Pilidium of Certain Nemerteans. 25V 



more or less parallel between the apical plate and the lower borders 

 of the lappets. The cells in the lappets likewise send off longitudinal 

 fibers along the floor of the lobe, as well as towards the borders of 

 the mouth. This complex system of muscles enables the lappets to 

 be moved readily in any direction. 



Figure 3, PI. xxxv, represents somewhat diagrammatically the 

 arrangement of the muscle-cells and fibers which are seen in a side 

 view of a pilidium. The nearly parallel vertical fibers constitute 

 one of the lateral muscle-bands. The relation of the muscle-cells to 

 the cells of the apical plate is shown in Fig. 6, PI, xxxv. As seen 

 in the figure, the muscular fibers are attached between the epithelial 

 cells rather than directly to their faces. Likewise the arrangement 

 of the muscle-cells and mesenchyme-cells in the lappets, and the 

 relation of the fibers to the epithelial cells is shown in Fig. 4, PL 



XXXIV. 



On the anterior and posterior faces of the pilidium we find com- 

 paratively few fibers, and these are much branched. They run 

 mostly horizontally or obliquely instead of vertically, as on the 

 lateral faces. They anastomose freely with each other, with the 

 fibers from the apical plate, and with others passing inward to the 

 digestive canal. Those on the anterior face are represented in Fig. 

 4, PL xxxv. 



In the mature pilidium one of the largest and most conspicuous 

 muscle-bundles is that running froiB the apical plate to each side of 

 the anterior border of the esophagus and thence to the lappet. This 

 muscle is well developed in C. marginatus, where it consists of 

 several strong fibers. It is represented in C. leidyi and M, ccBca 

 merely by a very few delicate and widely separated fibers. Two of 

 these fibers are represented in Fig. 3, PL xxxiv. They serve both to 

 hold the esophagus in place and to withdraw the apical plate. 

 They doubtless also aid in rocking the plate and thus moving the 

 flagellum. 



From the region of the mouth a series of scattered muscular fibers 

 runs radially in all directions. A few are fastened to the anterior 

 and posterior margins of the body, but most of them pass into 

 the lappets, at the edges of which they are attached. Besides these 

 there is a horizontal bundle running longitudinally along the floor 

 of the body-cavity on each side of the mouth. One of these muscle- 

 bands is represented in dotted lines in Fig. 3, PL xxxv, (b). Their 

 fibers are fastened at the anterior and posterior margins of the body, 

 and anastomose freely with other fibers. 



Trans. Conn. Acad., Vol. X. August, 1899. 



17 



