Peter Ax deceased on 2 May 1913. For more than 20 years, until 1993, he was editor of Zeitschrift für Morphologie der Tiere, later renamed Zoomorphologie, then Zoomorphology. We will remember him as an influential zoologist strictly using evolution as the common base of systematics, as an enthusiastic and stimulating teacher, and as an extraordinary scientist.

Peter Ax was born in Hamburg. After he finished the high school, he was forced to military service in the last days of World War II. Captured by the Russian Army, he once told me, he had tremendous luck to appear too weak and thus unfit for being transferred into captivity to Russia so that he finally was released into freedom. After having returned into the largely destroyed City of Hamburg, he decided to study biology at the University of Kiel in 1946. To decide freely what to do, he told me, was part of an immeasurable freedom he and his friends felt during the time after the war. It created an atmosphere of intense willingness to learn and to work hard at the subject of one’s choice. In Kiel, he soon joined the group of Adolf Remane, who was the leading German zoological systematist by that time. In Kiel, Adolf Remane and his group focussed on studying the biodiversity and ecology of the interstitial system of marine sediments in the Kiel Bight (Kieler Bucht), located in the Western part of the Baltic Sea. Within the scope of this research focus, Peter Ax studied the interstitial flatworm community, which marked the beginning of his continuous interest into free-living Platyhelminthes as well as into the interstitial realm in general throughout his entire scientific life. In 1950, he finished his Ph.D. thesis on the flatworm community in the Kiel Bight; this thesis was published 1 year later (Ax 1951). A postdoc position as assistant professor at the chair of Adolf Remane allowed him to extend his studies on the biodiversity, morphology, and ecology of interstitial flatworms to the Baltic Sea as well as to sandy beaches of the North Sea, the eastern Atlantic coast and the Mediterranean. At the marine biological station of Tvaerminne (Finland), he became familiar with the platyhelminths inhabiting brackish water environments. This special platyhelminth fauna remained one focus of his research and dominated his work particularly in the last period of active research (Ax 1959, 2008).

During the postdoc stage, he became a specialist for interstitial platyhelminths and an expert for the Otoplanidae, a group of proseriate platyhelminthes that inhabit high energy regions on beaches. In 1955, he submitted an impressive systematic revision and review on Otoplanidae as habilitation thesis and was rewarded receiving the venia legendi for zoology (i.e., the formal allowance by the University to teach this field of biology), a prerequisite to receive for a professor position. In the same year when his habilitation thesis was published (Ax 1956a), he described the first representative of a thus far unknown animal group, the Gnathostomulida, initially as a representative of a new platyhelminth taxon (Ax 1956b). His discovery of a new kind of animal organization made Peter Ax realize that the meiofauna houses a community of unexpected and largely unknown diversity with many new types of animal organization still left to discover, types of organization that could cast new light on animal evolution (Ax 1960). His publications in 1956 thus sketched and forecasted the three main areas of research he established during the following years, biodiversity of meiofaunal flatworms and other taxa, zoological systematics, and unravelling animal evolution by using morphological data.

In 1961, he accepted the chair of Systematic Zoology at the University of Göttingen. Although different German universities (Gießen, Kiel, Bochum) subsequently offered him the chair for Systematic Zoology, he stayed at the University of Göttingen, where he established a vivid and successful group that initially focussed on interstitial animals. He and a large group of highly committed students studied the meiofauna of the island of Sylt and in particular on a small beach soon called “Hausstrand”, which became one of the best studied beaches of the world. The studies on the interstitial fauna of the isle of Sylt revealed a tremendous diversity of meiofaunal organisms with numerous new species, several of them named in honour of Peter Ax. Also, these studies gave a detailed account on the thus far unknown biology and ecology of interstitial organism, particularly their migration, reproductive cycles and community structure (Ax 1966, 1969). In the early 70s, Peter Ax extended his studies onto the Galapagos Islands. Again, an enormous number of new species were described, and ecological parameters on the meiofaunal community were gathered. The geological history of the Galapogos Island caused him to reconsider general aspects of speciation and the evolution of meiofaunal organisms (Ax 1977). Most studies were published in the “Mikrofauna des Meeresbodens”, a journal series that was published by the Academy of Science and Literature in Mainz (Akademie der Wissenschaften und der Literatur, Mainz), from 1970 onward, with Peter Ax as chief editor. After 90 volumes, the name of the series changed into “Microfauna Marina”, and the journal’s scope was no longer restricted exclusively to the biodiversity and ecology of meiofaunal organisms, and also contained studies into the ultrastructure of interstitial organisms and other larger species.

Since all species descriptions depend on thorough morphological studies, Peter Ax always put a strong focus on comparative morphology and used these data for reconstruction of ancestral patterns in platyhelminthes (Ax 1961, 1963). He himself primarily used histology and light microscopy as main sources for describing and analysing the structural components of organisms, but the smallness of the interstitial organisms allowed adding electron microscopy to detailed analysis of the structure of organs by serial sectioning. During the 80s, Peter Ax and his group became well known for ultrastructural analyses of marine invertebrates. Systematic and comparative studies into morphology and ultrastructure always went along with the attempt to understand their evolution and adaptive value and to uncover the phylogeny of animals. At the end of the 70s, Peter Ax’s evolutionary interpretation of the results of comparative morphology was strongly influenced by the ideas of Willi Hennig.

