204 



Locomotion : The locomotory function, as in the larvae of other echino- 

 derms, is performed by the ciliated band. The posterolateral arms attain 

 maximum development in all planktotrophic larvae. During larval movement, 

 the wave created by beating of the cilia passes along the band (Strathmann 

 et al., 1972). 



Water currents along the outer edge of the arms are directed toward the 

 narrowed basal part of the larva, pushing it forward. The brittle star larva does 

 not have great maneuverability since the bands of the anterolateral and postoral 

 arms participate basically in feeding. Moreover, in many brittle stars these are 

 much shorter than the posterolateral ones. Nevertheless, by changing direction 

 of beating of the cilia, the brittle star larva is capable of halting and turning. 

 A change in the direction of beating of the cilia along the greater part of the 

 ciliated band can produce a reverse movement of the larva. Swimming on the 

 bottom of the dish in which they were reared, the larvae were capable of 

 performing jerky movements with their oral opening down; the arms of brittle 

 star larvae are rigidly fixed. 



Nervous system and sensory organs: Mortensen (1937), describing the 

 various larval stages of Ophiocoma lineolata, mentioned that nervous ele- 

 ments are present in different parts of the larval body. Unfortunately, he 

 provided no description of these structures. Other information on the larval 

 nervous system in brittle stars is lacking in the literature. 



Pluteus II stage 



Skeleton : Fully developed larvae of brittle stars have four pairs of arms. 

 The last pair, the posterodorsal, appears after the anterolateral arms on the 

 ventral side. The rays supporting them are processes of the anterolateral rods. 

 The basal rods distally bear one or two pairs of transverse processes of 

 varying lengths. In larvae of the genus Ophiocoma, these transverse processes 

 are absent and the basal rods interlock (Figures 156, 157). 



Feeding: McBride (1907) noted in the larva of this stage that the oral 

 opening is bordered by the adoral ciliated band (Figure 158), in addition to 

 the preoral band and lateral band, connecting the transverse areas of the 

 anterolateral and postoral bands. 



Respiration, transport of metabolites, and excretion : At this stage, the 

 coelomic structures are more complex. The left anterior coelom is connected 

 through the pore canal with the external environment and possibly partici- 

 pates in the excretion of products of metabolism. Studies on the development 

 oi Ophiothrix fragilis (McBride, 1907) and Ophiocoma nigra (Narasimhamurti, 

 1933) showed that soon after the formation of the pore canal, separation of 



