246 WILSON. 
DESCRIPTION OF PLATE IV. 
FIGs. 20, 21, 22, 23. Series of long. sec. from a stage somewhat younger than 
Fig. 18. Fig. 20 is through the first pair of mesenteries and filaments; Fig. 23 is 
through the line a, 4 in Fig. 18; and the other sections fall between. C, 4. 
Fic. 24. Long. sec. through a larva such as Fig. 19; on the right, through one of 
second pair of filaments, on the left through an intermesenteric chamber. D, 4. 
Fic. 25. From a larva with two long and two very short filaments, a, 4, c, are 
three half-sections from the same side of the larva. 4 is through one of second pair 
of mesenteries with its filament; @ and 4 are on opposite sides of this mesentery. D, 4. 
Fic, 26. Trans. sec. through one of primary pair of filaments and mesenteries of 
a stage like Fig. 19. F, 4. 
Fics. 27, 28, 29. Series of trans. sec. from a larva like Fig. 19. Fig. 27 is through 
the cesophagus, 28 is just above the lip, and 29 is below the cesophagus. The sec- 
tions are very slightly oblique. The third pair of mesenteries exist as longitudinal 
ridges of the supporting lamella. .Z. is the reflected ectoderm. D, 4. 
