Comparative Biochemistry and Physiology Part A 133 (2002) 605–609 1095-6433/02/$ - see front matter  2002 Elsevier Science Inc. All rights reserved. PII: S1095-6433 Ž 02 . 00192-7 Studies on the cardiac physiology of Onychophora and Chilopoda  W. Hertel , C.S. Wirkner *, G. Pass a b, c Institut fur Allgemeine Zoologie und Tierphysiologie, Friedrich-Schiller-Universitat Jena, D-07743 Jena, Germany a ¨ ¨ Vergleichende Zoologie, Institut fur Biologie, Humboldt-Universitat zu Berlin, Philippstrasse 13, D-10115 Berlin, Germany b ¨ ¨ Institut fur Zoologie, Universitat Wien, A-1090 Vienna, Austria c ¨ ¨ Received 15 January 2002; received in revised form 10 May 2002; accepted 8 July 2002 Abstract Cardiac physiology in Onychophora and Chilopoda has not yet been studied by electrophysiological methods. Here we present ECGs and the first intracellular recordings from dorsal vessel muscle cells in Scolopendra cingulata, Lithobius forficatus and Peripatopsis sp. In Onychophora, a myogenic automatism generates the heart function. In the Chilopoda examined, heartbeat is triggered by a neurogenic automatism that is superimposed over a basic myogenic rhythm, which occurs under experimental conditions. The neuropeptide proctolin and the head peptide have no significant effects in the species investigated. The results are discussed from the viewpoints of comparative physiology and phylogenetics.  2002 Elsevier Science Inc. All rights reserved. Keywords: Circulatory organs; Heart; Neurogenic automatism; Myogenic automatism; Pacemaker potential; Proctolin 1. Introduction The variety of circulatory organs in arthropods with respect to both functional morphology and physiology is astonishing. While chelicerates, crus- taceans and insects have been studied in consid- erable detail (reviews by Jones, 1977; Miller, 1985; Pass, 2000; Richter, 1973; Watson and Groome, 1989; Wilkens, 1999), myriapods and onycophorans have received only little attention until recently. In Chilopoda, the circulatory organs are relative- ly complex (Wirkner and Pass, 2002). The vas-  This paper was originally presented at ‘Chobe 2001’; The Second International Conference of Comparative Physiology and Biochemistry in Africa, Chobe National Park, Botswana – August 18–24, 2001. Hosted by the Chobe Safari Lodge and the Mowana Safari Lodge, Kasane; and organised by Natural Events Congress Organizing (information@natural- events.com). *Corresponding author. Tel.: q49-30-2093-6006; fax: q49- 30-2093-6002. E-mail address: christian.wirkner@web.de (C.S. Wirkner). cular system is made up of two central longitudinal vessels, the dorsal vessel and the ventral vessel, which are connected in the first trunk segment by the maxilliped vessel arch. Numerous peripheral vessels branch off from these central vessels and supply the extremities and hemolymph sinuses in the body cavity. Both central longitudinal vessels have a muscular wall and are thereby capable of actively pumping hemolymph, while the peripheral vessels are non-contractile. In Onychophora, the circulatory organs are simpler: they consist of a dorsal vessel to which only antennal arteries are connected (Pass, 1991). Investigations on the physiology of the hearts of onychophorans and myriapods are rare. Both types of heart automatism have been described for myriapods on the basis of pharmacological criteria only: a heart with myogenic automatism in Diplo- poda and a neurogenic triggered heart in Chilopoda (Sundara Rajulu, 1966, 1967; Sundara Rajulu 1968). For Onychophora, Sundara Rajulu and Singh (1969) suggest a neurogenic automatism. A