1. The nature of the testacea is the same as that of creatures without shells, as the cnidæ[166] and sponges, which inhabit the holes in rocks. There are two kinds of cnidæ, some which live in holes in the rocks, and cannot be separated from them, and other migrating species which live upon the smooth flat surface of the rocks. (The patella also is free and locomotive.) In the interior of the sponges are found the creatures called pinnophylaces, and the interior is closed with a net like a spider's web, and small fish are captured by opening and closing this web, for it opens as they approach, and closes upon them when they have entered. [166] Actinia. 2. There are three kinds of sponges; one of them is thin, the other is thick, and the third, which is called the Achillean sponge, is slender, compact, and very strong; it is placed beneath helmets and thigh-pieces, for the sake of deadening the sound of blows; this kind is very rare. Among the compact kinds, those which are very hard and rough are called tragi. They all grow upon the rock or near the shore, and obtain their food from the mud. This is evident, for they are full of mud when they are captured. This is the case with all other fixed things, that they derive their food from the spot to which they are attached. 3. The compact species are weaker than those which are thin, because their point of attachment is smaller. It is affirmed that the sponge possesses sensation; this is a proof of it, that it contracts if it perceives any purpose of tearing it up, and renders the task more difficult. The sponge does the same thing when the winds and waves are violent, that it may not lose its point of attachment. There are some persons who dispute this, as the natives of Torona. The sponge is inhabited by worms and other living creatures, which the rock-fish eat when the sponge is torn up, as well as the remainder of its roots. But if the sponge is broken off, it grows again, and is completed from the portion that is left. 4. The thin sponges are the largest, and they are most abundant on the Lycian coast; the compact sponges are softer, and the Achillean are more harsh than the others. On the whole, those that inhabit deep places with a mild temperature are the softest, for wind and cold weather harden them, as they do other growing things, and stop their increase. For this reason the sponges of the Hellespont are rough and compact; and, altogether, those beyond Malea, and those on this side, differ in softness and hardness.