8. Again, there are some creatures which are stationary, while others are locomotive; the fixed animals are aquatic, but this is not the case with any of the inhabitants of the land. Many aquatic animals also grow upon each other; this is the case with several genera of shell-fish: the sponge also exhibits some signs of sensation, for they say that it is drawn up with some difficulty, unless the attempt to remove it is made stealthily. Other animals also there are which are alternately fixed together or free, this is the case with a certain kind of acalephe; some of these become separated during the night, and emigrate. Many animals are separate from each other, but incapable of voluntary movement, as oysters, and the animal called holothuria.[6] Some aquatic animals are swimmers, as fish, and the mollusca,[7] and the malacostraca, as the crabs. Others creep on the bottom, as the crab, for this, though an aquatic animal, naturally creeps. [6] Perhaps some species of Zoophyte. [7] Cephalopods. 9. Of land animals some are furnished with wings, as birds and bees, and these differ in other respects from each other; others have feet, and of this class some species walk, others crawl, and others creep in the mud. There is no animal which has only wings as fish have only fins, for those animals whose wings are formed by an expansion of the skin can walk, and the bat has feet, the seal has imperfect feet. Among birds there are some with very imperfect feet, which are therefore called apodes; they are, however, provided with very strong wings, and almost all birds that are similar to this one have strong wings and imperfect feet, as the swallow and drepanis;[8] for all this class of birds is alike both in habits and in the structure of their wings, and their whole appearance is very similar. The apos[9] is seen at all times of the year, but the drepanis can only be taken in rainy weather during the summer, and on the whole is a rare bird. [8] Perhaps Sand martin. [9] Swift. 10. Many animals, however, can both walk and swim. The following are the differences exhibited by animals in their habits and their actions. Some of them are gregarious, and others solitary, both in the classes which are furnished with feet, and those which have wings, or fins. Some partake of both characters, and of those that are gregarious, as well as those that are solitary, some unite in societies and some are scattered. Gregarious birds are such as the pigeon, stork, swan, but no bird with hooked claws is gregarious. Among swimming animals some fish are gregarious, as the dromas,[10] tunny, pelamis,[11] amia.[12] [10] Some migratory fish. [11] A kind of tunny, still called palamyde at Marseilles. [12] A kind of tunny, Les Bonitons (Camus.)