400 Ralph S. Lillie 



Piate 22. Arenicola cristata. 



Fig. 1 . Portions of three successive adult nephrìdia, showing general appear- 

 auce after exposure by removal of transverse muscles. X 2. 



Fig. 2. First adult nephridiura of left side, with iiephrostome turned upward 

 and spread open so as to show structure of dorsal and ventral lips. 

 X 7. 



Fig. 3. Second adult nephridium of left side, seen in its naturai position from 

 above; showing nephrostome, with slip of transverse muscie attached 

 to the anterior border of its dorsal lip; also position ofnephrostomial 

 blood-vessel and gonad. X 5- 



Fig. 4. Fourth nephridium of left side with nephrostome freed from its attach- 

 ment and turned outward so as to show the outer portion of the 

 ventral lip. X 5. 



Fig. 5. Portion of transverse section of wall of glandulär portion of nephri- 

 dium. Explanation in the text. X SUO. 



Fig. 6. Portion of transverse section of epithelial wall of the vesicular ventral 

 lip of nephrostome, showing the abrupt transition to the membranous 

 portion of the same. X 800. 



Fig. 6a. Longitudinal section of first nephridium of a larva of 5 — 6 mm. , cut 

 tangentially to body- wall, showing terminal vesicle, membrane con- 

 necting nephrostome to body-wall, and nephridial blood-vessel; also 

 structure of the excretory cells. Section passes below the nephro- 

 stome. X 800. 



Fig. 7. Sagittal section through a larva of the swarming stage (somite V 

 in process of formation), showing general characteristics of tissues at 

 this stage. Nerve cord differentiated in ventral ectoderm; septa 

 incomplete dorsally; early lumen of stomach indicated; mesoderm of 

 the extreme posterior region in procesa of differentiation. X 800. 



Fig. 8. Sagittal section through larva at beginning of crasvling stage (somite 

 VI in process of formation), showing early septa, growing zone, and 

 early proctodaeum. Absorption of yolk has progressed considerably 

 since stage of Fig. 7. X 800. 



Fig. 9. Horizontal section through posterior region of larva in which somite VII 

 is in process of formation, showing early differentiation ofmesodermal 

 striictures, septa, ventral blood-vessel, chloragogen cells, nephridia. 

 X 800. 



Piate 23. Arenicola cristata. 



Fig. 10. Oblique section through posterior region of larva in which somite IX 

 is in process of formation, showing large posterior mesoblast nucleus, 

 growing zone, early septa, early nephridial rudiments and ventral 

 blood-vessel. X 800. 



Fig. 11. Longitudinal section through body-wall of a larva in which somite VIII 

 is in process of formation, showing earlj^ pronephridia of somites IV 

 and 5. Goblet-coUs in ectoderm also shown. X 800. 



Fig. 12. Cross section through ventral portion of intersegmental region 2—3 

 of a larva in which somite VII is just appearing, showing large myoblast 

 cells ali similar to one another in appearance. X 800. 



