240 ARTHUR E. SHIPLEY. 
the tentacular lophophore, is naturally not represented in 
Onchnesoma, as the tentacles are absent. 
At the sides the brain is continued into two nerves which 
pass round the mouth embedded in the tissue, just where the 
retractor muscle is attached to the csophagus (fig. 9); they 
fuse together on the ventral surface, and form the ventral 
nerve-cord, which shows no sign of its double origin (fig. 8). 
The portion of this cord which lies in the introvert is oval in 
cross section ; that which lies in the body is round. In Phy- 
mosoma and in Sipunculus the ventral nerve-cord is supported 
by numerons strands of muscle continuous with the skin, 
which permitted the introvert to be extended or withdrawn 
without any strain being placed on the cord; but in Onch- 
nesoma the cord is closely attached to the skin, and in the 
region of the introvert is almost embedded in the muscular 
layer. 
As is the case in other Sipunculids, the ganglion-cells are 
arranged on the ventral surface, the fibres on the dorsal. The 
nerve-cord gives off numerous branches into the body-wall, 
whose course I was not able to follow; but Koren and 
Danielssen have traced them into a fine ganglionated network 
amongst the muscles, Xc. 
The nerve-cord extends to the posterior end of the body. 
THe NEPHRIDIUM. 
There is only a single nephridium in Onchnesoma, and its 
position is not very constant; it may lie either to the right or 
to the left of the nerve-cord, but its external orifice is always 
a little below the ring-like thickening which marks the 
junction of the proboscis and the body. 
In its main features the nephridium resembles the same 
organ in Phymosoma varians, with the exception that there 
is no distinction between glandular and non-glandular regions, 
The external orifice leads straight into the lumen of the gland, 
which is as arule somewhat pear-shaped. The internal opening 
is close to the external; it has a flattened, funnel-shaped border, 
and is ciliated, 
