4. The other fish are seen both in their egress and ingress. The trichia is only seen as it enters, and is not observed to leave again; and if one is captured at Byzantium, the fishermen purify their nets, for it is unusual for them to return. The reason is this: these are the only fish that swim up into the Ister, and when this river divides they swim down into the Adriatic. The following is a proof; for the converse happens here, and they are never captured entering the Adriatic, but as they leave it. 5. The tunnies, as they enter, swim with their right side to the shore, and leave with their left side to the shore; and some persons say that they do this because they see better with their right eye, and their sight is naturally dim. The rhyades move during the day, and in the night remain quiet and feed, unless the moon is bright, in which case they continue their journey and do not rest themselves. And some persons engaged about the sea say that after the winter solstice they do not move, but remain quiet wherever they may be till the equinox. 6. The coliæ are taken as they enter, but not as they return. The best are taken in the Propontis before the breeding season. The other rhyades are captured more frequently as they leave the Pontus, and are then in perfection. Those that swim near the shore are the fattest when captured; and the farther they are away, the more lean they are; and frequently, when the south wind blows, they swim out in company with the coliæ and mackerel, and are taken lower down rather than at Byzantium. This is the nature of their migrations. CHAPTER XVI. 1. Land animals have also the same disposition for concealment. For in winter they all hasten to conceal themselves, and appear again when the season becomes warmer. Animals conceal themselves to guard against the excesses of temperature. In some the whole race is concealed; in others only a part of them. All the testacea conceal themselves, as those which are marine, the purpura, whelk, and all that class; but the state of concealment is more conspicuous in those which do not adhere to rocks; for these also conceal themselves, as the pectens. Some have an operculum on their exterior, as the land snails; and the alteration of those that are not free is inconspicuous. They do not all conceal themselves at the same period; for the snails are torpid during the winter, the purpura and whelk for thirty days under the dog star, and the pectens at the same period. Most of them conceal themselves in very cold and very hot weather.