HABITS, ETC., OP CEREBRATULUS LACTEUS. 171 



Each cell then sends out a process which develops into a 

 muscle-fibre. There are two sets of these lappet muscles. 



a. Radial Muscles of the Lappets. — These consist of 

 fine fibres extending radially from the edge of the lappets to 

 the circumoral ring (fig. 15). They are all of the same size, 

 and are arranged at approximately equal distances from one 

 another, each micromesencyte forming but a single fibre. 



They anastomose little, if at all, but each fibre at the edge 

 of the lappets divides into from two to five fibrils. 



Each fibril is attached to a fine mammillary process on the 

 inner surface of a cilia cell, which looks like a miniature of 

 one of the papillary muscles of the chord83 tendinese of the 

 human heart. These muscles correspond to the " large 

 muscles of the lappets noted by Salensky in Linens (45), 

 but differ from them in that they do not radiate from a 

 single point. 



There is a small group (fig. 15) radiating from the same 

 point as in Salensky's figure, but it does not constitute an 

 important part of the muscle. The actual process of attach- 

 ment by which one of the muscle fibres is fastened to the 

 pilidium wall is difiicult to watch, but it is easy to determine 

 in these lappet cells that the muscle fibrils are connected 

 with the cytoplasmic reticulum. When one of the threads 

 sent out by a mesenchyme cell comes in contact with an ecto- 

 derm cell it first flattens out a little on the surface of that 

 cell. 



The substance of the thread, which is actively moving 

 protoplasm, then fuses with the ectosarc of the cell, and 

 through it becomes attached to the reticulum in the interior. 

 That it is certainly thus attached is seen upon the disruption 

 of the pilidium under irritation, when muscle fibrils may 

 sometimes be found with a single ectoderm cell still attached 

 to either end. 



These of course were interparietal muscles. If now the 

 cells be held in position by pressure of the cover-glass the 

 muscle-fibre will go on contracting for some time. 



By increasing the power used until the reticulum of the 



