6. This is evident in castrated animals; for if a small portion only of the body is destroyed, the animal becomes a female; so that it is plain that if a very minute portion in the original composition of an animal becomes changed, if that portion belongs to the origin of the species, it might become either male or female; or, if taken away altogether, the animal might be neuter. And so, either way, it might become a land or aquatic animal, if only a small change took place ... it happens that some become terrestrial and others aquatic animals, and some are not amphibious which others are, because in their original generation they received some kind of substance which they use for food. For that which is natural is agreeable to every animal, as I have said before. CHAPTER III. 1. When animals are divided in three ways into aquatic and land animals, because they either breathe air or water, or from the composition of their bodies; or, in the third place, from their food, their manner of life will be found to agree with these divisions. For some follow both the composition of their bodies and the nature of their food, and their respiration of either water or air. Others only agree with their composition and food. 2. The testacea which are immoveable live by a fluid which percolates through the dense parts of the sea, and being digested because it is lighter than the sea water, thus returns to its original nature. That this fluid exists in the sea, and is capable of infiltration is manifest, and may be proved by experiment; for if anyone will make a thin waxen vessel, and sink it empty in the sea, in a night and a day, it may be taken up full of water, which is drinkable. 3. The acalephe (actinia) feeds upon any small fish which may fall in its way. Its mouth is placed in the centre of its body. This organ is conspicuous in the larger individuals: like the oyster, it has a passage for the exclusion of its food, which is placed above. The acalephe appears to resemble the internal part of the oyster, and it makes use of the rock, as the oyster does of its shell. (The patella also is free, and wanders about in search of food.) 4. Among the locomotive testacea, some are carnivorous, and live on small fish, as the purpura, for this creature is carnivorous, it is therefore caught with a bait of flesh: others live upon marine plants. The marine turtles live upon shell-fish, for which purpose they have a very powerful mouth; for if any of them take a stone or anything else, they break and eat it. This animal leaves the water and eats grass. They often suffer and perish, when they are dried up as they float on the surface, for they are not able to dive readily.