The animal holds on tightly to the rocks by its large creeping disc-like foot, but may be removed without injury by forcing a knife-blade under the margin of its shell. When examined it will be found that it has not a well-formed head like the majority of the gasteropods, and both eyes and tentacles are wanting. The gills form a series of lamellæ round the posterior end of the body, between the edge of the foot and the mantle; and it is interesting to note that the Chitons further justify the low position assigned to them among the gasteropods by their possession of a simple, central, tubular heart, similar to that of worms. Perhaps the commonest of the British species is _Chiton cinereus_. Its colour is a dull grey, but the ground is variously mottled, often in such a manner as to give it a protective resemblance to its surroundings. _C. ruber_ is the largest of our species: its shell is variously mottled with shades of yellow and brown; _C. fascicularis_ is bristled. Another rather common species (_C. lævis_) is distinguished by the glossy appearance of the dorsal portion of the shell. It will have been observed that the chitons differ from the majority of gasteropods in that their shells and bodies are both bilaterally symmetrical, and the same is true of the next family--_Dentaliadæ_, which derive their name from the tooth-like form of their conical shells. They are popularly known as the Tooth Shells, and although they generally live beyond low-water level, they may sometimes be seen alive on the beach, and the empty shells are often washed up by the waves. The shells (fig. 165) are curved, and open at both ends, the narrower extremity being the posterior. The mouth is circular, and the outer surface is quite smooth or grooved. [Illustration: FIG. 164.--CHITON SHELLS] [Illustration: FIG. 165.--SHELLS OF _Dentalium_] In these animals, too, the head is imperfectly formed, without eyes or tentacles. The foot is conical and pointed, with two symmetrical side lobes; and the gills, also two in number, are symmetrically disposed. The margin of the mouth is fringed, and the animal is attached to the shell near the posterior end. The _Dentaliadæ_ are carnivorous, subsisting on minute molluscs, foraminifera, &c., and generally live on sandy or muddy bottoms, in which they sometimes bury themselves. Our next family includes the familiar Limpets, and is designated _Patellidæ_ on account of the resemblance of the conical shell to a little dish. In these the apex of the cone is not central, but situated more or less towards the anterior; and the muscular impression within is shaped like a horseshoe, with its open end turned to the front.