231 



In the late auricularia, the hydrocoel has six diverticulae, lying initially 

 on the left side. These diverticulae later produce the primary tentacles and the 

 polian vesicle (Figure 180). The change in shape and size of diverticulae in 

 Stichopus japonicus is accompained by pulsation of the walls (Figures 181, 

 182). 



Figure 180: Holothuria impatiens (Mortensen, 1938). 

 General view of the late auricularia. 



Locomotion : The locomotor fiinction, as in all echinoderms, is performed 

 by the ciliated band. The larva is capable of moving horizontally, turning in 

 various directions, and remaining stationary. All of these complex movements 

 are accompanied by a change in direction of beating of the ciliated band. 

 Strathmann's (1971) observations have shown that beating of the cilia is not 

 synchronous throughout the entire length of the band; there is a strict alter- 

 nation of sections in which beating occurs in opposite directions. Due to such 

 alternation of direction of beating of the cilia, a running wave is created, 

 which involves the water layer surrounding the larva. The resultant water 

 currents produce a movement of the larva with its preoral lobe directed 

 forward and performance of other locomotory maneuverings. 



Nervous system and sensory organs : Metschnikoff (1869) detected a 

 larval nervous system in the auricularia of Labidop lax digitata. In the ecto- 

 derm of the perioral field there are two bands of cilia situated in an arch. 

 These bands comprise two rows of cells, under which lie the bi- and tripolar 

 neurons. Mortensen (1937, 1938) also mentioned that nervous elements are 

 situated in the perioral field of auriculariae of various species of sea cucum- 

 bers. Unfortunately, no detailed information is available in the literature on 

 the structure of the larval nervous system. 



