The dynamics of an incompressible turbulent air jet from an annular source into otherwise quiescent surroundings are investigated. Focussing on a slender annulus with an open core, the development of the jet is examined using planar and stereoscopic particle image velocimetry (PIV) from the source to 16 (outer) source diameters downstream. Unique to these annular sources, the jet induces a flow through the open core - in other words, the core is `ventilated'. Our measurements indicate that the volume flux drawn through the core exceeds that from the source by approximately 20%. Based on the streamwise development of the jet, we identify four distinct regions: (i) an internal region of induced flow that is bounded by the jet; (ii) a near-field planar-jet-like region; (iii) a transition region; and, ultimately, (iv) a far- field round-jet-like region. We explore the evolution of the jet towards self-similar behaviour based on cross-stream profiles of time-averaged velocity and turbulence statistics within these different regions. Four distinct length scales are shown to characterise the streamwise extents and behaviours within the regions identified. Finally, to highlight the role of the open-core (ventilated) annular geometry on jet development, comparisons are made with PIV measurements of turbulent jets issuing from circular sources. These comparisons reveal that the ventilated geometry significantly enhances dilution in the near field.