201 



principal pairs of arms — posterolateral, anterolateral, and postoral (Figure 

 153) — the time of development of the postoral rods may be taken as the 

 termination of the formation of pluteus I stage. 



Feeding : At this stage, until the complete development of the anterolateral 

 and posterolateral arms, the ciliated band passes along the margin of the 

 preoral depression. The ciliated band is involved in obtaining food for the 

 developing larvae only in species with planktotrophic larvae. According to 

 Strathmann (1971), a part of the ciliated band between the anterolateral arms 

 makes a loop above the preoral field towards the transverse section of the 

 band of postoral arms after their development. Beating of the cilia produces- 

 a water current over the mouth opening and extends beyond the limits of the 

 preoral field. Beating of the cilia of the posterolateral arms intensifies this 

 current. As in the larvae of sea stars and sea urchins, the ciliated band in the 

 ophiopluteus functions as a sieve which filters the food particles from the 

 plankton (Figure 154). The lateral bands are made up of a single layer of 

 cells. Studies conducted by Strathmann (1971) showed that in brittle stars 

 each cell of the ciliated band bears one cilium. The ciliated bands on the arms 

 are three to five cells wide; at some places the number of cells increases and, 

 as a result, the band becomes broader. The preoral and aboral fields bear few 

 cilia. Secretory cells have been detected in the tips of the arms of ophioplutei 

 but, as in echinoplutei, these cells are few in number. Very likely, the 



Figure 153: Pliiieus bimacideatits (Metchnikoff, 1869). 

 Pluteus 1 stage. General view, 

 anterolateral arms; b — basal rods; pi — posterolateral arms; po — postoral arms. 



