Appendicularians. a An individual of the appendicularian species Oikopleura dioica. Anterior is to the top right. b Illustration of the animal and its house as depicted in (a). Black arrows indicate the direction of water currents that flow into the house by the beating of the tail. Red arrow indicates the movement of food particles toward the mouth. c Lateral view of the anatomy of Fritillaria pellucida. The tail has been rotated 90° to showcase the single band of 10 interdigitated muscle cells (m) on the one side of the tail (other side not shown). a, sensory axons; c, caudal ganglion; cr, gill slit ciliary ring (spiracle); g, gonad; gl, gland cells; h, heart; m, muscle cells; n, notochord; o, esophagus; p, pharyngeal gland (endostyle); r, receptor cells of the lower lip; s, stomach; u, upper lip. Scale bar: 100 μm. a and b Adapted from adapted from Bouquet et al.  and Stolfi and Brown . c Adapted from Bone et al.