172 CHAS. B. WILSON. 



cell becomes visible^ it can be seen that the threads of this 

 reticulum are apparently continuous with the fibrils, and 

 that every contraction of the latter pulls the reticulum 

 adjacent to the point of attp-chment out of place. As soon 

 as the contraction ceases the reticulum returns to its former 

 shape. The ground substance of the cytoplasm is appa- 

 rently affected only as it is compelled to accommodate itself 

 to the changes of shape in the reticulum. The cytoplasm of 

 the thread, therefore, at first fuses imperceptibly with that 

 of the cell to which it is being attached, but on becoming 

 fibrillated the fibrils attach themselves to the - more sub- 

 stantial reticulum. 



These radiating fibres are very contractile, and may often 

 be seen to assume the same corkscrew shape as the apical 

 muscle when excessively contracted. They serve to produce 

 the opening and closing motion of the lappets already de- 

 scribed. 



b. Locomotor Muscles of the Cilia Rows. — These 

 correspond closely to the circumoral muscles in origin and 

 mode of development. The cilia rows lie at the junction of 

 the aboral and oral ectoderm where the mesenchyme cells 

 originate. 



When the lappets first start some of these cells are taken 

 along with them in their downward growth, and retain their 

 original position just inside the cilia cells (fig. 11). From a 

 portion of these micromesencytes are developed the radial 

 muscles just described, but most of the fibres unite to form a 

 muscle strand running along inside the cilia cells. This 

 strand increases as the lappets enlarge, and unites at either 

 end with the circumoral muscle (fig. 77). Its fibres divide into 

 fibrils which terminate in the individual cells of the cilia rows. 



The strand can be plainly seen in all longitudinal or dia- 

 gonal sections which cut through the edge of the lappets, 

 and it may be detected in specially perfect cross-sections, 

 where of course the fibres appear as dots. These are the 

 only muscles of the larva distinctively for locomotion (cf. 

 p. 159j. 



