152 



brevis takes 15 days for this process (Onoda, 1938). McBride (1903) noted 

 that sea urchins of the genus Echinus require 7-8 days to complete the 

 piuteus II stage. A similar period is necessary for Salmacius bicolor (Aiyar, 

 1936). Fundamental changes in the skeleton of the larvae of sea urchins occur 

 at this stage. 



Skeleton : In addition to the anterolateral and postoral arms, posterodorsal 

 arms, develop in the piuteus II stage on the dorsal side between the anterolateral 

 and postoral arms. In some sea urchins the rods in the pairs of arms are simple 

 but in others complex and fenestrated. The rudiment of the dorsal arch is 

 situated at the level of the upper third of the stomach. In the heart urchins 

 the ends of this arch elongate very quickly, providing the beginning for the 

 rods of the preoral arms. These rods are simple in all sea urchins (Figure 107). 



Figure 107: General view of piuteus II stage. 



A — Parasalenia gratiosa (Onoda, 1938); B — Echinocardium mediterraneum (Fenaux, 1972). 



al — anterolateral rods; asp — aboral spicule; b — basal rods; int — inner transverse 



rods; pd — posterodorsal rods; pi — posterolateral rods; po — postoral rods; pr — preoral 



rods; sb — secondary basal rods . 



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

 further complication in the structure of the coelomics sacs occurs in the 

 larvae. In particular, the left axohydrocoel divides into the axocoel and 

 hydrocoel — rudiment of the ambulacral system of the definitive sea urchin 

 (McBride, 1903). 



