THE Lin: -HI STORY OF NUCULA DELPHINODONTA. 391 



EiG. 53.— A pair of adult gill plates. The suspensory merabiaue, the 

 continuous cliitinous trough, the longitudinal muscle, and the walls of the 

 plates that join the plates next in succession have all been cut across in 

 removing the plates from the gill. (Drawn from a study of sections.) 

 X 250." 



PLATE 25. 



Figs. 54, 55, and 56.— Stages in the development of the labial palps. The 

 palps have been carefully drawn, but for the sake of clearness they have in 

 each case been represented with the outer palp on the right side turned away 

 from the corresponding inner palp. The foot is represented as cut off, and the 

 specimen is turned so that the mouth can be seen between the palps. X 125. 



Fig. 57.— The posterior portions of the right outer and inner palps of an 

 adult specimen. The two palps are represented as spread apart and placed in 

 a position that corresponds with Fig. 56. X 65. 



Figs. 58, 59, and GO.— Successive sections of the labial palps of a specimen 

 that has six pairs of gill jjlates. The sections are taken transverse to the 

 embryo. The stage is much the same as is re!)resented by Fig. 5t. Fig. 

 58 is near the mouth, Fig. 59 is near the posterior end of the outer palp, and 

 Fig. 60 is posterior to the outer palp. X 150. 



Fig. 61.— Transverse section of an embryo with four pairs of gill plates 

 (see Fig. 45, Plate 24) taken through the mouth, x 20U. 



Fig, 62.— Transverse section of an embryo with four pairs of gill plates, 

 taken just anterior to the stomach. X 200. 



Fig. 63.— Sagittal section of the antero-dorsal portion of an embryo that 

 has eight pairs of gill plates. X 150. 



Fig. 64. — Horizontal section of the foot of an embryo that has six pairs of 

 gill plates, taken just veutral to the mouth. X 150. 



Fig. 65.— Horizontal section of tiie foot of an embryo that has six pairs of 

 gill plates, taken through the mouth, x 150. 



Fig. 66. — Transverse section of the palp appendage of an adult specimen. 

 X 200. 



Fig. 67.— a nearly transverse section of an embryo that has five pairs of 

 gill plates, taken through the heart. X 200. 



Fig. 68.— A diagonal section of an embryo that has nine pairs of gill plates, 

 taken through the heart. X 90. 



Fig. 69.— a diagonal section of an adult specimen, taken through the heart. 

 X 45. 



