11 



In the pieeerebial reo-jon the vascular system consists 

 of two laciinjie (PI. I., tig. 2), which join at the tip of the 

 \\vad over the rhynchoiheuni. Tliese two cephalic laciinse, 

 together with the proboscis sheath, are surrounded by the 

 nervous ring. About the level of the ventral com- 

 missure of the l)rain they unite in the mid-ventral line 

 lielow the proboscis sheath, and from the commissure so 

 formed is given oft' the median dorsal vessel (or more 

 l)ro])erly lacuna). Further back the lateral lacuna' form a 

 second ventral communication, whence spring the two 

 small l)uccal vessels which join the vasculai' network in the 

 (esophageal region. Soon after this the lateral lacuniF 

 widen out greatly, and surround the hinder portion of the 

 cerebral ors-ans. -lust behind the mouth theie is a con- 

 tinuous network of (esophageal lacunae surrounding the 

 ventral surface of the (eso})hagus (PI. II., fig. 2), though 

 the lateral lacunie can still be recognised as the largest 

 and most dorsal of the spaces seen in transverse section. 

 Thev are in close proximity to the })roboscis sheath, and 

 the nephiidial tububvs come into contact with them (PI. 

 II., fig. 2, c.rf.). 



The median dorsal vessel pierces the wall of 

 the probi)scis sheath directly after its formation 

 at the level of the l)rain, and lies in the median ventral 

 line. Directly over it the i hynchoccelomic epithelium, 

 which alone sepaiates the vessel from this cavity, assumes 

 a cohimnar appearance. The (esophageal lacunar network 

 extends nearly to the end of the (esophageal region. It 

 then terminates, and the lateral lacuna:^ become much 

 smaller and trausfoinied 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-vential line. At about the same 

 level the median dorsal vessel leaves the proboscis sheath. 



