Aristotle [ca. 350 BCE] (De animalibus, Book 4, 6.1-5): [1] [Book 4, 1.1] The creatures called tethya have a most distinct character, for in these alone is the whole body concealed in a shell. Their shell is intermediate between skin and shell, so that it can be cut like hard leather: this shell-like substance is attached to rocks; in it there are two perforations, quite distant from each other, and not easily seen, by which it excludes and receives water, for it has no visible excrement as other testacea, neither like the echinus [sea urchin], nor the substance called mecon. [Book 4, 1.2] When laid open, there is first of all a sinewy membrane lining the shell-like substance, within this the fleshy substance of the tethyon. Unlike any other creature, its flesh, however, is alike throughout, and it is united in two places to the membrane and the skin from the side, and at its points of union it is narrower on each side; by these places it reaches to the external perforations which pass through the shell; there it both parts with and receives food and moisture, as if one were the mouth, the other the anus, the one is thick, the other thinner. [Book 4, 1.3] Internally there is a cavity at each end, and a passage passes through it; there is a fluid in both the cavities. Besides this, it has no sensitive or organic member, nor is there any excrementitious matter, as I said before. The color of the tethyon is partly ochreous, partly red. [Book 4, 1.4] The class acalephe is peculiar; it adheres to rocks like some of the testacea, but at times it is washed off. It is not covered with a shell, but its whole body is fleshy; it is sensitive, and seizes upon the hand that touches it, and it holds fast, like the polypus does with its tentacula, so as to make the flesh swell up. It has a central mouth, and lives upon the rock, as well as upon shell-fish, and if any small fish falls in its way, it lays hold of it as with a hand, and if any eatable thing falls in its way it devours it. [Book 4, 1.5] One species is free, and feeds upon anything it meets with, even pectens and echini; it appears to have no visible excrement, and in this respect it resembles plants. There are two kinds of acalephe, some small and more eatable, others large and hard, such as are found near Chalcis. During winter their flesh is compact, wherefore in this season they are caught and eaten; in summer time they perish, for they become soft; if they are touched they soon melt down, and cannot by any means be taken away. When suffering from heat, they prefer getting under stones. - [Cresswell translation, 1887]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.45): Kylos is a fish of the sea, as Aristotle says, and it is attached to stones. The shell in which it hides is like a vessel and is very rough. The form of his body is like flesh. This type of fish senses anything that approaches it. It clings to stones with its hind feet, and with its fore feet, which it uses as if for hands, whatever fish passes over it becomes its prey. But if something passes which it believes it cannot overcome, it swells and contracts and clings more strongly to the stone to which it is attached. Its mouth is in the middle of the body. There are two kinds of this fish: one has a small body and is eaten, and for this reason men hunt it in the winter and it is eaten in the summer, for they are preserved for a long time by being infused with salt. The second kind is large and distinguished by whiteness; but it is so pestilential, that if any one touches it during the heat of summer, its form dissolves and deteriorates. - [Badke translation/paraphrase]