While he was at Lanzarote he began to take an interest in a second group of lowly animals besides the siphonophores, namely the sponges. When the general reader hears the word “sponge” he must modify his ordinary ideas a little. In the present instance he must not think of the plants, belonging to the fungi-group, such as the morel and cognate forms, that are often called “sponges” in common parlance. He must think rather of the sponge he uses in his bath. The bath-sponge is a structure made up of very tough, elastic, horny fibres. This structure is originally the skeleton, as it were, of certain animals that are known as “sponge-animals” or, briefly, sponges; they have nothing to do with the spongy mushrooms I spoke of. At the same time these socially-living sponges are such curious creatures that it was disputed for a long time whether they were real animals or not. There was a second controversy in regard to them as to where the “individual” began—what was a single animal, and what a co-operative colony of animals. The latter point alone would have been enough to direct Haeckel’s attention to this group after he had, in the case of the siphonophores, gone so deeply into the mystery of combined individuals, forming a new “state-individual.” His own opinion eventually was that as a matter of fact in the majority of cases the whole sponge is a stock or colony of separate sponge individuals closely connected together. They had not, indeed, anything like the ingenious method of division of labour that we find in the social medusæ; in fact, the sponges are in all respects much more lowly organised animals than the medusæ. But they were certainly true animals. And in the middle of his efforts to prove this Haeckel travelled into an entirely new field of research, lying far beyond the theory of individuality. As there is an enormous number of different sponges, he had confined his studies from the first to a single group of them that might be taken as typical. He chose the calcispongiæ (calcareous sponges), which had been the least studied up to that time. As the name obviously implies, these sponges form their internal framework or skeleton, not of elastic horny fibres like the common bath-sponge, but of solid calcareous needles or spines. They secrete these out of the soft substance of their bodies just as the radiolaria do their pretty siliceous houses. Haeckel was engaged for five years, from 1867 to 1872, in a profound and careful study of the natural history of the calcispongiæ. Then he published the results in his _Monograph on the Calcispongiæ_, consisting of two volumes of text and an atlas of sixty fine plates.