Cytological and anatomical aspects of the late gastrula and trilobed larva of Terebratalia transversa. (A) Light micrograph of a late trilobed larva with ocelli (OC) on the dorsal side of the apical lobe (AL). Vesicular bodies (VB) and other epidermal cells line the border between the apical and mantle lobe (ML). The mantle lobe has four chaetal sacs with long chaetae (CH). The posterior pedicle lobe (PD) will attach the larva to the substrate at metamorphosis. (B) Ventral view of a bilateral late gastrula stage labeled for acetylated α-tubulin with a long ciliary tuft (AT) produced by specialized cells with recessed ciliary rootlets (ATC). (C, D) Partial frontal z-projections depicting aspects of late larval anatomy such as the cilia of the anterior transverse ciliated band (CB) and sensory neurons (SN1 and SN2) within the apical organ (AO) that send axonal fibers into the central anterior neuropil (NP). The larva is nonfeeding, but does develop a blind-ended gut (G). (E, F) Complete z-projections of the histaminergic nervous system of the late trilobed larva. Cell borders and some larval muscles are stained with phalloidin. There are at least 70 histaminergic cells in the apical organ (AO), approximately 60 histaminergic cells in the broad ventral region of the apical lobe (AVC), and also approximately 30 histaminergic cells (MVC) in a mid-ventral region (V) in the mantle lobe. All scale bars = 25 μm.