244 



A. 1ZUKA. 



for each, into the efferent vessels of the rete before spoken of. The two 

 efferent vessels in question and the single descending vein from the vascular 

 networks of the proboscis unite into one in the fourth segment and thus 

 form the anterior end of the ventral vessel. 



2. Of the Nervous System. 



The brain (fig. S), situated in the hind part of the prastomium, 

 represents a flattened, 4-lobed body invested all around by a muscular 

 tunic. Its anterior ed^e is slightly indented in the median line ; the 

 posterior edge shows a somewhat deeper median indentation The lateral 

 edges also show a rounded indentation which divides each half of the brain 

 into a larger anterior and a smaller posterior lobe 



There are in all twelve, symmetrical}' disposod pairs of nerves 

 proceeding from the brain, each nerve arising with a distinct root. Fig 8 

 represents the arrangement of the cephalic nerves in the fully protruded 

 state of the proboscis. For the sake of reference the cerebral nerves may 

 be numbered I — XII , according to the order of succession of their origin 

 on either side of the median plane. The nerves I — IX. may all be said 

 to arise from the arched anterior margin of the anterior cerebral lobe. The 

 three nerves I — III. form a group on either side of the anterior median 

 indentation. N. I., soon after its origin, divides into two branches, both 

 which proceed into the proboscis. N. II. innervates the tentaculm, and N 

 III. the dorsal wall of the head 



Laterally from the group just mentioned, the nerves IV., V. and VI 

 form another group. N. IV. and N VI together supply the wall of the 

 subtentaculum, while N. V. in the middle between them proceeds to the 

 terminal boss of the same. 



N. VII arises from a position ventral to the above group of three 

 nerves It goes into the proboscis. 



N VIII. and N. IX. (figs. 8, 9 and 10), arising from the antero-lateral 

 corner of the brain, are situated one above the other, so that only one of 

 them presents itself to view when seen either from above or from below. A 



