149 



When the pyloric sphincter is opened, the food particles from the stom- 

 ach enter the small intestine. Defecation is accompanied by a contraction of 

 the intestine, which happens when the pyloric sphincter is closed and the anal 

 sphincter is open (Figure 104). The larvae of sea urchins feed on various 

 species of microalgae whose diameter does not exceed 85 |im, namely, spe- 

 cies of Amphidinium, Phaeodactylum, Dunaliella, and others. 



Figure 104: Division of the intestinal canal into sections in Echinarachnius parma 



(Fewkes, 1893). 



asph — anal sphincter; csph — cardiac sphincter; e — esophagus; hg — hind gut; 



psph — pyloric sphincter; s — stomach. 



Although phytoplankton is the main food of sea urchin larvae, the plutei 

 of the heart urchin Echinocardium cordatum can obtain a large part of their 

 daily ration from soluble organic matter even at its fairly low concentration 

 in water (Vyshkvartsev and Sorokin, 1978). The pluteus of Dendr aster 

 excentricus stores nutrients in the cells of the stomach wall (Chia and Burke, 

 1978). 



Respiration, transport of metabolites, and excretion : The larva has no 

 special organs of respiration. Respiration is performed by the entire body 

 surface. Hemal and excretory systems are also absent. Transport of nutrients 

 within the larval body occurs through the primary and secondary body cavi- 

 ties. Excretion of metabolic products, after formation of the madreporic pore, 

 apparently proceeds with the participation of coelomic sacs. 



At this stage in the larvae of sea urchins, the coelomic sacs enlarge along 

 the esophagus and divide into two: the anterior — axohydrocoel and the 

 posterior — somatocoel. The left axohydrocoel broadens anteriorly, becomes 

 thin-walled, and forms an ampulla in which the stone canal opens. The 

 madreporic pore opens on the left side of the larva and connects the 

 axohydrocoel with the external medium (McBride, 1903) — Figure 105. 



