Figure 1
From: Hydrocoel morphogenesis forming the pentaradial body plan in a sea cucumber, Apostichopus japonicus
Normal development of the sea cucumber Apostichopus japonicus and schematic illustration of its hydrocoel. (a) The developmental process during auricularia larva and juvenile stages. Development proceeds from left to right. Auricularia, doliolaria, metamorphosis and pentactula are ventral views, anterior to the top, and juvenile is dorsal view, anterior to the top. (b) Schematic illustration of hydrocoel in each developmental stage. The orange part represents the hydrocoel or its derivative, i.e., the water vascular system. Auricularia and metamorphosing larvae are ventral views, and doliolaria, pentactula and juveniles are left lateral views, anterior to the top. The hydrocoel emerges on the left side of the archenteron in an auricularia, and 10 lobes are formed from the hydrocoel. After the lobe formation, the hydrocoel becomes horseshoe-like shaped to form a ring canal surrounding the digestive tract. Then, the larva metamorphoses into a doliolaria. Five of the lobes (I–V) extend anteriorly and become the water vascular canals of the tentacle of a juvenile, and another five lobes (i–v) are formed between the water vascular canals of the tentacle extend posteriorly and become radial canals. The numbering identities for hydrocoel lobes are based on the application of Loven’s axis to sea cucumbers14.
