No. I.] MESENCHYME IN CERTAIN LARVAE. 



21 



Other cells become fastened to the walls of the digestive 

 tract along its anterior border. The branches of these cells 

 anastomose with each other and with the first cell, and from 

 them are developed the strong muscle which enables the larva 

 to retract the apical plate with its tuft of cilia. This muscle 

 becomes attached at first to the wall of the digestive tract, 

 as figured by Verrill {Marine Nemerteans of New Engla7id, 

 p. 417). But as soon as the branches begin to anastomose it 



Fig. 4. — Side view of pilidium larva, showing mesenchyme cells in position to form the 

 apical muscle. Zeiss cam. luc. x 575 diams. 



develops along the line of mesenchyme cells seen in Fig. 4, and 

 becomes fastened to the anterior border of the lappets. In a 

 similar way a transverse muscle is formed just in front of the 

 apical plate. 



This consists of a single large mesenchyme cell which subse- 

 quently develops long processes reaching from one side of the 

 pilidium to the other. In later development the whole internal 

 surface of the umbrella is covered with a loose meshwork of 

 anastomosed mesenchyme cells, which give the larva so much 



