In this position we might find digestive cells. Localised cell division would perhaps be required to ensure an adequate food supply. Local multiplication of cells in this area would make it larger, and it is possible that to fit in more cells the animal formed a dent with the new cells pushing upwards into the hole in the middle of the ball of cells and we might even find an inpushing, a cavity, a gut lined with digestive and absorbing cells. All cells would retain a cilium, which would be used for locomotion or for food transport. The size of both hydroids and planktonic animals was, and still is, limited by the effectiveness of the one cilium per cell set-up.
Cilia are small, and have a limited power output.
The next advance we see, in the plankton appears to be the formation of a band of compound cilia, which are more effective. A downstream band would give better swimming, allow an increase in body size and waft food particles towards the gut opening. The downstream ciliary collecting band or archaeotroch naturally goes around the opening of the gut. The least efficient feature of this animal is now the one way gut. Let us suppose that a series of secondary openings, gastrial pores, appeared around the apical organ.