166 



OHAS. B. WILSON. 



formation and attacliment of new processes and the letting 

 go of old ones J until they are approximately parallel (fig. 14) . 

 The strands then come together and fuse into larger ones, 

 while the cell bodies, which now contain little beside the 

 nuclei, travel slowly along the strands, and seem finally to lie 

 on the under surface of the apical plate, or upon the wall of 

 the intestine.^ 



The removal of the nuclei leaves the strands in the condi- 

 tion of clear muscle-fibres (fig. 15). Occasionally the body of 

 a cell is all used up in forming processes, and then the nucleus 

 may be flattened slightly and retain its place in the fibre. 



The muscle is thus fastened first to the wall of the intes- 

 tine (fig. 13), and is so figured by Yerrill (46, p. 417). 



But it soon divides and passes down on either side of the 

 intestine, and fastens to the anterior border of the lappets, 

 separating into fine fibres at the point of junction (fig. 15). 



In a good light the fibres may be seen to be attached to 

 delicate papillae, which are raised slightly from the inner 

 surface of the ectoderm cells. The muscle is now no longer 

 connected with the stomach, for a strong contraction draws 

 the apical plate down until it almost, if not quite, touches the 

 stomach wall. This is the longest, the largest, and the 

 strongest muscle in the pilidium, as well as the first one to be 

 formed, and it is chiefly through its violent contractions that 

 the cells of the pilidium wall are pulled asunder under 

 irritation. 



In such a case, after the muscle has shortened and thick- 

 ened all its structure will allow, a still further shortening 

 is produced by the assumption of a corkscrew shape, 

 exactly as in the contractile stem of Yorticella. This is 

 the only means by which the muscle can draw the apical 

 plate down far enough to touch the stomach wall. 



2. Interparietal Muscles. — The first of these consists 

 of a single cell, which lodges usually just in front of the 

 apical plate (fig. 17). Long processes are developed at 



^ Mrs. G. F. Andrews kindly gave me this final location of the nuclei, from 

 her own observations upon Cerebratulus. 



