Having practiced on jellyfish and sea squirts, I embark to design the alien chordate analog.It is clearly related to the active “salp”, but it does not use the jet propulsion, relying on swimming by undulation of somatic muscles instead. Its top channels are still pulsing water front to back through them, but their apertures are tiny and there are no valves. This is a structure dedicated to ventilation of gills; even its intakes are no longer in the same atrium as the digestive ones, because food particles would clog it. The thing also has a developed heart, on the dorsal side, associated with gills.
The bottom pair of channels is still digestive, with a gill rudiment increasing the digestive area. They are associated with the atrium, and the atrium still has a ring of in-facing bristles. But there is only one channel; the left channel has become a digestive gland, while the right one has been dedicated to absorption of nutrients. Only the one gonald associated with the right channel survives, and the right channel’s exhaust has relocated to the front of the left one’s and serves purely for egg laying. Here we have a beginning of highly broken symmetry.The creature has two “brains”, barely bulges on two neural cords: the ventral cord runs through the length of its body, while the dorsal one is much shorter. The ventral cord is mostly motor functions, while the dorsal one is primarily sensory and behavioral functions. (Its caudal end also controls heartbeat and breathing). It’s larger than the ventral one; in this fauna, the “brain” and the “spinal cord” are two discontinuous structures joined by twin neural tracts.
I have given the creature four simple eyes, just on a whim. The alien would retain the feature, eventually, even though outwardly it seems to have only two.