ON THE DEVELOPMENT OF NEBALIA. 433 



PLATE 21. 



Fig. 56. — Transverse section througli segments I and II in Stage F, 

 showing shell flap, optic and antennulary ganglia, central mass of nervous 

 tissue and labrum. X 238. 



Fig. 57. — Transverse section just behind that shown in Fig. 50, showing 

 divisions of the central mass of nervous tissue, the stomodseum, and the first 

 appearance of jointing in the autennules, x 238. 



Fig. 58. — Transverse section through Stage F, behind that shown in 

 Fig. 57. X 238. 



Fig. 59. — Transverse section through Stage F, behind that shown in 

 Fig. 58. X 238. 



Fig. go. — Transverse section through Stage F, behind that shown in 

 Fig. 59. This figure shows the cuticle. X 238. 



Fig. 61. — Transverse section through stomodaeal region in Stage F. 

 X 288. 



Fig. G2. —Transverse section through ventral side of Stage F, between 

 segments VI and VII, showing rudimentary muscles, x 238. 



Figs. 63, 64. — Transverse sections through the first tlioracic segment in 

 Stage F, showing the ventro-lateral out-piishings of the mesenteron, genital 

 cells, and shell thickenings. Fig. G3 shows the first thoracic ganglion at its 

 largest part, x 238. 



