FIG. 15.--Sense-organ of _Limnocodium_ 97 FIG. 16.--The Freshwater Jelly-fish of Lake Tanganyika 98 FIG. 17.--Sir Harry Johnston’s specimen of the Okapi 99 FIG. 18.--Bandoliers cut from the striped skin of the Okapi 99 FIG. 19.--Skull of the horned male of the Okapi 100 FIG. 20.--The metamorphosis of the young of the common Eel 101 FIG. 21.--A unicellular parasite of the common Octopus, producing spermatozoa 102 FIG. 22.--The _Coccidium_, a microscopic parasite of the Rabbit, producing spermatozoa 102 FIG. 23.--Spermatozoa of a unicellular parasite inhabiting a Centipede 103 FIG. 24.--The motile fertilizing elements (antherozoids or spermatozoa) of a peculiar cone-bearing tree, the _Cycas revoluta_ 104 FIG. 25.--The gigantic extinct Reptile, _Triceratops_ 106 FIG. 26.--A large carnivorous Reptile from the Triassic rocks of North Russia 107 FIG. 27.--The curious fish _Drepanaspis_, from the Old Red Sandstone of Germany 107 FIG. 28.--The oldest Fossil Fish known 108 FIG. 29.--The skull and lower jaw of the ancestral Elephant, _Palæomastodon_, from Egypt 109 FIG. 30.--The latest discovered skull of _Palæomastodon_ 110 FIG. 31.--Skulls of _Meritherium_, an Elephant ancestor, from the Upper Eocene of Egypt 111 FIG. 32.--The nodules on the roots of bean-plants and the nitrogen-fixing microbe, _Bacillus radicola_, which produces them 114 FIG. 33.--The continuity of the protoplasm of vegetable cells 116 FIG. 34.--Diagram of the structures present in a typical organic ‘cell’ 117 FIG. 35.--The Number of the Chromosomes 119 FIG. 36.--The Number of the Chromosomes 120 FIGS. 37 to 42.--Phagocytes engulphing disease germs--drawn by Metschnikoff 136-7 FIG. 43.--A Phagocyte containing three Spirilla, the germs of relapsing fever, which it has engulphed 137 FIG. 44.--The life-history of the Malaria Parasite 142 FIG. 45.--The first blood-cell parasite described, the _Lankesterella_ of Frog’s blood 144 FIG. 46.--Various kinds of Trypanosomes 145