AMPHINEURA 



485 



it. The girdle varies much in character, its upper surface being either 

 horny, covered with scales, or densely beset with spines. The foot is 

 usually broad and flat, and is both a creeping surface and a sucker to 

 attach the animal to the rocks; in a few forms the foot is much reduced 



Fig. 745 



Fig. 744 — Ch(Etoderma nitidulum (Cambridge Natural History). 1, mouth; 2, 

 anus. Fig. 745 — Chiton squamosus. A, dorsal aspect; B, diagram showing mouth 

 (1), brain (2), pleurovisceral nerve chord (3), pedal nerve chord (4), gills (5), and 

 anus (6). 



12 II 



and narrow, especially in certain vermiform species. The mantle (Fig. 

 746) is a fold which projects over the side of the foot enclosing a deep fur- 

 row along each side and at the hinder end of the body, which is the mantle 

 cavity (5). In this cavity at the sides of the body are from 6 to 80 pairs 

 of gills (6) and the genital and kidney 

 pores, which are also paired. The mouth 

 is at the fonvard and the anus at the 

 hinder end. The head is rudimentary 

 and tentacles are absent. Special sense 

 organs are poorly developed, the most 

 important being peculiar sense buds 

 called aesthetes, which are present in 

 definite groups in the shell; in certain 

 species these organs are modified to 

 form eyes. 



The mouth opens into the pharynx, 

 in which a radula is present; a pair of 

 salivary glands join the pharynx. The 

 digestive tract is made up of a short 

 cesophagus, a stomach with a pair of 



livers, and a long coiled intestine. The nei^ous system (Fig. 745) is char- 

 acterized by the lack of definite ganglia, except two small pairs near the 

 pharynx, most of the ganglion cells being distributed throughout the larger 

 nerves. The heart consists of a tubular ventricle (Fig. 746,2) and a pair 

 of auricles (3), and lies in a pericardium (1), which represents the hinder 



Fig. 746 — Diagram of a cross sec- 

 tion of a chiton (Lang). 1, peri- 

 cardium ; 2, ventricle ; 3, auricle ; 

 4, branchial vein-; 5, mantle cavity ; 

 6, gills ; 7, foot ; 8, pleurovisceral 

 connectives ; 9, branchial artery ; 10, 

 ccelom ; 11, intestine; 12, gonad; 

 13, 14, nephridium ; 15, pedal nerve 

 chords. 



