In Velella the pneumatophore becomes of complex structure and sends air-tubes, lined by a chitin and resembling tracheae, down into the compact coenosarc, thus evidently serving a respiratory as well as a hydrostatic function.
In their most primitive form they are seen in Velella as " gonosiphons," which possess mouths like the ordinary sterile siphons and bud free medusae.
Thus in Velella free medusae are produced, which have been described as an independent genus of medusae, Chrysomitra.
In forms with a compact coenosarc such as Velella, Physalia, &c., the separate cormidia cannot be sharply distinguished, and such a condition is described technically as one with " scattered " cormidia.
- Upper sur- sisting of a bract, siphon, tentacle and face of Velella, showing gonophore; when free it is known as pneumatophore and sail.