xv NERVOUS AND EXCRETORY SYSTEMS 417 



nerves arise from it, and pass to the fringe surrounding the 

 mouth and to neighbouring parts. At the sides, the brain 

 is continued into two stout nerve-cords which encircle the oeso- 

 phagus, and meeting, fuse together in the median ventral line 

 to form the ventral nerve-cord (Fig. 211). The latter is of the 

 same diameter throughout, and shows no signs of segmentation ; 

 it is oval in section, and consists of small ganglion cells heaped 

 up on the ventral surface, i.e. next the skin, and of numerous 

 fibres situated dorsally. The cord gives off many nerves, which 

 usually arise in pairs. These pass into the skin, and forming 

 rings, run round the body, and give off finer nerves as they go. 



The nerve-cord is supported by numerous strands of muscle 

 which pass to it from the skin. These are especially long in the 

 region where the introvert joins the trunk, and thus allow free 

 play to the nerve-cord when the former is being protruded or 

 retracted. 



Sipunculus is not well provided with sense-organs, but in an 

 animal which lives buried in sand we should not expect to find 

 these very highly developed. On the introvert there are certain 

 patches of epithelium bearing long stout cilia, which have been 

 regarded as tactile in function, and there is a tubular infolding 

 reaching the brain, which almost certainly has some sensory func- 

 tion. Ward * has termed this " the cerebral organ." It consists 

 of a duct lined with ciliated cells, which opens to the exterior in 

 the middle dorsal line outside the tentacular fringe. The duct 

 leads down to the brain, and expands at its lower end into a 

 saucer-shaped space, covering that portion of the brain where 

 its substance is continuous with the external epithelium. In 

 Phymosomn this cavity is produced into two finger-shaped pro- 

 cesses, which are sunk into the brain and are lined by cells 

 crowded with a dense black pigment. 2 They are probably rudi- 

 mentary eyes, perhaps distinguishing only between darkness and 

 light. The pits appear to be absent in Sipunculus nudus, but 

 Andrews states they are found, although without pigment, in 

 S. gouldii. 3 



Excretory System. The excretory organs or " brown tubes " 

 are typical nephridia, that is to say, they consist of tubes 



1 Bull. AIus. Harvard, vol. xxi. 1891, p. 143. 

 - Shipley, Quart. J. Micr. Sei. vol. xxxi. 1890, p. 1. 

 3 Stud. Johns Hopkins Univ. vol. iv. 1887-90, p. 389. 

 VOL. II 2 E 



