THE CHITONS COAT-OF-MAIL SHELLS - ORDER POLY PLACOPHORA. Shell composed of eight overlapping plates, supporter by a muscular, leathery girdle, which extends beyond the plates and folds under, forming the margin of the convex body shield; body flattened, oval; foot, the whole ventral surface of the body; mantle encircles the body; gills, multiple, forming a continuous fringe between mantle and foot; head distinct from body; ten tacles wanting; eyes mostly wanting; radula well developed; sexes distinct; reproductive organs paired; eggs laid in ropes or clusters; kidneys paired. Mollusks nocturnal, sluggish in movements, curl up when disturbed; subsist chiefly on vegetable diet. Habitat, rocky shores. Distribution, world-wide, in temper ate and tropical seas. Eaten by poor classes in some localities..
The chitons are unique among mollusks. The shell is com posed of eight separate but overlapping plates. By this shell peculiarity all chitons may be instantly recognized by the most casual observer. The name, "coat-of-mail shells," is a very good one. All other mollusks have one or two valved shells, with the rare exception of shell-less forms.
The eight plates form a dorsal shield which, inverted, looks like a boat. The girdle is the leathery skin in which the plates are securely embedded. It extends beyond the wings of the shell plates, forming the thin-edged horny border of the shield. Underneath it extends to the body, which lies in the concave of the arching plates. The flat ventral surface of the body is the foot. The mantle is a muscular fold between the foot and the inner edge of the girdle. It is best seen when the foot muscles contract. The expanded foot throws the mantle into folds in the narrow oval groove. The gills are fringe-like, external, attached in the groove between the mantle and the foot. The head is scarcely more than a tapering extension of the body. It bears no 237 The Chitons. Coat-of-Mail Shells organs but the mouth. The mantle edge covers it completely, like a hood.
The chitons are an ancient family, geologically speaking. Thirty-eight genera, including about two hundred and fifty living species, are distributed over a large part of the world. Nine
tenths of these species live along shore in less than twenty-five fathoms of water, feeding upon the marine vegetation of this zone. Of these forms the greatest number are found under stones, between high and low tide levels, where the bottom is oozy and muddy. Chitons are rarely found on sandy coasts. A few species venture out to too fathoms depth; still fewer forms are dredged in The collector of shells needs a few instructions before he goes for chitons. These curious creatures are nocturnal in habits. They congregate by night where seaweeds are decaying, and return on slow foot to rest in certain familiar rock crevices by day. When the stone to which a chiton is attached is lifted, the mollusk seems to be grown fast to it, so strong is the suction of the broad foot. A quick thrust of a blunt knife under the shield is necessary to dislodge it. The disturbed mollusk will curl up into a ball, like a pill bug, if handled now. No use to try to straighten it by force. The stubborn muscles will break before they will relent. But drop the specimen into a bucket of salt water, and it will soon assume its natural position.
The business-like collector goes prepared, carrying small, smooth wooden slats and a ball of soft cotton twine. He deftly slips the specimens, one by one, to a place on the wet slat, to which they gratefully attach themselves, and are promptly bound with coils of the twine. Thus they are dried in proper position for museum or cabinet specimens.
The girdle is a muscular belt, which helps to hold the plates in place, and forms the margin of the shield. The leathery covering is variously coloured and marked. It may be striped or marbled, with smooth, horny surface, or covered with down, or scales, or tufted hairs,or stout limy spicules and knobs,like the sea urchins. Sometimes the margin has a deep posterior slit or notch. Sometimes the width is so great that the plates are entirely covered over by the girdle. In other forms the girdle is narrow.