j^gQ John H. Gerould, 



Fig. 61. Optical section of yolk membrane of an unfertilized egg. 



Fig. 62. Trochophore forty-eight hours old, showing circlet of long 

 preoral cilia, proctodaeum, etc. 



Fig. 63. Longitudinal section through the proctodaeum of a larva, 

 immediately after metamorphosis, showing a slight invagination. 



Fig. 64. Trochophore of the same stage that is shown in Fig. 62, 

 viewed from the side, and slightly compressed to show the coelom, in 

 which yolk granules have already appeared. The dorsal pair of retractors 

 have begun to act, and cause a twitching of the body wall at the point 

 indicated by the arrow. The larval cuticula is present beneath the yolk 

 membrane. 



Fig. 65. Side view of larva immediately after metamorphosis and sisty 

 hours after fertilization of the egg. The specimen was somewhat com- 

 pressed. The future introvert lies in front of the arrow. Partial intro- 

 version now takes place. 



Fig. 66. Side view of a larva, five days old. 



Plate 9. 



The figures are all of Pli. gonklii. 



Fig. 67. Parasagittal section of an embryo during metamorphosis. 

 Masses of yolk granules are breaking away from the degenerating proto- 

 troch cells and are passing backward into the newly formed coelom. The 

 dorsal and ventral retractors are represented in a state of partial retraction. 

 280 : 1. 



Fig. 68. Surface view of a slightly older stage than that shown in 

 Fig. &1 (forty-three hours old), showing the casting off of the yolk mem- 

 brane. 280:1. 



Fig. 69. Trochophore of thirty-six hours. This figure has the same 

 magnification as Fig. 70, 72, 73, 76 and 77, and is introduced to show 

 the relative size of the trochophore and larva. 230 : 1. 



Fig. 70. Larva, six and a half days. The yolk granules and other 

 corpuscles that are found in the coelom have been almost entirely omitted 

 in the drawing, which was made from a preparation. 230 : 1. 



Fig. 71. Larva, nine days. 280: 1. 



Fig. 72. Larva, seventeen days. Yellow excretory cells begm to 

 appear on the inner surface of the body wall. 230 : 1. 



Fig. 73. Larva, twenty days. 230 : 1. 



Fig. 74. Yellow excretory cells, which project into the coelom from 

 the inner surface of the wall of the body. Areas of adhesion to the body 

 wall are indicated by*. 680 : 1. 



• Fig. 75. Optical section of the nephridium of a larva of twenty-four 

 days. The ciliated nephrostome is shown. No nephridiopore could be 

 detected. Area of attachment is at*. 680 : 1. 



Fig. 76. Larva, thirty days. 230:1. 



