656 HAROLD HEATH, The Development of Ischnochiton. 



Fig. 48. Embryo of about the age shown in Fig 49, showing the 

 position of the cerebral ganglia and apical sense organ. 



Plate 35. 



Fig. 49. Side view of well advanced embryo, showing migration 

 of the mouth, position of the growth zone (gs) and the cerebral ganglia. 

 The groove extending from the mouth to the prototroch indicates the 

 position of the second quartette (sq). 



Fig. 50. Same embryo as Fig. 49. Anterior view. The grooves 

 indicate the position of the second quartette while the intervening areas 

 are third quartette products. 



Fig. 51. Later stage seen from ventral side. The mouth has 

 reached the prototroch and the foot is distinctly outlined. 



Fig. 52. Embryo immediately before escaping from the chorion 

 seen from the side. In the region of the shell large cells (mucous ?) 

 alternate with the very narrow bands of cells which ultimately secrete 

 the calcareous salts. The shell has just commenced to advance onto 

 the head vesicle. Mucous cells occur in the head vesicle and various 

 parts of the body. 



Fig. 53. Embryo 2 days after hatching, dorsal side, showing 

 shape of head vesicle and the increased area of the lime secreting cells 

 which alternate with the mucous cells. The velum has been cast off. 



Fig. 54. Free-swimming larva. The shell has advanced onto the 

 head vesicle to a greater extent than in Fig. 52. 



Fig. 55. Embryo about 10 days after hatching. The head vesicle 

 is flattened preparatory to forming the proboscis. Tegmental sense 

 organs (aesthetes) appear in the shell. 



Fig. 56. Later stage (13 days) showing later stages in the meta- 

 morphosis. The outlines of the proboscis now appear. 



Fig. 57. End of 14. day, showing fusion of three portions of 

 the shell. 



Fig. 58. Ventral view of embryo of about the stage shown in 

 Fig. 56. The mouth lies posterior to its final position. Opening of 

 foot-gland occupies anterior border of the foot. 



Fig. 59. Embryo 15 days after hatching. The mouth occupies its 

 permanent position and the metamorphosis is complete. 



Fig. 60. Dorsal view of embryo represented in Fig. 59. 



