ice, adhering to the coast. The Sea of Azof has frequently disappeared beneath a thick crust of ice, and even the whole of the Black Sea has been frozen over in winters of exceptional severity. The cold surface waters, together with those conveyed into the Black Sea by large rivers, descend to the bottom, and prove most detrimental to animal life. Echinodermata and zoophytes are not met with at all in the Black Sea; certain classes of molluscs, already rare in the Levantine Sea and the Archipelago, are likewise absent; and the total number of species of molluscs is only one-tenth of what it is in the Mediterranean. Fish are numerous as far as individuals go, but their species are few. In fact, the fauna of the Black Sea appears to resemble that of the Caspian, from which it is cut off, rather than that of the Greek seas, with which the Sea of Marmara connects it. In addition to the species which have found a second home in the Mediterranean, there are some that must still be looked upon as visitors. Such are the sharks, which extend their incursions to the seas of Sicily, to the Adriatic, and even to the coasts of Egypt and Syria. Such, also, are the larger cetacea—whales, rorquals, and sperm whales—whose visits, however, are confined now to the Tyrrhenian {30} basin, and become less frequent from century to century. The tunny-fish of the Mediterranean are also visitors from the coasts of Lusitania. First-rate swimmers, they enter through the Strait of Gibraltar in spring, ascend the whole of the Mediterranean, make the tour of the Black Sea, and return in autumn to the Atlantic, after having accomplished a journey of some 5,600 miles. In the opinion of the fishermen the tunnies go upon their travels in three immense divisions or shoals, and it is the central shoal which visits the coasts of the Tyrrhenian Sea, and consists of the largest and strongest fish. Each of the three divisions appears to be composed of individuals about the same age. For mutual protection they swim in troops, for they are preyed upon by enemies innumerable. Dolphins and other fish of prey follow their track, but their great destroyer is man. In the summer the tunny fishery, or _tonnaro_, is carried on in numerous bays of Sicily, Sardinia, Naples, and of Provence. Enormous structures consisting of nets enclose these bays, and they are ingeniously arranged so as to close gradually around the captured fish, which, passing from net to net, find themselves at last in the “chamber of death,” where they are massacred. Millions of pounds of flesh are annually obtained from these floating “slaughter-houses,” yet the tunny appears year after year in multitudes, and on the same coasts. There may have been a slight decrease in the number, but their closely packed masses still invade the “Golden Horn” of Byzance and other bays, as they did when first they attracted the attention of Greek naturalists. [Illustration: Fig. 7.—THE PRINCIPAL FISHERIES OF THE MEDITERRANEAN.