150 



Figure 105: Scheme of development of coelomic sacs in Echinus esculentus (McBride, 1903). 

 A — prism stage; B — pluteus I stage; C — pluteus II stage, 

 he — hydrocoel; mp — madreporic pore; sc — stone canal. 



Locomotion : The ciliated band performs a locomotor function. At this 

 stage, locomotion occurs along the anterolateral and postoral arms and the 

 peristomal depression. When the larva moves, the cilia in some sections of 

 the band remain erect while those in other areas bend to one side or the other. 

 Thus, a wave is created which traverses the entire ciliated band. The larva can 

 change the direction of its movement and turn; in this process a change in the 

 direction of beating of the cilia occurs, which is preceded by a brief pause. 

 The direction of water currents created by beating of the ciliated band is 

 opposite to the direction of movement of the larva. When moving forward 

 with the arms, the current is directed by the wave motion of sections of the 

 band on the outside of the arms to the base of the larva. When the larva is 

 motionless, beating of the right sections of the band is directed in the direc- 

 tion opposite to beating of the left sections of the band. When the larva turns, 

 beating of the cilia is noticeable only on the outer side of the direction of 

 turning. Movement of the pluteus in a reverse direction is accomplished by 

 beating of the cilia of the ciliated band in a forward direction. 



The speed of horizontal movement of the larvae of Strongylocentrotus 

 dreebachiensis, S. purpuratus, and Dendraster excentricus is 0.3—0.5 mm/sec 

 at 10-15°C (Strathmann, 1971; Strathmann et al., 1972). 



Nervous system and sense organs : The first mention of the presence of 

 nervous elements in the larvae of sea urchins is found in the work of McBride 

 (1903), who speaks of nerve fibrils situated above the adoral band. However, 

 no information whatsoever on the structure of the larval nervous system was 



