 cephalic extremity. 



Figs. 18, 19, 20, 21. From a single embryo, showing various outlines assumed while swimming. 



Figs. 22, 23, 24. Different views of another embryo. 



Figs. 25, 26, 27. Different embryos showing first appearance of caudal or peduncular segment. 



Figs. 28 to 36. Different embryos at the stage when the caudal segment becomes conspicuous. 



Figs. 29, 30. Representing the same embryo contracted and expanded. 



Figs. 37, 38, 39, 40, 41. Embryos just attaching themselves by their caudal segment. 



Figs. 42, 43. Drawn from the same embryo : the first showing the embryo stretched to its utmost, the sec- 

 ond the same contracted. (These motions would often follow each other rapidly.) 



Figs. 44, 45. Embryos showing first appearance of ventral? area by the bulging of the thoracic ring. 



Figs. 46 to 53. Embryos in various stages showing widening of thoracic ring, and its gradual growth 

 toward enclosing the cephalic ring. 



Figs. 54 to 61. Successive stages of the embryo showing formation of dorsal and ventral areas by the 

 folding and growth of the thoracic ring. In Fig. 61 the head is still seen projecting from the dorsal and ven- 

 tral folds of the thoracic ring. 



Figs. 62 to 77. Succeeding stages of embryos drawn in various positions. The deciduous setae appear in 

 these stages. 



Fig. 68. Dorsal and ventral plates of embryo, the contents having been removed by Parama3cia. 



NOTE. The cilia constantly clothing the embryo have been purposely omitted except in a few cases, to 

 save trouble in engraving. On the first appearance of the dorsal and ventral folds of the thoracic ring, the 

 cilia disappear from that region. Figs. 18 to 77 were drawn with a -fa objective of Smith and Beck ; the 

 embryos measure about the T J^ inch in diameter. 



