688 DAVID HILT TENNENT. 
Fic. 17.—Transverse section through germ tube, in the wall of which an 
active differentiation of germ cells is going on. In the lumen of the tube lie 
germ cells and germ balls in various stages of development. 
PLATE 40. 
Fic. 18.—Section of germ ball. 
Fic. 19.—Section of germ ball passing through the rudimentary pharynx. 
Fic, 20.—Section of young germ ball. 
Fic. 21.—Section through actively dividing germ ball. 
Fic. 22.—Longitudinal section through the end of a germ tube in which 
a portion of the tube has become specialised as a germ-producing organ, 
“ ovary.” 
Fic. 23.—Anterior end of Bucephalus haimeanus, in optical section, 
drawn from living cercaria. The mucous secretion of the glandular complex 
has been forced out into the invaginated portion of the anterior end of the 
body as a plug. 
Fie. 24,—Dorso-ventral longitudinal section of the posterior end of 
Bucephalus haimeanus. The cuticle, etc., are not shown. The plane of 
the section passes slightly to one side of the median line. 
Fic. 25.—Median dorso-ventral longitudinal section of the posterior end 
of Bucephalus haimeanus. 
Figs. 26—28 represent individuals in which the tails have been lost. In 
such individuals the bladder-like portion collapses, and the muscular portion 
exhibits considerable changes of form. 
Figs. 26—29 were drawn from clay models made for the purpose of 
showing the changes in form undergone by the posterior end of the body of 
Bucephalus. 
Fic. 26.—Seen from dorsal surface. Middle piece elongated. 
Fig. 27.—Seen from dorsal surface. Middle piece contracted and broadened. 
Fie. 28.—Dorso-lateral view. The underside of the middle piece is seen to 
be concave, the concavity being caused by the collapse of the bladder-like 
portion. 
Fic. 29.—Middle piece and a portion of the appendages in dorsal view. 
Fie. 80.—Dorso-ventral longitudinal section of the posterior end of the 
body and midale piece, showing the muscles by means of which the move- 
ments of the middle piece are governed. 
Fie. 31.—Surface view of cuticle of Bucephalus haimeanus, drawn 
from living specimen. 
