Name: 4-arm pluteus larva stage
ECAO ID: 0000470
Definition: "The 4-arm pluteus larva is mainly characterized morphologically by the presence, on the ventral side, of four extended arms (two anterolateral arms at the level of the oral hood and two postoral arms) and on the dorsal side by a sleeked apex. The larva now possesses a completely tripartite and functional digestive tract composed of an esophagus, a stomach, and an intestine, respectively limited by a cardiac sphincter, a pyloric sphincter, and an anal sphincter. Around the esophagus, the right and left coelomic pouches have further started to grow in size and elongate. The first morphological indicator of a left-right asymmetry becomes apparent at that stage, the left coelomic pouch being slightly more elongated than the right one. Furthermore, a thin protrusion of non-skeletogenic mesenchyme cells can be seen extending from the left coelomic pouch to the anterior ectoderm. This is the primary pore canal, which opens to the exterior environment by the hydropore. The larva further now possesses, all around the esophagus, a network of contractile muscles allowing it to swallow and feed. Finally, the apical tuft previously observed at the animal pole has by now disappeared, leaving behind only the concentrated stretch of intermediate size cilia borne by the ciliary band."