The _spleen_, which is substantially a lymphatic gland, may be mentioned here, as it is constantly situated in the immediate vicinity of the stomach, generally near its cardiac portion. With the exception of _Branchiostoma_, it is found in all fishes, and appears as a rounded or oblong organ of dark-red colour. In the Sharks frequently one or more smaller pieces are detached from the principal body. In the Dipnoi a thin layer of a very soft substance of brownish-black colour below the mucous membrane of the stomach and upper part of the intestine has been regarded as the homologue of the spleen (Fig. 57, _m_). In most _Teleostei_ the spleen is undivided, and appended by its vessels and a fold of the peritoneum to the pyloric bend of the stomach or the beginning of the intestine. [Illustration: Fig. 57.--Upper part of Intestine of Ceratodus. The anterior wall of the intestine is opened, the liver (_c_) and gall-bladder (_e_) being drawn forward. A slit is made at _n_, through which part of the next compartment of the spirally wound intestine may be seen. _é_, Mouth of ductus choledochus; _f_, stomach; _i_, adipose agglomeration; _l_, first compartment of intestinal spire; _m_, spleen; _oe_, lower part of œsophagus, opened; _p_, double pyloric fold; _q_ _q_, glandular patches.] CHAPTER IX. ORGANS OF RESPIRATION. Fishes breathe the air dissolved in water by means of gills or branchiæ. The oxygen consumed by them is not that which forms the chemical constituent of the water, but that contained in the air which is dissolved in water. Hence fishes transferred into water from which the air has been driven out by a high temperature, or in which the air absorbed by them is not replaced, are speedily suffocated. The absorption of oxygen by fishes is comparatively small, and it has been calculated that a man consumes 50,000 times more than is required by a Tench. However, some fishes evidently require a much larger supply of oxygen than others: Eels and Carps, and other fishes of similar low vitality, can survive the removal out of their elements for days, the small quantity of moisture retained in their gill-cavity being sufficient to sustain life, whilst other fishes, especially such as have very wide gill-openings, are immediately suffocated after being taken out of the water. In some fishes noted for their muscular activity, like the _Scombridæ_, the respiratory process is so energetic as to raise the temperature of their blood far beyond that of the medium in which they live. A few fishes, especially such as are periodically compelled to live in water thickened into mud by desiccation and vitiated by decomposing substances, breathe atmospheric air, and have generally special contrivances for this purpose. These are so much habituated to breathing air that many of them, even when brought into pure water of normal condition, are obliged to rise