254 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



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 lie 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 ccelomic space. 

 Each 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 neuroglial 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 oesophageal 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 fibrillse may be recognised. The cells 



