Oxygen and Early Arthropod Micro-Benthos Evolution Figure 3. Chronostratigraphy for the Cambrian. Correlation of regional stratigraphies (modified from [58]) provides the key to understanding the bradoriid and phosphatocopid ranges reconstructed for Figure 4. doi:10.1371/journal.pone.0028183.g003 vertical migration of Vargula, [35]). Experiments with Cylindroleberis mariae, which possesses a system of gas transfer (ventilatory system and a cardiovascular system - dorsal heart and hemolymph network within the valves and well-developed book-gills), also demonstrated the inability of myodocopid ostracods to regulate their ventilatory and circulatory activities [31]. However, their PLoS ONE | www.plosone.org 4 December 2011 | Volume 6 | Issue 12 | e28183