CHAPTER LVIII: THE CHITONS. 

 COAT-OF-MAIL SHELLS 



Order Polyplacophora 



Shell composed of eight overlapping plates, supporteo 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 



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