18 ARTHUR E. SHIPLEY. 



The fibres are on the dorsal^ the cells on the ventral side of 

 the cord. 



Along each side of the nerve-cord runs a longitudinal band 

 of muscle-fibres, the cord and its pair of muscles being together 

 enclosed in a special peritoneal sheath. The space between 

 the sheath and the cord is filled with a peculiar connective 

 tissue (fig. 29), which has been regarded by some observers as 

 clotted blood, the cord being said to lie in a blood-vessel. My 

 preparations afford no evidence in support of this view ; and I 

 am strongly of opinion that the substance lying between the 

 nerve-cord and its peritoneal investment is, as above stated, 

 connective tissue. 



By contraction of the muscles within the peritoneal sheath 

 the nerve-cord may become crumpled, so that while the sheath 

 is perfectly straight the cord within it presents the appearance 

 shown in fig. 28. 



The nerve-sheath is attached to the ventral body wall by a 

 series of mesenteric cords, each of which contains, not only a 

 prolongation of peritoneal epithelium, but also a central axis 

 of connective tissue (figs. 28 and 29) . 



The peripheral nerves form, as in Sipunculus, a series 

 of rings encircling the body, and lying between the circular and 

 the longitudinal muscles. In the region of the introvert a 

 nerve-ring lies beneath each ring of hooks, at the base of the 

 circular muscle which supports them (figs. 1 and 2). 



Each nerve-ring is connected with the ventral cord by a 

 single short nerve, which runs from one to the other in the 

 middle ventral line. 



The lophophoral nerve runs along the base of the tentacles, 

 one on each side of the lophophore. Each gives off a series of 

 small nerves, one of which passes up the axis of each tentacle, 

 lying immediately beneath the ciliated groove (figs. 2, 5, 

 and 17). 



In addition to the sense-pits on the brain there are a number 

 of ectodermal structures on the introvert, which are probably 

 sensory in function, and may well be described here. These 

 bodies are arranged in circles parallel to the rows of hooks 



