Figure 12

Scenario of phoronid body plan evolution. Different elements of the nervous system are indicated by different color: orange – apical organ, yellow – cerebral ganglion; cyan – the commissure, blue – nerve tract, which innervates the tentacles, pink – neurites and perikarya of the nerve plexus. (A) Hypothetical pelagic ancestor with tentacles and ciliated epidermis. (B) Creeping ancestor. (C) The formation of ventral pouch, which is exploited as a tool to dig into soft substrata to avoid danger. The apical organ undergoes the reduction. (D) Appearance of actinotroch larva. Abbreviations: ao – apical organ; cg – cerebral ganglion; dao – degenerating apical organ; ep – epistome; gp – dorsolateral groups of perikarya; m – mouth; t – tentacle.