252 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



In the precerebral region the vascular system consists 

 of two lacunae (PL I., fig. 2), which join at the tip of the 

 head over the rhynchodaeum. These two cephalic lacunae, 

 together with the proboscis sheath, are surrounded by the 

 nervous ring. About the level of the ventral com- 

 missure of the brain they unite in the mid-ventral line 

 below the proboscis sheath, and from the commissure so 

 formed is given off the median dorsal vessel (or more 

 properly lacuna). Further back the lateral lacunae form a 

 second ventral communication, whence spring the two 

 small buccal vessels which join the vascular network in the 

 oesophageal region. Soon after this the lateral lacunae 

 widen out greatly, and surround the hinder portion of the 

 cerebral organs. Just behind the mouth there is a con- 

 tinuous network of oesophageal lacunae surrounding the 

 ventral surface of the oesophagus (PL II., fig. 2), though 

 the lateral lacunae can still be recognised as the largest 

 and most dorsal of the spaces seen in transverse section. 

 They are in close proximity to the proboscis sheath, and 

 the nephridial tubules come into contact with them (PL 

 II., fig. 2, ext.). 



The median dorsal vessel pierces the wall of 

 the proboscis sheath directly after its formation 

 at the level of the brain, and lies in the median ventral 

 line. Directly over it the rhynchocoelomic epithelium, 

 which alone separates the vessel from this cavity, assumes 

 a columnar appearance. The oesophageal lacunar network 

 extends nearly to the end of the oesophageal region. It 

 then terminates, and the lateral lacunae become much 

 smaller and transformed into the lateral vessels which at 

 first lie just above the level of the lateral nerves, but soon 

 take up a position ventral to the intestine, and not far 

 removed from the mid-ventral line. At about the same 

 level the median dorsal vessel leaves the proboscis sheath, 



