256 W. B. Coe — Development of the 



They later collect in more or less constant numbers in certain 

 definite localities, where they send out especially long processes and 

 attach themselves permanently to the epithelium. When they are 

 no longer freely movable in the segmentation cavity their processes 

 grow out into long fibers. They are then recognized as muscle- 

 elements. In the early pilidium we find a dozen or more such cells 

 immediately beneath the apical plate. Others fasten themselves 

 firmly to the walls of the body and to the digestive tract, and a con- 

 siderable number to the lower borders of the lappets. 



Fibers now grow out in certain definite directions. At the end of 

 two or three days, they have elongated and anastomosed with each 

 other to such an extent that they form a practically continuous sys- 

 tem of interdependent muscles. Some of these eventually become 

 arranged in several definite muscular bands, while others form an 

 irregular network connecting these bands, as Salensky (28), Biitschli 

 (6), and Wilson (32) have described for other species. The mus- 

 culature in M. ccBca and C. leidyi consists (a) of two principal mus- 

 cular bands extending from the apical plate to the two lappets ; {b) 

 of a pair of bands extending longitudinally along the floor of the 

 pilidium at the sides of the mouth ; (c) of a few fibers connecting 

 the apical plate with the anterior border of the esophagus ; [d) of a 

 few fibers with numerous bfanches lying on the anterior and poste- 

 rior faces of the embryo; (e) of a few surrounding the digestive tract 

 and serving to connect this organ with the body-walls. 



Of these bands the lateral pair connecting the apical plate with 

 the lappets is by far the most extensive. In close proximity to the 

 crowded, columnar cells of the apical plate lie about six or eight 

 muscle-cells on each side. From each of these cells proceed one or 

 two long, slender, branched fibers which pass close beneath the 

 body-epithelium downwards until they either anastomose with fibers 

 from similar cells in the lappets or become attached directly among 

 the epithelial cells at the lower margin of these lobes (Fig. 2, PI. 

 XXXIV ; Fig. 3, PI. xxxv). One or two other fibers from each cell 

 pass among the columnar cells of the plate and serve to anchor the 

 muscle-cell in place. From the dorsal surface the arrangement of 

 these cells and fibers is shown in Fig. 2, PI. xxxv. Those fibers 

 running nearly parallel down the right and left sides constitute the 

 lateral bands. 



The muscle-cells in the lappets send similar pi-ocesses toward the 

 apical plate. These, in part, anastomose with the fibers from the 

 apical plate. Thus there are from eight to sixteen fibers running 



