174 
The Brain is situated in the prostomium, and even 
in large specimens (250 mm. long) is only about a milli- 
metre in length. It consists of a pair of anterior lobes 
placed well forward in the prostomium, a pair of posterior 
lobes which he below the nuchal organ, and an inter- 
mediate region which connects the anterior and posterior 
lobes (figs. 46, 47). 
The anterior lobes are short but broad—in fact this is 
the broadest part of the brain; behind these lobes the 
brain gradually tapers. Their shape may be seen from 
fig. 46. They are separated in front by a coelomic space. 
Kach gives off anteriorly and dorsally a series of nerves 
to the epithelium of the prostomium, and ventrally nerves 
to the upper lip and neighbouring part of the eversible 
buccal mass. The anterior part of these lobes consists of 
small clusters of cells, separated from one another by 
fibrous tracts and by neuroghal tissue. Further back the 
delicate neuropile which forms the core of the anterior 
lobes is well seen covered by clusters of cells and fibrous 
tissue. Bundles of nerve fibrils may be traced from the 
bases of the prostomial epithelial cells into the neuropile. 
Larger unipolar ganglion cells are found just outside the 
neuropile, and particularly on the side nearest the middle 
line. These cells are more numerous immediately in 
front of the point of union of the anterior lobes, and for 
a short distance behind that point. The cesophageal con- 
nectives arise from the anterior lobes at the point where 
the neuropile reaches its greatest development. ‘The eyes 
are found on the dorsal side of this part of the brain. 
Passing backwards along the middle region of the brain, 
it is seen that the ganglion cells become more restricted 
to the dorsal and lateral faces, the middle and ventral 
parts being composed largely of neuropile, in which also 
neuroglial cells and fibrille may be recognised. The cells 
