Arthur Dendy, zoologist, was the son of John Dendy, silk manufacturer, and his wife Sarah, born Beard. He was educated at Owens College, Manchester, obtaining the degrees Bachelor of Science (1884) and Master of Science (1887). He first joined the editorial staff compiling the reports of the Challenger Expedition and then worked as an assistant in the Zoological Department of the British Museum (Natuarl History) in 1887, studying sponges. From 1888 to 1894 he was demonstrator and assistant lecturer in biology at the University of Melbourne, Australia. During this period, in 1891, he obtained the degree Doctor of Science (DSc) at Victoria University, Manchester. He published ten major papers on the anatomy and systematics of southern Australian sponges, in which he described 87 new species. This research laid the groundwork for his subsequent complete revision of the phylum Porifera, on which he became a world authority.
In 1894 Dendy became professor of biology at Canterbury College, University of New Zealand. From 1886 to 1900 he published about 65 scientific papers on systematic zoology and comparative anatomy. In addition to his work on sponges these dealt with, among others, Australian species of Peripatus, land planarians of Victoria (Australia), and land planarians of New Zealand.
Early in 1903 Dendy arrived in South Africa to fill the newly created chair of zoology at the South African College, Cape Town. That same year the University of the Cape of Good Hope admitted him to the DSc degree on the basis of his DSc from Victoria University. He delivered his inaugural lecture on 11 August 1903 on "The study of animal life, with illustrations from the fauna of New Zealand". In 1904 he and several other professors at the college successfully applied for recognition as extra-mural teachers by the Universities of Aberdeen, Edinburgh and Glasgow, so that the college would be in a position to present a recognized first year education for medical students. He also served on a committee which investigated and reported on the future of university education in the Cape Colony. Joining the South African Association for the Advancement of Science in 1903, he served on its publication committee that year, and as secretary to the committee for Section B. At the (first) annual meeting of the association in Cape Town that year he read a paper on "The nature of heredity", in which he expressed himself tentatively in favour of the inheritance of body modifications acquired during an organism's lifetime.
Dendy left the South African College in May 1905 to take up an appointment to the chair of zoology in King's College, University of London. Before leaving he left a number of books to the College library and to the Departments of Botany and Geology. In later years he wrote Outlines of evolutionary biology (1912, with later editions to 1938), and The biological foundations of society (1924). He was elected a Fellow of the Linnaean Society, the Zoological Society of London, and the Royal Society of London, and became a member of the British Assiciation for the Advancement of Science in 1902. In December 1888 he married Ada Margaret Courtauld, with whom he had three daughters and a son.