FIGURE 1.

(A) Illustration of a primary and a secondary receptor cell in tunicates. The primary receptor is a peripheral neuron, whose soma (indicated by the blue nucleus) is in the epidermis. The secondary receptor (orange) is, vice versa, a dedicated receptor that transmits its input to a brain sensory neuron. (B) Chordate evolutionary tree. * The monophyly of Phlebobranchia is disputed [see (DeBiasse et al., 2020)]. Stolidobranchia species are defined as Pleurogona (with gonads in the lateral body wall), whereas Phlebobranchia and Aplousobranchia are defined as Enterogona (with gonads close to the gut). (C) Adult individual of the ascidian Molgula socialis (right view). (D) Young colony of Botryllus schlosseri composed by two adult zooids and their buds. Dorsal view. (E) Ciona robusta larva at stage 28, 18 h post fertilization at 20°. Arrowheads: two anterior papillae. Ascidian larvae are composed of an anterior cephalenteron, i.e., a body part including both head structures (such as the brain) and trunk structures (such as the gut), and a posterior tail. The cephalenteron is usually called a “trunk”.