147 



tigating the development of Tripneustes gratilla in Japan, Onoda (1936) es- 

 tablished that the pluteus I stage formed by the fourth day. For this very 

 species in the Mediterranean Sea, Fenaux and Fenaux (1974) found that at a 

 temperature of 21°C the pluteus I stage develops within 40 hrs. The longest 

 period of development required for attaining this stage, 8 days, was observed 

 in Heliocidaris crassispina (Onoda, 1931). All the structures of the larva are 

 formed in the pluteus I stage and simply grow in size in later stages. 



Skeleton : The distinctive feature of the larvae of sea urchins is the larval 

 skeleton, which provides support for the larva. The pluteus I stage has two 

 pairs of arms — anterolateral and postoral. The anterolateral arms are almost 

 parallel to each other and support the apical part of the pluteus — the oral 

 lobe; the postoral arms diverge at an obtuse angle to each other. The 

 anterolateral rods of the larval skeleton are always simple while the postoral 

 ones in some species may be complex triradiate and perforated. The basal 

 rods are situated at the base of the larval body, they often join to form 

 complex structures. Sometimes, besides the principal basal rods, one or two 

 pairs of secondary basal rods also develop. Above the stomach, from the base 

 of the basal rod to the center of the larva, there may be an inner transverse 

 rod. In heart urchins another unpaired aboral rod is seen ( Figure 103). The 

 skeleton structure is of systematic importance and many researchers are en- 

 gaged in its study. 



Feeding : Unlike larval sea stars, the body of sea urchin larvae is bor- 

 dered by a single ciliated band. The length of the outer cilia is 25—30 )im. 

 The width of the band is 3— 5 cells and each cell contains one cilium. Few 

 secretory cells are found only on the tips of the arms (Strathmann, 1971). 



In the larvae of sea urchins the ciliated band between the anterolateral 

 arms makes a ventral loop to the side of the transverse section of the postoral 

 part of the general ciliated band (Figure 103). In the region of the peristome 

 water currents are created by beating of the cilia of the ventral loop and the 

 transverse part of the postoral band. Two currents are generated: an incurrent 

 into the peristome and an excurrent from it. As in the larvae of sea stars, the 

 region of the peristome in plutei bears cilia. 



The mechanism of capture of food particles by cilia of the ciliated band 

 and transport of these particles to the oral opening in planktotrophic larvae 

 is common to all classes of sea urchins (see above). In echinoplutei the speed 

 of movement of the food particles along the ciliated band is 0.5 mm/sec at 

 13°C. As soon as the food particles are trapped by the cilia, they are moved 

 along the ciliated band to the peristomal field. In this region they move at a 

 speed of almost 0.5-1.4 mm/sec. Cilia of the peristomal field transfer the 

 food particles to the adoral band, which forms a loop that extends along the 



