We have thus far been dealing with snails whose shells were formed in a spiral coil. Quite a number of mollusks are not protected by such a shell, its place being taken by a flat, shield-like disk, or several distinct plates placed side by side. The most familiar of the first is the limpet or Patella, which is a depressed, conical, oval disk, looking not unlike a miniature shield. They live on rocks, to which they cling with great tenacity. The animal seems to have a pretty clear idea of local geography, for it invariably returns to the same place after its excursions for food and the rock in some localities has been hollowed out to a considerable depth by the continuous dwelling thereon of the limpet. The large foot is very strong and it is almost impossible to dislodge the shell from the rock when the animal becomes alarmed and is aware that danger is near. While grazing along the sides of a rock covered with fine sea-weed, it will leave a track like a worm and will clean off quite an area in a very short space of time. Another species is the key-hole limpet (Fissurella), distinguished by having a slit or foramen in the apex of the shell. The shells of Fissurella are generally rougher than those of Patella, and as a rule they live in warmer seas. In the limpet we find a departure from the general form of both animal and shell, both being bilaterally symmetrical, that is, having both sides alike. In the mollusks which have been presented thus far, the body has been twisted in the form of a spiral, making one side different from the other and causing the organs of one side to become atrophied. In the limpets the organs are paired, as they are supposed to have been in the ancestors of the living mollusks. The most peculiar of all the mollusks, so peculiar, indeed, that they constitute a separate order (Polyplacophora) are the Chitons, or coat-of-mail shells. The shell is made up of eight separate pieces or plates, each locking with the other, the whole supported by and buried in a coriaceous mantle which forms a margin all the way around. This must not be confounded with the true mantle of the animal, for it is only a part of the shell. It is beset with bristles, spines or hairs, which add much to the peculiar appearance of this mollusk. The Chitons live for the most part on rocks at low water and are said to be nocturnal in habit, feeding only at night. Their movements are slow and they appear to be very sluggish in all their actions. When detached and taken from their rocky homes they have the provoking (to the collector) habit of rolling up and are sometimes very difficult to straighten out again. There are about two hundred and fifty living species, found in all parts of the world.