Name: 8-arm pluteus larva stage
ECAO ID: 0000500
Definition: "The 8-arm pluteus larva is mainly characterized morphologically by the presence on the ventral side of the larva of eight arms and by the development within the larva, on the left side of the stomach, of the adult rudiment. This stage is relatively long. It includes distinct changes occurring to the larva as well as within the larva, although the larva consistently exhibits 8 arms. In addition, it should be highlighted that the sequence of events taking place at the level of the larva compared to that occurring on the left side of the stomach to form the rudiment are heterochronic, meaning that differences are observed among individuals in the relative timing of these events. From the larval point of view, at that stage, on the anterior side, the dorsal arch has extended out of the oral hood, thereby forming two additional arms above the mouth, i.e. the preoral arms. In parallel, across the apex, the epaulettes have continued developing, thereby generating four large structures rich in cilia and perpendicular to the ventro-dorsal axis. At the tip of the apex, the suture of the body rods has also eventually ruptured, considerably enlarging the larval apex and reducing the angle between the postoral and posterodorsal arms. In parallel, inside the larva, the left coelomic pouch has also by now been partitioned into three parts: the axocoel (upper part, close to the esophagus), the hydrocoel (middle part) and the somatocoel (lower part, next to the stomach). Likewise, on the opposite side, the right coelomic pouch has also been partitioned into a right axohydrocoel and a right somatocoel, although the right axohydrocoel remains in a rudimentary state compared to its left counterparts. In addition, at the basis of the posterodorsal arm on the left side, a small portion of the ectoderm has thickened, invaginated, and came into contact with the left hydocoel. This structure is called the vestibule and together with the left hydrocoel and the left stomatocoel they have further subsequently fused to form the adult rudiment. The formation of the rudiment has overall taken some time, giving rise first to a pentaradial disc and then to a pentaradial complex structure harboring podia (i.e. tube feet) and spines. In parallel, on the right size of the larva, a variable number of pedicellariae (one to four) and endoskeletal elements (i.e. the five genital plates) have further formed, which will also later contribute to the composition of the adult body."
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