Stage 28
This is the Mid-Swimming Larva Stage (CirobuD:0000051; 20-22 h). The papillae elongate and their basal part expands. The trunk is square in shape. The ciliary network belonging to the epidermal sensory neurons (ascidian dendritic network in tunic, or ASNET; CirobuA:0000892) becomes more complex (ATEN, DCEN; RTEN, and VCEN). The preoral lobe (CirobuA:0000901), a wide anterior body cavity between the pharynx and the anterior epidermis, is recognizable. Here, round mesenchyme cells are present (mech). Spaces among the epithelia (haemocele, CirobuA:0000888) become larger; here, mesenchyme cells represent the haemocyte (CirobuA:0000571; synonym of blood cell) precursors. According to Parrinello and co-authors (Parrinello et al., 2020), haemocytes in adult animals are stem cells and granulocytes. The latter include clear granulocytes (precursors to clear vesicular granulocytes), microgranulocytes, and vacuolar granulocytes (including unilocular granulocytes and globular granulocytes). Tunic cells (CirobuA:0000751) are also recognizable in the inner compartment of the tunic. They differentiate from mesenchyme cells. Coronet cells are well-differentiated on the left side of the sensory vesicle. The otolith (CirobuA:0000671) senses gravity and gravitaxis is observed at this stage (Bostwick et al., 2020).