Although we have to deal principally with the species that belong more or less to the shore--the _littoral_ fishes--we should like to refer briefly to one or two interesting features of those that live at great depths. It will be readily understood that much light is lost as the rays penetrate into deep water, so that the bottoms of deep seas must be more or less darkened. To allow for this loss, we find that the species living at moderate depths are provided with larger eyes to enable them to see their prey and their mates; but at still greater depths, where the sun’s light cannot penetrate, the fishes are either blind, or are possessed of luminous organs which enable them to see their way. Again, as the sea is so thinly populated at such great depths, the carnivorous species do not find abundant food always at hand, hence they are often provided with such mouths and stomachs as will allow them to make the best of favourable opportunities, some being capable of swallowing a fish quite as large as themselves. We often find fishes roughly classified into fresh-water and salt-water species, and although such a division is at times convenient, it must be remembered that some of the former migrate into brackish and even into salt water, while some of the latter ascend estuaries and rivers either for the purpose of obtaining suitable food, or for the deposition of their eggs. The fishes that frequent our coasts may be classified into two main groups, those with cartilaginous skeletons (_Elasmobranchii_), and the bony fishes (_Teleostomi_). Both these are divided into family groups, and we shall deal more or less briefly with all the important families that include common British marine fishes, but giving more attention to those species that are truly littoral in habit--species that may be found in the rock pools or under stones at low tide, and which may be obtained by the amateur angler working from rocks, piers, &c. The cartilaginous fishes include the Sharks, Dogfishes, and Rays. They have pouchlike gills, five or more on each side, each one opening to the exterior by a separate slit. The skin generally contains bony elements that are toothlike in structure and often in form; the mouth is usually on the under side of the head, and the tail is nearly always of the heterocercal kind. They are all carnivorous creatures, and often exceedingly voracious; and are represented in our seas by the Rays and Dogfishes. Rays or Skates (family _Raiidæ_), of which there are six or seven British species, are readily known by their broad flattened rhomboidal bodies, with the mouth on the under side of the head, a longitudinal fold on each side of the tail, and pectoral fins extending quite or nearly to the front point of the head.