When the English Government came to publish the results of the _Challenger_ expedition in a monumental work (of fifty volumes), he was entrusted with the work on the siphonophores, the corneous sponges, and all the radiolaria in the collection. For ten years, from 1877 to 1887, Haeckel devoted every available hour to the work of selecting the radiolarian shells with his microscope from these specimens of the deep-sea deposits, and naming, describing, and drawing the new species. When he began his task 810 species of radiolaria were known to science. When he came to his provisional conclusion, ten years afterwards, though his material was not yet exhausted, there were 4,318 species and 739 genera. They are described in the splendid work that he wrote for the _Challenger_ Report. It consists of two volumes of text (in English) with 2,750 pages and 140 large plates, with the title, _Report on the Radiolaria collected by H.M.S. Challenger_. In the preparation of these plates (and in the illustration of all his later works) he had the very valuable assistance of the gifted Jena designer and lithographer, Adolph Giltch. A good deal of new information with regard to the living body of the radiolaria had come to light since 1862. In particular it had now been settled beyond question that they consisted merely of a single cell. There was, therefore, a good opportunity of reconstructing the _Monograph_ of 1862 with the new and more comprehensive work. The chief contents of the English work (with a selection of the plates) were then published in German, and appeared in 1887 and 1888 as the second, third, and fourth parts of the _Monograph on the Radiolaria_. A sort of supplementary essay on the methods of studying the radiolaria and cognate “plancton” animals was published separately with the title of _Planctonic studies_ (1890). Though it was a moderate and tactful criticism of the methods of some of his colleagues in this kind of work, it was “refuted” by them in a way that it would be difficult to qualify—in other words, it was fruitlessly assailed with charges of the most general but most unpleasant character. In the English Report we find two other volumes afterwards from Haeckel—the volume on the siphonophoræ in 1888, and the _Report on the Deep-sea Keratosa collected by H.M.S. Challenger_ in 1889; these again opened up new chapters in zoology. The _Challenger_ work is the crown of Haeckel’s studies as a specialist. To some extent the conclusion of it closes an epoch in his life. We will only touch briefly on what he has done since. It has not yet passed into the region of history.