A description has already been given of the dorsal eyes in Chiton (p. 187), the nervous system (p. [202]), the branchiae (p. [154]), the radula (p. [228]), and the generative system (p. [126]).
Fig. 264.—Girdles of various Chitonidae. A, Radsia sulcata Wood, × 2; B, Maugeria granulata Gmel., × 3; C, Enoplochiton niger Barnes, × 3; D, Acanthochiton fascicularis L., × 4; E, Tonicia fastigiata Sowb., × 4.
The recent Chitons are thus classified by Dr. W. H. Dall:—
Section I. Chitones Regulares.—Anterior and posterior valves of similar character.
A. Leptoidea.—Insertion plates obsolete, or, if present, unslit; Leptochiton, Hanleyia, Hemiarthrum, Microplax.
B. Ischnoidea.—Insertion plates sharp, smooth, fissured; with eaves; Trachydermon, Callochiton, Tonicella, Schizoplax, Leptoplax, Chaetopleura, Spongiochiton, Ischnochiton, Callistochiton.
C. Lophyroidea.—Insertion plates broad, pectinated, projecting backward; Chiton, Tonicia, Eudoxochiton, Craspedochiton.
D. Acanthoidea.—Insertion plates thrown forward; Sclerochiton, Acanthopleura, Dinoplax, Middendorffia, Nuttallina, Arthuria, Phacellopleura.
Section II. Chitones Irregulares.—Posterior valve abnormal, or with a sinus behind.