1 8 WILSON. [Vol. II. 



to be made. The drawings in Figs, i and 2 were taken from 

 individuals which had been paralyzed with magnesium sulphate. 

 These cells are isolated from everything except the liquid in 

 which they lie, and, consequently, if there be any stimulus 

 previous to contraction, it must be given through the medium 

 of the liquid or it must arise spontaneously in the cell itself. 



By careful watching, the contraction can be seen to begin in 

 the cell body and travel outward along the branches, though 

 the contractile wave moves so quickly that it practically begins 

 at all points simultaneously. As a result of its action the 

 protoplasm draws together, the cell body becomes more 

 spherical, all the branches, large and small alike, become 

 shorter and thicker, and the whole meshwork of fibrils is drawn 

 in until it occupies much less area than formerly. 



When it first begins, the contraction is comparatively weak 

 and results simply in a shortening of the branches and fibrils, 

 but as it proceeds it becomes rapidly stronger and stronger. 

 This increase in contraction cannot manifest itself in any 

 further shortening of the branches, for they have already 



shortened all they are capable of 

 doing. The only way in which the 

 two ends of any branch can now be 

 brought nearer together is by a 

 bending or folding of the fibers 

 upon themselves, and this is what 

 actually occurs. At the close of 

 contraction (Fig. 3) the smaller 

 ^, „ , , branches and fibrils have been drawn 



r iG. 3. — 1 he same two cells at the close 



of contraction. Leitz objective No. 7, in SO much that they are twisted into 

 ye-piece o. 3. ^ corkscrcw shapc for their entire 



length. The large branches have also contracted so strongly 

 that their surface becomes wavy or sinuous in outline. The 

 bodies of the cells remain spherical, but become so opaque 

 that neither nucleus nor vacuoles are visible. 



This twisting or plication of the muscle fibers, whereby their 

 retractive power is increased, is also shown in the retractor 

 muscle of the velum (Fig. i), and will be noticed later in 

 certain muscles of the nemertean larva. The same thins: has 



