5. Among marine creatures the carabi and astaci cast their skins either in spring or autumn, after having deposited their ova; and carabi have been sometimes taken with a soft thorax, because their shell was ruptured, while the lower part, which was not ruptured, was hard. For the process is not the same in them as in serpents. The carabi remain in concealment for about five months. The crabs also cast their old skin, certainly those which have soft shells; and they say that those which have hard shells do the same, as the maia and graus. When they have cast their shells, the new shells are first of all soft, and the crabs are unable to walk. They do not cast their skins once only, but frequently. I have now described when and how animals conceal themselves, and what creatures cast their skin, and when they do so. CHAPTER XX. 1. Animals are not all in good health at the same season, nor in the same degrees of heat and cold. Their health and diseases are different at different seasons in various classes, and on the whole are not alike in all. Dry weather agrees with birds, both in respect of their general health and the rearing of their young, and especially with pigeons; and wet weather, with few exceptions, agrees with fish. On the contrary, showery weather generally disagrees with birds, and dry weather with fish; for, on the whole, abundance of drink does not agree with birds. 2. For the birds with crooked claws, generally speaking, as it was before remarked, do not drink. But Hesiod was ignorant of this circumstance; for in relating the siege of Nineveh he represents the presiding eagle of the augury drinking. Other birds drink, but not much; neither do any other oviparous animals with spongy lungs. The sickness of birds is manifest in their plumage; for it is uneven, and has not the same smoothness as when they are well. 3. The generality of fish, as it was observed, thrive the most in rainy years; for not only in such seasons do they obtain a greater supply of food, but the wet weather agrees with them as with the plants that grow on land; for potherbs, even if watered, do not grow so well as in wet weather. The same is the case with the reeds that grow in ponds; for they never grow, as we may say, except in rainy weather. 4. And this is the reason why so many fish migrate every summer into the Pontus; for the number of rivers which flow into it render the water fresh, and also bring down a supply of food, and many fish also ascend the rivers, and flourish in the rivers and lakes, as the amia and mullet. The cobii also become fat in the rivers; and on the whole, those places which have the largest lakes furnish the most excellent fish.