160 



left postoral rods as well as the plate which appeared at the base of the larval 

 body, are transformed into genital plates. In all regular urchins the madraporic 

 or second genital plate forms around the appendage of the dorsal arch. Only one 

 genital plate, the third, is not associated in its development with the larval 

 skeleton (Figures 114 and 116). 



As described above, in the larvae some species of regular urchins, at the 

 place of connection of the rods supporting the arms and at the base of the 

 larval body, growing pedicellariae are attached by their bases to the skeletal 

 plates. These are complex capitate formations attached to the plate by a short 

 stalk. The bulb is composed of three articulating blades with a dentate mar- 

 gin. The stalk is similar in structure to the rods of the juvenile individual and 

 has a tetrahedral base (Figure 116). These pedicellariae have a special system 

 of muscles for opening the blades. On the surface of each blade sensory cells 

 occur among the epithelial; each sensory cell bears one cilium surrounded by 

 a ring of microvilli. In structure and function these pedicellariae are quite 

 similar to those of the adult, although their development is completed in the 

 larva much before the rapidly developing processes of metamorphosis. The 

 larval pedicellariae, in all probability, perform a defensive function in the 

 settling individual (Burke, 1980). The mesenchymal cells of the rudiment of 

 the pedicellariae are differentiated into skeletogenic and myogenic cells. The 

 ectodermal cells of the rudiment produce the covering epithelium of the 

 pedicellariae, nerve, and sensory cells. The larval pedicellariae begin to fiinc- 

 tion very early in the larva. During metamorphosis they are shifted to the 

 aboral surface of the definitive individual. The plates, or ftiture bases, as 

 already mentioned, are transformed into genital plates. The juvenile spines 

 present on the settling urchins perform a defensive and partly locomotor 

 function, then later disappear (Ubisch, 1913). 



The mouth apparatus and its skeleton form in regular urchins after the 

 juvenile has settled. 



Formation of test in the sand dollars (subclass Irregularia, order 

 Clypeasteroida): In the development of Echinarachnius parma (Gordon, 1928; 

 Kryuchkova, 1979a), Scaphechinus mirabilis, and Scaphechinus griseus 

 (Kryuchkova, 1979a), the second and third interambulacral plates and their 

 spines are the first to appear (Figure 117). Subsequently, the ambulacral 

 plates develop. The ocular plates develop independent of the larval skeleton. 



The genital plates, except for the second, that is, the madreporite, appear 

 after settling and are not associated with the larval skeleton. The madreporite 

 forms around the projection of the dorsal arch of the larva. Besides the five 

 genital plates, two additional ones develop on the aboral side. These plates, 

 like the genital ones, fijse to form the single complex madreporic plate of the 

 adult (Figure 118). 



