Pilidium of Certai7i Nemerteans. 255 



The Muscular System. 



In the living pilidium the muscles appear as clear, highly refrac- 

 tive fibers, which are most commonly slightly curved. In optical 

 cross-section each fiber appears as a bright dot. They run in a more 

 or less irregular course from the muscle-cells to their attachment in 

 the epithelium, or to an anastomosis with another fiber. When the 

 embryo is killed they contract forcibly, thereby reducing greatly its 

 size. This is in part accomplished by forcing water out of the 

 digestive canal. 



In contracting the fiber not, only shortens in length, but also 

 becomes wavy and twisted upon itself, or coiled up in a spiral like 

 the stem of a Vorticella. Fig. 3, PI. xxxv, shows the fibers in a 

 living pilidium, while Fig. 5, PI. xxxiv represents similar fibers after 

 the embryo has been killed with corrosive sublimate. 



The nuclei of the muscle-cells are mostly clear, oval bodies about 

 .003™"^ in length. Around the nucleus is a very small quantity of 

 perfectly clear protoplasm, from which two, three, or more contrac- 

 tile fibers extend in various directions. 



The origin of the muscles from undifferentiated mesenchyme is 

 easily followed because of the transparency of the embryos. The 

 cells of the mesenchyme, as stated above, appear in the segmenta- 

 tion cavity in the earliest stages of gastrulation. Some of them seem 

 to originate directly from the entoderm, while others are derived 

 from a pair of primary mesoderm cells set apart as such in the blas- 

 tula. Never have I seen any indication that any of them came from 

 the ectoderm, as Hubrecht (16) describes in Desor's larva.* 



At the time of their first appearance the mesenchyme cells are 

 very large and few in number. During gastrulation they multiply 

 rapidly, as may be seen from the many karyokinetic figures which 

 they contain. At the time of appearance of the lappets which dis- 

 tinguish the pilidium from the gastrula their number is very consid- 

 erable. Some of them become irregular in shape, as described by 

 Metschnikoff (25), and as Wilson (32) has pointed out in C. lacteus. 

 If watched carefully, some of these irregular cells will be seen to 

 send out amoeboid processes, and exhibit a great deal of indepen- 

 dent motion. They not only change their shape, but actually move 

 about in the cavity of the body by means of their pseudopod-like 

 processes. 



* Arnold (1) has recently reinvestigated the development of Desor's larva, and 



finds that the mesoderm originates on both sides of the blastopore at the point where 

 entoderm and ectoderm come together. 



