BY PROFESSOR W. A. HASWELL. 515 



Fig. 13. — Two cystogenic cells ( x 950). 



Fig. 14. — Cells supposed to be possibly of the nature of polar bodies ( x 1500)* 



Fig. 15,— Early free stage ( x 1000). 



Plate XX. 



Fig. 16. — Somewhat later stage than that represented in fig. 15 ( x 950). 



Fig. 17. — Stage with flattened posterior end ( x 1000). 



Fig. 18.— Stage with clefts and rudiments of pharynx and reproductive 

 system : entire specimen in optical section. 



Fig. 19. — Longitudinal section of similar stage : combined from two adjoin- 

 ing sections ( x 750). 



Fig. 20. — Transverse section of pharynx at the stage in which its wall has 

 become well established but the musculature has not become 

 formed (x700). 



Fig. 21. — Transverse section of pharynx at a somewhat later stage : the cells 

 still intact in the interior. 



Fig. 22. — Somewhat later stage in which the cells in the lumen have disap- 

 peared and the cuticle has become developed ( x 700). 



Fig. 23. — Transverse section of early Cercaria through the posterior region, 

 showing the intestinal epithelium in the condition of irregularly 

 distributed cells enclosed within the developing muscular layer 

 ( x 700). 



Fig. 24. —Transverse section of one of the rami of the intestine at the stage 

 in which the internal cells, though still few in number, have 

 become arranged in a definite layer ( x 1500). 



Fig. 25. — Section similar to that represented in fig, 24, but of a somewhat 

 later stage, in which the epithelium has become more fully 

 developed ( x 1500). 



Fig. 26. — Section of a small part of the epithelium of the intestine in the 

 mature Cercaria, to show the form and arrangement of the cells 

 (X 1500). 



Fig. 27. —Section through Cercaria in the region of the brain at a stage when 

 muscular tissue is only beginning to be formed ( x 750). 



Fig. 28. — Tail at an early stage in its development : viewed as a transparent 

 object ( X 750). 



Fig. 29. — Similar view of later stage. 



Fig. 30. — Transverse section of tail at a stage corresponding to that repre- 

 sented in fig. 23 ( x 700). 



Fig. 31. — Transverse section of later stage in which the cuticle has become 

 formed and the formation of the muscular fibres has begun. 



Fig. 32. — Cercaria stage {Bucephalus) of a species of Echinostomum ( x 240). 



Figs. 33-36.— Outline of head end of living Bucephalus in different conditions. 