Since then, Peter Ax became the most prominent advocate for phylogenetic systematics, a methodological approach that was already published by Willi Hennig (1950), when Peter Ax finished his Ph.D. thesis. Willi Hennig and his approach remained largely unknown until his work was translated and gained enormous impact on biological systematics in the USA (Hennig 1966), because it met and was combined with numerical taxonomy by Farris (1979). It was Peter Ax’s merit to have brought back systematization based on Hennig’s principles back to a wider audience in Germany (Richter 2013). In his clear, enthusiastic and sometimes provocative book on phylogenetic systematics (Ax 1984), he broke with the German tradition of systematization of the animal kingdom and especially with the widely accepted view of his academic teacher, Adolf Remane, on animal phylogeny. He wrote this book during a sabbatical, and I remember quite well the complete change of his lecture on animal systematics after this sabbatical. He did not use the categories of Linnean classification any longer, presented animal systematics as a hierarchical system of sister group relationships, and he argued against the use of taxon names for non-monophyletic groups. One of them was the “Turbellaria”, the group of Platyhelminthes he had scientifically worked on intensely since his Ph.D. thesis. Avoiding the categories of the Linnean classification and of non-monophyletic taxa later became a principal part of the editorial policy of the Zoomorphology.

Besides integrating phylogenetic systematics into his lecture, he trained and strongly influenced a new generation of systematists in phylogenetic systematics, by his books and by advocating phylogenetic systematics in scientific discussions and conference talks. Here, his clear and stringent argumentation in favour of phylogenetic systematization fascinated his audience followers. Ulrich Ehlers and Peter Ax exemplified this approach for the Platyhelminthes (Ehlers 1985). Hypothesizing a sister group relationship between Platyhelminthes and Gnathostomulida, Peter Ax once again returned to that group, he once described as a new type of metazoan organization (Ax 1985). Although this sister groups relationship could not be confirmed by subsequent studies, at least a common ancestry of Platyhelminthes and Gnathostomulida with other taxa of a clade called Gnathifera is still under discussion (Hankeln et al. 2014). In 1988, Peter Ax published a smaller student book on phylogenetic systematics, where he included some of the philosophical foundations on natural entities in living matter which are the rational base for any systematization of nature (Ax 1988). Considerations on including an epistemological backbone into phylogenetic systematics were central topics of several publications during his last years as active professor.

During his annual lecture on “The phylogenetic system of animals”, which lasted two terms, he presented phylogenetic relationships of animals in such a fascinating manner that his research group attracted a large number of students. I remember quite well that very moment during his lectures when a clear evolutionary structure in systematics became visible to me and I no longer felt helplessly lost in a system that structured diversity by categorical ranks. After retirement, he summarized his knowledge, recent studies and the theoretical background of phylogenetic systematics in three volumes of a book on multicellular animals, the Metazoa (Ax 1995, 1999, 2001; English translation Ax 1996, 2000, 2003). This book was the first attempt to unravel the animal phylogeny based on morphological data and to present it without using the traditional terms of the Linnean classification, but by presenting the hierarchy as an encaptic system of sister group relationships. The book illustrates the value of morphological data for phylogeny inference. Some of his hypotheses are still valid and have been confirmed by molecular approaches, and others were recently confirmed after a period of conflicting evidence that later turned out to result from bioinformatics problems or incomplete data sets.

Biodiversity of free-living interstitial platyhelminths, however, always remained a central focus in the work of Peter Ax, and his last paper was the description of three new species (Ax 2011). During the last years prior to retirement, he had begun to focus again on platyhelminths of the brackish water and became familiar with while working on the meiofaunal platyhelminths of the Baltic Sea at Tvaerminne marine biological station in Finland. I still remember him returning from a field trip to New Brunswick (Canada) where he had studied the platyhelminth meiofauna, highly excited because he found individuals of Coronhelmis lutheri (Ax 1951), a brackish water species he described in his Ph.D. thesis. The high degree of identity of brackish water platyhelminths between Baltic Sea and the East Canadian coast led him to hypothesize an amphiatlantic distribution of this fauna (e.g. Ax 1959; Ax and Armonies 1987, 1990). In the following years, he substantiated this hypothesis during a series of field trips to Iceland, Faroer Islands, Greenland, Alaska, South Carolina and other places that he undertook shortly before and after retirement. During the last years, he summarized his experience and enormous expertise in a large book on the brackish water Platyhelminthes of the Northern hemisphere (Ax 2008), a book he was proud of, because it summarized a field of research that kept him busy his entire scientific life, the biodiversity and morphology of interstitial platyhelminths.

During his time as a professor, Peter Ax created a vivid and stimulating research environment that offered the opportunity to work independently on a given topic. He never exerted pressure on any group member to come over with results, but warranted sufficient time to validate data and to create new ideas he always was willing to discuss. He was fairly open-minded to change his views at convincing evidence (Ax 1989). He could create such an atmosphere, because he always kept his own area of research continuously funded by the Academy of Science and Literature in Mainz and did not necessarily depend on the results of his co-workers. After retirement, he kept on working at the institute in Göttingen, focussing on research and writing books, free of any administrative duties and the necessity to teach, and enjoyed to share his ideas with his former students. I am grateful for having had the opportunity to know him, to work with him, and to perform research in his group. The memory of Peter Ax as an extraordinary comparative zoologist will last a long time.