66 Illinois State Laboratory of Natural Histonj. 



The sheath of the ventral nerve chain has a well-defined 

 outer and inner limiting membrane of modified connective 

 tissue, similar to that described for the cerebral ganglion. 

 From the inner one, in some sections, fibers may be seen pass- 

 ing in among the other tissues of the sheath, while occasional 

 strands of connective tissue extend from it across the cord, at 

 the sides of the median giant fiber, to the membrane of the 

 opposite side of the cord. The sheath is not as thick on the 

 first ventral mass as it becomes farther to the rear, and it lacks 

 here the muscle fibers, most of its substance being made up of 

 granular matter and of blood capillaries. The muscular tissue 

 of the sheath appears between the first and second ganglia, and 

 shows on ganglion 2 as a series of fibers next the inner 

 enclosing membrane of the sheath, the sheath being still made 

 up largely of the refracting granular material. Beneath the 

 slender anterior division of the intestine the muscular tissue of 

 the nerve cord becomes better developed, the fibers being large 

 and not so closely confined to the inner membrane of the 

 sheath. In the region of somites 19 and 20 the sheath is 

 largely made up of muscle. (Fig. 19, PL IV.) The fibers of 

 this muscle have exactly the same structure as those in the 

 body-wall, consisting of series of flattened fibrils, with central 

 space and interstitial protoplasmic substance. The sheath 

 becomes thinner again posteriorly and loses much of its mus- 

 cular character, the fibers appearing, as in front, as a series 

 along the inner membrane of the sheath. (Fig. 21, PI. IV.) 



It seems evident that the function of these muscles of the 

 nerve sheath is to adjust the cord to the very great changes 

 in the length of the body of the worm, and to accommodate 

 it to the abrupt bending of the body from side to side which 

 occurs during the creeping and burrowing operations of the 

 living worm. A sudden change in length from a foot to 

 six inches requires that the nerve cord be, by some 

 means, readily adjusted to so abrupt and pronounced a 

 change without taking harm or having its office interfered 

 with. The loii<;itudinal muscle fibers of the sheath doubtless 

 shorten the cord at such times and prevent its being thrown 

 into folds. Sections of the cord from greatly shortened 

 worms show an expanded condition, probably attributable to 



