153 



Locomotion : With increase in the number of arms, the ciliated band 

 elongates. After the complete development of the posterodorsal arms, the 

 larva can spread the postoral and posterodorsal arms and thereby come to a 

 stop. The arms are moved by muscles which appear at the site of joining of 

 the postoral and posterodorsal rods with the basal rods. These muscle fibers 

 were observed by McBride (1903) in larvae of the genus Echinus. Grave 

 (1902) found muscle fibers in larvae of Mellita testudinata at the posterior 

 end of the body, which were situated between the rods proceeding from here 

 (Figure 108). On contraction, the postoral and posterodorsal arms diverge to 

 the sides. 



Figure 108: Arrangement of muscle fibers in the posterior end of the Mellita testudinata 



pluteus (Grave, 1902). 

 bw — body wall; hg — hind gut; mf — muscle fibers; sr — skeletal rods. 



Nervous system and sensory organs : The nervous system remains un- 

 changed. In some species, pigment granules which perform a photosensory 

 fiinction begin to accumulate in the cells at the tips of arms. According to 

 Ryberg and Lundgren (1979), localization of the pigment cells can be ob- 

 served in the regions of active morphogenetic processes. Histochemical meth- 

 ods revealed a carotene-protein complex in the pigment cells. The most 

 common carotenoid in the sea urchins is echininon, which is similar in its 

 fiinction to the provitamin A. The pigment cells work as photoreceptors and 

 play an important role in the vertical migration of larvae. 



Pluteus III Stage 



Complete development of the larvae of sea urchins, according to many 

 investigators, requires different time spans. Larvae of the sand dollars 

 Astriclypeus manni (Onoda, 1938), Echinarachnius parma, Scaphechinus 

 mirabilis, and Scaphechinus griseus require only 5 — 7 days while 

 Echinarachinus brevis needs 38 days (Onoda, 1938); completely developed 

 larvae appear in sea urchins of the genus Echinus in 18-20 days (McBride, 



