6. The black substance is joined above to the origin of the teeth, this black substance is bitter and not eatable; in many animals there is either this substance or its analogue, for it is found in tortoises, toads, frogs, turbinated shells, and in the malacia; these parts differ in colour, but are entirely or nearly uneatable. The body of the echinus is undivided from beginning to end, but the shell is not so when seen through, for it is like a lantern, with no skin around it. The echinus uses its spines as feet, for it moves along by leaning upon them and moving them. CHAPTER VI. 1. The creatures called tethya[134] 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, nor the substance called mecon. [134] Ascidian mollusks. 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. 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 colour of the tethyon is partly ochreous, partly red. 4. The class acalephe[135] 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. [135] Actiniæ.