148 



Figure 103: General appearance of the pluteus I stage. 



A — Strongylocentrotus pulcherrimus (Onoda, 1936); B — Spatangus purpureas 



(Fenaux, 1972). 



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



po — postoral rods; sb — secondary basal rods. 



ventral side of the esophagus. Cilia of the aboral band transport food particles 

 into the oral opening, where they reach the esophagus. 



To remove inedible food particles and, possibly, to ingest large particles, 

 the oral opening in the echinopluteus I stage can be broadened through the 

 action of a pair of special muscles — the posterior dilators. One end of these 

 muscles is attached to the upper ventral part of the esophagus and the other 

 end to the larval skeleton at the place of divergence of the anterolateral and 

 inner transverse rods. 



Numerous cilia line the esophagus and pass the food particles to its 

 posterior part where they are stored until the circular esophageal muscles 

 begin to contract and the cardiac sphincter opens up. The food mass enters 

 the stomach where sorting takes place. Two types of cells have been observed 

 in the stomach: cells secreting digestive enzymes and absorbing and sorting 

 nutrients, and cells phagocytizing food particles as a whole (Burke, 1981). 

 Experiments conducted by Strathmann (1971) have shown that when carmine 

 and crystals of calcium carbonate together with algal culture reach the stom- 

 ach, the algae remain in the stomach while the carmine and calcium carbon- 

 ate pass through it unobstructed to the small intestine and are egested. The 

 separation of particles is observed until they reach the lower part of the 

 stomach. Clumps of algae circulate in the stomach for a long time. The time 

 of passage for the food through the digestive tract is, on average, 30 min. 



