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. 2015 Dec 1;92(2):161–172. doi: 10.1007/s12565-015-0317-7

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© The Author(s) 2015

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Fig. 1a,b — Two types of excretory system in metazoans. a Protonephridia. The primary urine-producing apparatus (PUPA) is formed by terminal cells and is connected directly to the modulating tubule. Protonephridia are found in the interstitial space. b Metanephridial system. The metanephridium contains only the modulating tubule. A part of the mesothelium is altered into podocytes to form the PUPA. The term “metanephridial system” is used to indicate the assemblage of the podocyte-based PUPA and metanephridium. These schematic drawings are based on Ruppert and Smith (1988)