The animal which I formerly named an _Echinorhynchus_, and to which I even applied the term _Porocephalus_, appears, on a closer inspection, according to Rudolphi’s better grounded opinion, to belong to the division of _Pentastoma_.[KN] It is found in the abdominal cavity and the wide-celled lungs of a species of _Crotalus_, which, in Cumana, occasionally infests even the interior of houses, and preys on mice. The _Ascaris lumbrici_[KO] lives beneath the skin of the common earth-worm, and is the smallest of all the species of Ascaris. _Leucophra nodulata_, Gleichen’s pearl animalcule, has been observed by Otto Friedrich Müller in the interior of the reddish _Nais littoralis_.[KP] It is probable that these microscopic animals are, in their turn, inhabited by others. All are surrounded by air, deficient in oxygen, and copiously charged with hydrogen and carbonic acid. It is extremely doubtful whether any animal could exist in _pure nitrogen_, although such an opinion did, formerly indeed, seem warranted with reference to Fischer’s _Cistidicola farionis_, since, according to Fourcroy’s experiments, the swimming-bladder of fish was presumed to contain air wholly devoid of oxygen. But the experiments made by Erman, and confirmed by myself, prove that the swimming-bladder of fresh-water fish never contains pure nitrogen.[KQ] In sea fish as much as 0·80 parts of oxygen have been found, while, according to Biot’s views, the purity of the air depends on the depth at which the fishes live.[KR] Footnote 77: p. 214—“_The united Lithophytes_.” According to Linnæus and Ellis the calcareous Zoophytes, (among which Madrepores, Meandrinæ, Astrææ, and Pocilloporæ especially produce mural coral-reefs,) are inhabited and invested by animalcules, which were long supposed to be allied to the Nereids belonging to Cuvier’s Annelida (jointed worms). The anatomy of these gelatinous animalcules has been made known by the acute and comprehensive researches of Cavolini, Savigny, and Ehrenberg. We have learned that, in order to understand the whole organism of the (so-called) rock-building animals, we must not consider the scaffolding which remains after their death, namely, the layers of lime formed into delicate lamellæ by a vital function of secretion, as foreign to the soft membranes of the food-receiving animal. Besides our increased knowledge of the wonderful formation of the living coral-stocks, a more correct view has gradually gained ground respecting the extensive influence which the coral world has exercised on the appearance of low island groups above the level of the sea, on the migration of land-plants, and the successive extension of the domain of the Floras, and, indeed, in some parts of the ocean, on the distribution of the human race and of languages.