Marshall—A Parasitic Ostracod. 
131 
ganglion. This ganglion consists of an upper fibrillar half and 
a lower part containing all of the nerve cells. 
The ventral nerve cord, PI. XIII, figs. 24 and 27 is composed 
of the infra-oesophageal and four other ganglia. The first and 
second ganglia, are fused, the others being distinct and entirely 
separated from each other. In both longitudinal and, trans¬ 
verse sections this separation is noticeable as is also the arrange¬ 
ment of the nerve cells. In the two nerves it is shown that the 
nerve cells are not abundant in the' inter-ganglionic spaces but 
that, excepting the first ganglion, they are present along both 
the dorsal and the ventral surfacs. As already noticed the first 
ganglion differs from the others in having the nerve cells en¬ 
tirely absent from its dorsal surface. The other ganglia, plainly 
show the nerve cells more abundant dorsally than ventrally, 
and it is in the dorsal masses that the separation of the ganglia 
from each other is most distinctly seen, PI. XIII, figs. 25 and 
27, as is also the division into a, right and a left. The fibrillar 
part of the cord passes continuously from one end of the chain 
to the other, PI. XIII, fig. 27. The nerve cells on the ventral 
wall are nearly continuous a. few being found in transverse sec¬ 
tions of the inter-ganglionic space, PL XIII, fig. 26. 
Eye :—The eye is situated equally distant from: the top' of the 
brain and the dorsal surface of the shell: a plane, passed trans¬ 
versely through the body, which touched the posterior edge of 
the eye would pass through the middle of the fore-brain. The 
triune structure is readily seen in a dorsal view, PI. XIII, fig. 
29, the two lateral parts being slightly larger than the median 
one. The basal surface of each of the three parts of the eye 
is covered with a layer of pigment, PL XIII, fig. 28, composed 
of a number of dark brown and black spherical granules, which 
are in some places massed so that they touch each other, while 
in other parts; they are more scattered. These cover the next 
layer, tapetum, fig. 28 t p (Claus, Exner) which is quite thick 
and composed of many minute' lamellae 1 so- close together that 
seen under a low power, the layer appears light yellow and ho¬ 
mogeneous. The lamellar structure is, however, easily made 
out under a high power and is; most distinct at the edge. The 
remaining space;, from the; outer surface of the tapeitumj to' the 
