All organ systems of mesothelid spiders are revised and supplemented with new information. This concerns particularly the eyes, which were studied by TKM comparing the data with previous results. Based on ultrastructure, histology and experimental observations a hypothesis on vision in heptathelid spiders is proposed. The ultrastructure of the mesothelid tarsal organ and the trichobothria is described for the first time. Both organs are equipped with numerous sensory cells. The respiratory capabilitiy of mesothelids is investigated and discussed in relation to mygalomorph and araneomorph spiders, which represent the two major taxa of opisthothelid Araneae. A special chapter is devoted to the morphology and function of the male palpal organ which is compared to respective organs in mygalomorph and araneomorph spiders. Its function is explained using morphological and experimental data. Ecological aspects deal with the habitats of Mesothelae and especially microbial diseases. In the chapter on taxonomy current problems concerning mesothelid species are critically discussed from a general zoological perspective.