A more detailed account of the anatomy of _Hydra_ will be found in any biological text-book, for instance in Parker's Elementary Biology; but it is necessary here to say something more as regards the nettle-cells, which are of great biological and systematic importance. A nettle-cell of the most perfect type and the structures necessary to it consist of the following parts:-- (1) A true cell (the cnidoblast), which contains-- (2) a delicate capsule full of liquid; (3) a long thread coiled up in the capsule; and (4) a cnidocil or sensory bristle, which projects from the external surface of the cnidoblast. A nerve-cell is associated with each cnidoblast. In _Hydra_ the nettle-cells are of two distinct types, in one of which the thread is barbed at the base, whereas in the other it is simple. Both types have often two or more varieties and intermediate forms occur, but generally speaking the capsules with simple threads are much smaller than those with barbed ones. The arrangement of the nettle-cells is not the same in all species of _Hydra_, but as a rule they are much more numerous in the tentacles than elsewhere on the body, each large cell being surrounded by several small ones. The latter are always much more numerous than the former. CAPTURE AND INGESTION OF PREY: DIGESTION. The usual food of _Hydra_ consists of small insect larvæ, worms, and crustacea, but the eggs of fish are also devoured. The method in which prey is captured and ingested has been much disputed, but the following facts appear to be well established.