HAPLODISCOS PIGER. 
7 
The yellow cells are, as has already been said, scattered 
quite irregularly throughout the body. Iu the protoplasmic 
tuuic they are numerous, lying generally freely iu a space 
which separates them from the protoplasm of the tunic itself. 
This relation is well seen in horizontal sections through the 
body wall, such as that represented in fig. 6. It is, of course, 
probable that the space surrounding each cell is a post- 
mortem effect produced by the action of reagents on the proto- 
plasm. In any case the appearance in sections is constant 
and characteristic. 
There is generally a considerable group of yellow cells 
above the brain (fig. 2). 
No distinct cell wall is discernible in the cells themselves, 
which appear to consist of a mass of protoplasm, sometimes 
solid and finely granular (fig. 6), more often vacuolated as in 
fig. 2. A rounded concretion was often observed in some part 
of the protoplasm, as iu the cell marked y, fig. 4. The 
nucleus is always situated close to one end of the cell, and is 
in sections somewhat coarsely granular. 
The systematic position of Haplodiscus is not easily 
determined. I regret that my limited opportunities of ex- 
amining fresh specimens did not permit me to form an opinion 
as to the presence or absence of an excretory system. But if such 
a system be present, it may fairly be assumed, from its absence 
iu sections and from the general character of the animal, that 
it is built up on the ordinary Platyelminth type. And, 
neglecting the excretory system, the other characters of Haplo- 
discus seem exactly such as might be looked for in a free- 
living Cestode, which, owing to the absence of a nutrient 
fluid in which to bathe the surface of the body, and from 
which to absorb food, had either retained or acquired a mouth. 
At the same time it seems easily conceivable that a Cestode 
or Trematode larva might, either normally or as the result of 
exceptional surrounding conditions, acquire reproductive glands 
of a simple type, and such a process would introduce into the 
life-history of the species in which it occurred a form which 
might easily present the characters of the animal before us. 
