664 DAVID HILT TENNENT. 
TRANSITIONAL STAGES. 
From the Silverside (Menidia menidia) I obtained forms 
which are intermediate between Bucephalus and Gastero- 
stomum. 
Encystep Forms. 
The encysted forms were within a rather clear capsule, 
through which the outlines of the body and its structure 
could be seen (fig. 48). The contractile sac of the water 
vascular system was greatly distended and filled with 
refractive granules. The cystogenous organ appeared as 
an opaque mass in which nuclei were scattered. 
Free Forms. 
Many of the free forms appeared similar to Bucephalus 
from which the tails had been removed. 
In other forms the sucking disc had taken the place of 
the cystogenous organ, the penis sheath had assumed definite 
form, and the spines were very noticeable (fig. 49). 
GASTEROSTOMUM GRACILESCENS. 
The body is about 1:0 mm. long and 0°5 mm. broad. It 
tends to be cylindrical in form, the anterior half of the body 
being of a rather uniform diameter with the anterior end 
rounded; the posterior half tapers gently to a pointed end 
(figs. 46 and 47). 
In the living worm comparatively little of the anatomy 
may be made out. It is easily seen that the body is covered 
with minute spines, arranged in longitudinal rows. Through 
the integument may be seen the penis sheath, the testes and 
the ovary, the vitellaria and the pharynx and gut. In 
specimens in which the uterus is filled with eggs absolutely 
nothing of the internal structure may be seen, the uterus 
