224 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 oesophagus (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 numerous 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 which permitted the introvert to be extended or withdrawn 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, &c. 



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 inPhymosomavarians, 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 a rule somewhat pear-shaped. The internal opening 

 is close to the external ; it has a flattened, funnel-shaped border, 

 and is ciliated. 



